Essential Guide to Metric Fasteners

History of DIN, ISO, and EN

Deutsches Institut für Normung (DIN)

The DIN began in Berlin in 1917. Its initial purpose was quality assurance with the aim to improve global communication. Its first standard, the DIN 1, set specifications for taper pins, a type of steel rod used in mechanical engineering. Having these requirements improves manufacturing efficiency and bolsters trade. While using these standards is voluntary, it offers opportunities for increasing exports and efficiency.

The organization is an independent non-profit with over 26,000 contributing experts. It acted quickly after its start, publishing its 3000th standard in 1927. In 1951, the DIN joined the ISO, working on the country’s behalf. In 1961, it organized the European Committee for Standardization (CEN). The DIN increased its scope in the 1970s to include the burgeoning technology industry.

The global reach of DIN came into fruition with its participation in the Transatlantic Free Trade Agreement (TAFTA) in 2013. It strengthened its relationship with China’s Standardization Administration of the People’s Republic of China (SAC) with its Beijing office in 2014. Today, it works in tandem with the ISO and the EU to support European business in the global economy.

To date, DIN has published over 80,000 standards that cover requirements in a variety of industries, from acoustics to safety guidelines to the drying stages of paints and varnishes. The organization operates as an adjunct to the ISO and EN if standards do not exist with the two systems. While its primary interest is in Germany, 85 percent of its standards are European or international.

European Standards (EN)

The CEN is the European equivalent of the DIN, covering the participating nations of the EU. Like the DIN, the main focus of EN standardization is the countries it represents in the global marketplace. Its mission mirrors it, with requirements to ensure environmental protection, worker safety, healthcare, and R&D across a full spectrum of industries, services, and products.

ENs supersede any existing standards of the member nations, with immediate adoption by the countries. It has 20 fields of study and over 440 technical committees, supported by over 60,000 experts in their respective industries. Interestingly, other non-European countries have joined the CEN, including Serbia and Turkey. The organization also has several affiliates in Israel, Egypt, and Ukraine.

The drive toward standards has played out on an international scale. To avoid conflict, the CEN and ISO drafted the Vienna Agreement in 1991, giving the latter primary authority. CEN works with the European Committee for Electrotechnical Standardization (CENELEC) to develop its voluntary standards. The organizations pride themselves on their transparent drafting and maintaining process.

On the global front, its mission is to promote European industries in the global economy while lending support to international cooperation goals. The CEN and CENELEC recently launched the Strategy 2030 program. Its mission is to increase awareness and optimize the global digital transition. It includes a plan for creating a resource-efficient economy for creating a more sustainable EU.

International Organization for Standardization (ISO)

The ISO got its start in 1926 as the International Federation of the National Standardizing Associations (ISA). The organization ended its brief reign in 1942 and later emerged as the ISO in 1946 with 25 founding countries. Like the DIN and EN, its standards are voluntary. Its role is to oversee the existing national standards to unify these requirements for simplified global trade.

Its mission reflects that of other organizations to foster the reliability, quality, and safety of products, processes, and services. The ISO published its first standard, then referred to as recommendations, in 1951. In 1960, it released the ISO 31 for SI— International System of Units. This milestone established set requirements for measurement still in use today.

The removal of trade barriers took a giant leap forward with the publication of freight container standards in 1968, building on its earlier success. In 1971, its focus turned to the environment with water and air quality technical committees. The ISO then pivoted to quality management in 1987 with the ISO 9000 standards. Consumers and businesses both benefited from its implementation.

The ISO paved the way for organizations to assess and control their environmental impact with ISO 14001. It provides guidance for developing environmental management systems. It was followed by the ISO 50001 energy management standard in 2011. The organization has successfully adapted to new technologies in today’s digital world.

It’s main focus has remained on keeping workers safe, with the publication of ISO 45001 in 2018, detailing occupational health and safety management systems. Today, there are 165 member countries, with over 250 technical committees. It is the largest organization of its type in the world.

The ISO itself is a model for cooperation and transparency. In 2019, it published a record 1,638 new standards, covering industries such as information technology, mechanical engineering, and food and agriculture.

