Guide to Metric Nut and Bolt Sizes

Reviewed by David Carmichael, Solution Engineer (October 2024)

Nuts and bolts come in all sorts of shapes and sizes and are essential in industry and the home. Because fasteners vary so much it’s important to choose the right one, which is why understanding the difference between metric nuts and bolts and other types is helpful. This guide explains what metric nuts and bolts are, how metric nuts and bolts are measured, and provides a guide to names and metric bolt sizes, including a metric bolt size chart.

Whatever their size, nuts and bolts usually fall into one of two categories: standard/inch and metric (imperial or metric bolt sizes).

You’ll want to know how to tell if a bolt is metric or standard, so just what is a metric bolt?

Metric nuts and bolt sizes are commonly abbreviated to the capital letter M followed by a number, as in ‘M8’ or ‘M12’. By contrast, standard/inch nuts or bolts are defined by a number between one and 12, or a fraction of an inch, followed by threads per inch (the number of threads on an inch of the thread length). This helps when it comes to how to identify metric bolt threads.

Metric nuts and bolts are subject to International Organization for Standardization (ISO) standards, DIN standards in Europe, and American Society for Testing and Materials (ASTM) standards in the US.

In the UK, the ISO standards are covered by British Standards BS 3643-1 and BS 3643-2. These standards contain details of metric bolt dimensions: basic thread dimensions, thread profiles, diameter and pitch combinations, tolerance principles, and information from up-to-date ISO metric screw thread standards. The second part of the standard also provides tolerances and diameters for coarse and fine pitch threads in tabular format. Adhering to standards helps to ensure nuts and bolts are cross-compatible with each other.

You may be wondering how to measure a metric bolt. When specifying nuts and bolts for your application, it’s important to understand metric nut dimensions and metric bolt dimensions to select the correct fastener for the job. Metric nuts and bolts are measured in millimetres. Although metric nuts and bolts are commonly referred to using just M and a number, the full description of the size includes the diameter, pitch, and length of the bolt, or just the diameter and pitch for the nut.

For example, the metric bolt specification in full might read as: M8-1.0 x 20, which means:

• M. The fastener is metric.
• 8. The nominal diameter of the fastener is 8 mm.
• 1.0. The thread pitch (distance between the threads) is 1 mm.
• 20. The length of the bolt is 20 mm.

As mentioned, the thread pitch is one of three key measurements in the description of a metric fastener. The thread pitch is determined by measuring the distance between corresponding points on threads that are next to each other. Pitch diameter, meanwhile, is the diameter of a hypothetical cylinder if it were to pass through the middle point of the minor diameter and the major diameter. If there is no thread pitch listed on a metric fastener, it has what is known as a ‘coarse thread’ (a large pitch).

The major diameter (also referred to as ‘outside diameter’) is the diameter of the widest part of the thread, measured over the highest parts of the thread at a right-angle to the axis.

By contrast, the minor diameter is the smallest diameter of the thread of the bolt or nut. It is measured across the root of the thread.

If you need to create a metric thread, you can use a tapping drill to create the female thread (as in a nut). The typical way of producing the thread is by drilling a small hole and using the right diameter of tap to produce the thread. The initial hole should be smaller than the major diameter of the thread. When producing metric threads, tapping drill diameter can be figured out by subtracting the pitch from the major diameter of the thread.

A clearance hole is drilled to allow a bolt to pass through, but not its head. If you want to know the correct clearance hole diameter to drill for your metric bolt, you can use a table that shows the correct diameter for a bolt with an M number, like the metric bolt dimensions chart below.

What are metric bolt sizes? You can assess the dimensions of metric bolts using this metric bolt chart. When it comes to how to read metric bolt sizes, we’ve got you covered:

To recap, when you’re specifying a metric nut and bolt, remember to consider:

• Metric nuts and bolts always begin with a capital M. This differs from standard/inch nuts or bolts, which begin with a number between one and 12, or a fraction of an inch.
• Metric nuts and bolts should meet ISO standards. In the UK, BS 3643-1 and BS 3643-2 cover the standards required.
• Thread pitch is a key measurement. Thread pitch is determined by measuring the distance between corresponding points on threads that are next to each other.
• Diameter is also a key factor to consider when it comes to how to measure the size of a metric bolt. The major diameter is the diameter of the widest part of the thread, and the minor diameter is the smallest diameter of the thread of the bolt or nut.

There’s a wide range of metric nuts and bolts for every application, so you’re sure to find the one you need, but don’t forget that nuts and bolts vary in toughness. The type of steel used in manufacturing influences strength, with nuts and bolts made from alloy steels the strongest.

Explore RS’ range of fasteners and fixings to find the right fastener for the job. RS also stocks a wide variety of hex nuts and nuts and washers.