Its reach extended into privacy information management, its risk management toolbox for cybersecurity, and medical device regulation for adapting to a changing world. Its crowning achievement was the FDA’s adoption of these quality standards for future regulation of these technologies.

Fastener Property Classes

Fasteners are generally manufactured in accordance with two distinct sizing systems. The first fastener system is measured in inches and is known as the imperial system, due to it’s institution by Imperial England in 1824. This system categorizes steel material according to “grades”. Although originally instituted in England, and spread during English colonization, most countries no longer use this system, with the exception of the United States, Liberia and Myanmar.

The second fastener system is the metric system, originally created in 1618, the metric system was developed specifically for standardization, i.e. it’s more intuitive nature (whereas the imperial system was rooted in tradition and based on using the human body for measurements, i.e. it’s less intuitive conversions). The metric system was designated as the standard measurement for lengths in 1960 according to the System of International Units, which specifies the standard units for a range of variables, such as temperature, pressure, etc and quickly became the dominant measurement system in the world. Metric fastener material is categorized by “classes”. The fastener industry uses the term property classes because they describe the properties of the fasteners; most notably, strength.

The Fastener Backstory

The threaded bolts and nuts of today are a remnant of the industrial revolution — when the first mass-produced fasteners appeared. More specifically, a British company began the serialized production of screw bolts in 1760. While others soon followed suit, a problem emerged — different companies were making different-sized fasteners, which made it far more difficult to assemble machinery with them.

Fast forward to 1841, after the institution of the imperial system, and a solution was found — all sizes of fasteners were standardized across Britain. The pitch of the thread was set at 55 degrees, and certain bolt diameters had a set number of threads.

Initially, the USA only copied the british imperial system, but after a couple of years, they made a slight change to their standard by altering the thread by five degrees. This seemingly innocent change would represent issues a century later when the Allies would wage two World Wars together. After WWII, Canada, Britain, and the United States adopted a common fastener standardization which was applied using imperial measurements.

The advent of the ISO system for all kinds of international measurements has ushered in an age of the metric system; though that still hasn’t happened in North America. And while threads have been completely standardized by metric designs, other variables have not — which is why we have metric property classes today, along with imperial grades.

Imperial Grades

The United States is home to two grading systems that still use imperial grades:

  • Society of Automotive Engineers (SAE)
  • American Society for Testing and Materials (ASTM)

Both of these associations use number scales for their bolts — higher tensile strengths are depicted with higher numbers. The SAE scale is used for diameters of fasteners up to an inch and a half, with Grade 8 being stronger than Grade 5.

Every single SAE fastener contains letters showing you who the manufacturer is, along with markings that denote tensile strength. If a bolt or a nut has no visible markings, that means you’ve got a Grade 2 — the very lowest tensile strength level according to the SAE system.

On the other hand, a fastener containing three radial lines shows tensile strength at Grade 5. Finally, a Grade 8 fastener has six radial lines.

When you’re dealing with ASTM fasteners, the situation is much easier — they use simple alphanumeric codes. So, for example, if you were dealing with an ASTM A325 bolt that’s standardly used with structural steel joints — you’d see the A325 letters on the bolt. As you might imagine, this makes them far easier to identify.

Metric Classes

Next up, we’ve got the metric fasteners — most often used for machinery and factory plants. Their scale is quite simple to understand, and it shows the bolt diameter, ranging from M5 to M30. It’s easy enough — the M30 bolt comes with a thread diameter of 30mm.

When it comes to the tensile strength of these fasteners, each of their heads should have a numerical marking depicting the strength and the ID code of the manufacturer.

These numerical markings show the maximum load-bearing capacity of the fastener; in other words, the biggest load per square millimeter that the fastener is capable of carrying before breaking. If you’ve got a class 8.8 bolt, that means that it’s capable of bearing 80kg per square millimeter. And the .8 number is there to tell you that the fastener will begin bending or stretching at 80% of the specified load. So, if you’ve got a 10.9 bolt, then it is capable of bearing 100kg per square millimeter and will begin bending or stretching at 90% of the specified load and so forth with class 12.9.

Small fasteners

It should be noted that many smaller fasteners (like screws) are not graded, because there’s no need for their strength to be specified. Of course, fasteners used for specialized purposes and those of larger sizes absolutely must comply with specific strength requirements. That’s why we have the property classes for metric fasteners across the globe, and grades for imperial inch fasteners in the U.S.

Understanding Metric Fasteners

Metric fasteners are named using the diameter, pitch, and length.

Fastener name: M12-1.25 x 20

  • The M indicates that the measurement for the fastener is metric.
  • The 12 tells you the diameter in millimeters.
  • 1.25 explains the distance between threads, also known as the pitch.
  • 20 tells you how long the fastener is in millimeters.

To determine the coarseness of metric fasteners you should know that as the number gets larger, the thread pitch becomes coarser. These different levels of coarseness can be used for different purposes.

For example, coarser fasteners are often used as car parts while finer pitch fasteners are more common in plumbing fixtures.

Converting Metric Pitch to Threads Per Inch

People accustomed to working with inch fasteners might get confused when switching to metric. That’s because inch fasteners explain how many threads per inch a fastener has instead of the space between threads.

But you can easily convert the pitch of a metric fastener to understand its threads per inch. Here’s the formula you’ll follow.

  1. Convert the pitch from millimeters to inches by multiplying the pitch by 0.03937.
  2. Divide 1 by the number you got in step 1.

To make things simple for you, we’ve added a table below that converts metric pitch to threads per inch.

Table 1. Fastener thread pitch conversion from millimeters to inches to threads per inch.

What to Do if Pitch Isn’t Indicated

A widely accepted way of referring to a fastener’s pitch is by indicating it is coarse or fine. In some rare cases, you might also see extra fine threads.

Some fasteners don’t indicate a pitch size. So what do you do if there isn’t a clear indication? Fasteners that exclude pitch information are generally classified as coarse.

What you have to watch out for is that fasteners classified as coarse that use threads per inch are generally even more coarse than metric fasteners that are classified as coarse.

For example, M14-2.0 screw is considered coarse. Its threads are spaced 2.0 millimeters or 0.079 inches. On the American scale, the 9/16-12 screw is considered comparable and has a coarse classification as well. However, its threads are spaced 0.083 inches apart.

In this example, the American screw would have a coarser pitch than the metric one. There are some rare instances where the comparable American equivalent is finer than its metric counterpart, so you’ll want to pay attention and use the conversion information listed above.

Comparing Length of Metric Vs. Inch Fasteners

One common ground for fasteners is how to measure the length. The only difference is that metric uses millimeters to indicate the length and American fasteners use inches. Here’s how to measure various types of fasteners.

  • Socket, low socket, cheese, hex, button and pan: measure the fastener’s length as everything under the head.
  • Flatheads: measure the entire fastener from head to the end.
  • Oval heads: measure from where the dome top and conical bearing meet down to the tip of the fastener.

Metric Fastener Materials

Once you understand the naming conventions for metric fasteners, you’ll have another decision to make before purchasing. Fasteners can be made from a variety of materials, including:

  • Steel
  • Stainless steel
  • Aluminum
  • Brass
  • Hot dip galvanized
  • Polyamide
  • Silicon bronze
  • And more…

Metric Fastener Thread Types

There are several metric fastener thread types that Eurolink Fastener Supply Service sources. These are some of the most common types.

  • UNF
  • UNC
  • BA
  • BSF

Metric Fastener Thread Types Explained

Metric threads are available in fine and coarse threads. In addition to the thread’s coarseness, you can select from a variety of materials for the fastener to be made of.

The various coarseness and materials each serve their own purpose. While you might use a very coarse thread for one industry, another industry might favor a finer thread for its purposes. Coarser fasteners are easier to find and more commonly used.

Fine threads are not well suited for high-speed assembly. However, they are often better for fastening hard material using less torque. The finer the thread, the less tendency the fastener has to loosen.

Here’s a definition of each of the most common thread types.


Unified Coarse (UNC) is the most common metric fastener thread type for the U.S. Agriculture is an industry that uses UNC frequently. That’s because these are strong, coarse threads.

The deeper, more generic threads make these fasteners easier to remove. However, they also have a high tolerance for the manufacturing industry.


Unified Fine (UNF) is another popular thread in the U.S. These tight, fine threads are common in the automotive and aerospace industries. You’ll see these threads most commonly used in steel and stainless steel.

They are heavy load bearing and better torque locking than UNC.


British Association (BA) are old-style British thread fasteners dating back to 1884. They have numbered diameters and were standardized in 1903. Most commonly, these thread types are used in electrical fittings.

However, because this is a British-style fastener, it’s slowly phasing out in favor of UNF and UNC since these are more universal metric styles. Additionally, you’ll struggle to find BA style fasteners in small screws.


British Standard Fine (BSF) is another type of British thread. You’ll find these threads in both steel and stainless steel. It’s most commonly used on vintage cars and machinery. Just like BA, it’s harder to find due to a heavy favor for metric threads.

As the name implies, these are finer thread bolts, screws, and nuts compared to BA fasteners.

Metric Fastener Materials and Coatings

Fastener materials and coatings are an essential aspect of the performance and longevity of your equipment. How and where you’ll the fastener will dictate what materials and coatings you should consider.

Fastener Materials

Fastener materials are generally stainless steel, steel, silicon bronze, brass, or aluminum. Here’s a look at each of these materials and what they’re good for.

Stainless Steel

One of the most corrosion-resistant materials for the price is stainless steel. It is an alloy with inherent properties that make it anti-corrosive. Additionally, you can rest easy knowing that if you scratch the metal while installing, it will maintain its anti-corrosive properties.

Because of its low carbon content, stainless is actually not as strong as regular steel. It cannot undergo the same heat treatment that makes steel so hard. You need to be very careful when installing stainless steel fasteners because they can seize up when you’re installing them. Another term for this process is galling.

Another characteristic of stainless steel is that it is less magnetic than regular steel. There are three different types of stainless steel you should be aware of:

  • 18-8 stainless: this type of stainless steel contains 18 percent chromium and 8 percent nickel. Out of all the types of stainless steel you’ll see on the market, this is the most popular.
  • Stainless 316: you’ll see this stainless steel used the most in salt water or chlorine because it is the most corrosion-resistant. It is more expensive than other stainless options, though.
  • Stainless 410: this grade of stainless steel is harder than 18-8 but is not as corrosion-resistant.


Steel is a very common material for popular fasteners. You can get fasteners that are entirely steel or get some with different treatments that could aid you in your use of the equipment.

Generally, steel fasteners come in four grades. While other grades exist as well, they are not nearly as common or easy to find. Here’s a look at the major steel grades and what you need to know about them.

  • Grade 2: you’ll find this grade at most hardware stores. It’s extremely common and not very expensive. This grade generally has no headings or markings on them.
  • Grade 5: these steel fasteners go through more hardening to increase their strength. The automotive industry favors these bolts because of their strength. To identify these bolts, look for three radial lines evenly spaced on the head.
  • Grade 8: grade 8 bolts are even harder than grade 5. These strong bolts are popular for automotive suspension because of their extreme strength. Look for bolts with six radial lines evenly spaced on the head to identify this grade bolt.
  • Alloy Steel: these bolts are very strong but also fairly brittle. They are not plated, which leaves them with a dull black look.

Bronze/Silicon Bronze

Bronze fasteners are made of copper and tin with a little bit of silicone. That’s why they are also sometimes called silicon bronze. The most common use for these fasteners is in seawater.

These highly corrosion-resistant fasteners are very strong compared to stainless steel. That’s why you’ll see them in boat construction since they’re reliable but also at low risk of corrosion.

Bronze fasteners are more expensive than other types of fasteners, but their strength and durability make them long-lasting.


Brass is made up of copper and zinc. Although it is corrosion-resistant, its use cases are limited because it is so soft. These fasteners are also electrically conductive. It is an attractive fastener, though, which is why people still use it despite its challenges.


Aluminum is inherently corrosion resistant, just like steel. That means that scratches won’t affect its corrosion resistance if something happens during transport or installation. These fasteners are very lightweight.

However, their lightweight property also means they are softer than other fasteners. Adding silicon can help make these fasteners stronger.

Fastener Coatings

In addition to the materials a fastener is made of, you can also choose from various coatings that will impact their durability and use. There are three main fastener coatings to choose from.

Zinc Plating

Zinc helps with making a fastener more corrosion resistant. The zinc plating gives the fastener a silver or gold appearance. While the coating can help make the fastener more corrosion resistant, it does break down in saltwater environments, leading to rust.

Hot Dip Galvanizing

Hot dip galvanizing adds a thick coating of zinc to the metal. It helps make the fastener extremely corrosion resistant.

Because the coating is so thick, it is not compatible with standard nuts. You’ll need to use galvanized nuts as well, which are larger to accommodate for the thick coating. These fasteners are ideal for outdoor conditions, especially in seawater.


Chrome plating provides both corrosion resistance and an attractive, shiny finish. But because of the use of chrome, these fasteners are more expensive than zinc. Chrome is often added to stainless steel fasteners to add even more corrosion resistance. The great news about using chrome-plated stainless steel is that they are still corrosion-resistant after the chrome is scratched or damaged.

Bolt Measuring Guide

Metric Bolt Measurement

  1. Metric vs. imperial
  2. Shank diameter
  3. Shank length
  4. Thread pitch

When searching for your parts, you’ll come across two different types of bolts. The first type uses the metric system. The second, is based on the American standard of imperial units. The differences might seem subtle, but basing your measurements off the wrong system can lead to major headaches down the road.

Here’s what you need to know about each:

  • Metric – measured by counting the number of threads per inch.
  • Imperial – measured by the distance between threads

Knowing how your prospective bolts are measured is the first step in ordering the perfect part for your project.

Once you’ve chosen metric or imperial, it’s time to dig in and start measuring the bolts.

The size of a metric bolt is determined in millimeters using its:

  • Diameter
  • Pitch
  • Length

For example, the metric bolt size M4- 0.7 x 20. This translates as:

  • M = Metric thread designation
  • M4 = Diameter of the fastener
  • 7 = The pitch in millimeters, i.e., the measurement from one thread to the next
  • 20 = length measured in millimeters

Shank Diameter

The Shank Diameter, frequently called the Major Diameter, is the diameter of a bolt measured in millimeters. The Shank Diameter is roughly the same as the Major or Thread; therefore, this measurement is suitable for fully threaded bolts.

Shank Diameters range from M1 to M100 (and sometimes even greater), though the vast majority of stocked diameters are between M4 and M42.

Some diameters are referred to as 2nd or 3rd preference class. This means that they are relatively rare or non-standard diameters.

Sizes such as M3.5, M14, M18, M22, M27 and M33 are considered 2nd preference class.

Sizes such as M7, M11, M15, M25, M26, M28 and M39 are 3rd preference class.

The higher the preference class, the more likely the bolts will be difficult to source from stock, especially stateside stock. Some bolts are not available in the 3rd preference class diameters at all.

Shank Length

Shank Length measurement depends on whether the bolt is designed to be countersunk or sit above the surface. It measures how far into the material the fastener is intended to go. Understanding what is needed for the finished project is crucial in determining the Shank Length needed.

If the bolt head is meant to sit above the surface, the measurement is from the underside of the head to the bottom of the bolt.

However, if the bolt head is intended to be countersunk, the measurement takes in the entire length of the screw.

Thread Pitch

While imperial fasteners typically use thread count, metric bolt measurement uses pitch. Thread Pitch is measured in millimeters by the distance between each thread.

For example, on an M4- 0.7 x 20 bolt, the thread pitch is defined as a 0.7 mm distance between each thread.

Thread pitch generally increases with bolt diameter. For example, whereas the M4 diameter bolt has a standard coarse thread pitch of 0.7 mm, an M6 would have a standard coarse thread pitch of 1.0mm. Thread pitch continues to increase with diameter until M64. Above M64, the standard coarse thread pitch remains at 6.0mm.

Bolts can be coarse thread, fine thread or extra fine thread. Generally coarse thread is most common, some bolts are frequently stocked with a fine thread option (such as DIN 961, being the fine thread version of DIN 933). Extra fine thread bolts will be the most rare. Fine thread and extra fine thread options may be even more limited for 2nd and 3rd preference class diameter bolts.

Bolts That Omit Pitch Definition

Many metric bolt measurements fail to include the thread pitch. They can appear as M8 X 40. If no pitch is specified, this means the bolt thread is coarse.

Our example above of M8 x 40 has a diameter of 8mm and a length of 40mm. Without the thread pitch being specified, the account manager will assume that coarse thread is required, therefore the customer is looking for a M8 – 1.25 X 40 bolt, as 1.25mm is the standard coarse thread pitch for the M8 diameter.

Generally, if the thread pitch is included, the account manager will check to see if the required thread pitch is coarse or fine thread. For example, if a customer requests a M8 – 1.0 X 40 bolt, then the customer is looking for a fine thread version of that bolt. The thread pitch could not be omitted in this case, because it is a defining requirement for that fastener.

Metric Bolt Measurement Standards

While different standardizations, such as DIN, ISO and JIS, may or may not require different dimensions for their respective bolts, the measuring of the bolts is generally the same, with only very few exceptions. The rare exceptions generally only affect how they are defined and not necessarily a change in the dimensions themselves. This is clearly seen in some pins.

Other Important Measurements

Some bolts have special features that can be required for proper sourcing. For example, hex head cap screws have a hexagon above the shank. The head itself is often measured by the WAF (width across the flats – the width across the head from one side to the adjacent side).

WAF can actually differ between standardizations, such as between DIN 933 and ISO 4017. At most sizes, there is no difference, but at the M10, M12, M14 and M22 sizes, the WAF is actually slightly different, which can affect it’s usefulness in some applications.

Another example is flange diameter, with some bolts have lesser or greater flange diameter (washer-like ring that extends out from under the head).

Flanged Hex Head Bolts

Tapered Socket Cap Bolts

Dome/Button Head Cap Screws

Flat Washers

Countersunk Body Washers

Common Uses of Metric Fasteners

Metric Fasteners for the Automotive Industry

Car frames date back to the 1940s, when metric fasteners were the go-to. Even today’s modern cars continue to use metric fasteners as the standard because they have various decimal dimensions instead of inches like standard measurements.

Even American-made cars use metric bolts. This universal use makes it easier for mechanics and dealerships to work on vehicles using one set of sockets.

Metric fasteners are also marketed globally and are measured by the millimeter. This high-quality and precise production process makes it a dependable option for the automotive industry. It also makes it simpler for the auto industry to source these consistent fasteners.

Aviation Industry Metric Fastener Usage

Airplanes are enormous transportation devices where if one small thing goes wrong, the consequences can be devastating. Airplane manufacturers use metric fasteners because of their strength and durability.

While fasteners come in a wide variety of materials with various strengths, those that you find in aviation tend to be the most durable and resilient type.

Why Manufacturing Uses the Metric System

Over the last 50 years, the U.S. has become more reliant on the metric system. Manufacturers started taking note of this shift and knew that sourcing materials for the upkeep of their facilities would be more affordable and attainable if their plants used metric fasteners.

In 1972, P&G began to slowly shift its various manufacturing to metric machinery. With the shift, the parts necessary to maintain the machinery would be available worldwide. As a global company, this was great news for the company.

As the company built new plants and replaced old machinery, they opted for metric machines. This trend is not isolated to P&G. Xerox is another global company that recognized the need to use the metric system when their products expanded outside the U.S.

Sourcing Hard-to-find Metric Fasteners

Metric fasteners provide the wide range of sizes, styles, and materials you need to get the job done. But some lesser-used sizes and types might be hard to source.

Eurolink Fastener Supply Service partners with you to source these more unique metric fasteners. We provide regular recurring orders and quick one-time orders to get the job done.

Contact us to start an order or request a quote.


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