Understanding Metric Fasteners (2024)

Metric fastener sizes are specified by diameter, pitch and length, in millimeters (mm). Sizing for nuts will appear as diameter and pitch. If the pitch isn't specified, the fastener is assumed to be coarse thread by default. Here is an example of how metric sizing:

M10-1.0 x 20

Taking a closer look at this description, let’s explore its meaning:

  • M = This designates the fastener is a metric size

  • 10 = The nominal diameter in millimeters

  • 1.0 = The thread pitch, or distance between threads, in millimeters

  • 20 = The fastener’s length, in millimeters

Now, let’s look at an example illustrating how a coarse thread fastener would be labeled:

If the size is M10 x 25, that would mean that the diameter is 10 mm. Since there is no thread pitch listed, the fastener is assumed to have a coarse thread. The 25 indicated a length of 25 mm. A matching nut would be labeled M10, which also doesn’t require a thread pitch since it’s a course thread fastener.

If you generally work with fasteners using imperial measurements (inches, etc.), the pitch notation can be a confusing. Just remember that thread pitch means the distance between adjacent threads, so in the example at the beginning of this article, the 1.0 thread pitch indicates that the distance between one thread and another is 1.0 millimeter.

Imperial measurement fasteners are classified by threads per inch, so a 1/4-20 bolt has 20 threads per inch (TPI), while a 10-24 screw has 24 threads per inch. You can determine the threads per inch of a metric fastener by multiplying the millimeters by 0.03937, and then dividing 1 by that number.

At Mudge Fasteners, we’re here to help. If you have questions about any of the dimensions of metric fasteners, or need help with conversions, just contact us. We have also created some handy brochures to assist you, including our metric line card and metric conversion chart with selection guide.

Understanding Metric Fasteners (2024)


What does m6 M8 M10 mean? ›

Designation. A metric ISO screw thread is designated by the letter M followed by the value of the nominal diameter D (the maximum thread diameter) and the pitch P, both expressed in millimetres and separated by a dash or sometimes the multiplication sign, × (e.g. M8-1.25 or M8×1.25).

What does m8x1 25 mean? ›

The "M" in the designation stands for "metric," indicating that the bolt follows the metric system of measurement. The first number, 8, represents the bolt's nominal diameter in millimeters, while the second number, 1.25, represents the distance between each thread crest.

What does M10 mean on a bolt? ›

Bolt Sizes Chart & Metric Bolt Dimensions
Thread SizeMajor Diameter (mm)Thread Pitch (mm)
36 more rows

What does M8 mean on a bolt? ›

Each size is denoted by the letter 'M' followed by a number. This number indicates the outer diameter of the thread, measured in millimetres. As an example, a hex bolt with a thread size of M8 would have an outer diameter of 8mm.

What does M10 mean? ›

Metric threads are designated by the letter M followed by the nominal major diameter of the thread and the pitch in millimeters. For example M10 x 1.0 indicates that the major diameter of the thread is 10mm and the pitch is 1.0mm. The absence of a pitch value indicates that a coarse thread is specified.

How do I know if I have M8 or M10? ›

Thread Diameter: M8: The "M" stands for metric, and "8" refers to the nominal diameter of the bolt's thread in millimeters. So, an M8 bolt has a thread diameter of 8mm. M10: Similarly, an M10 bolt has a thread diameter of 10mm. M12: An M12 bolt has a thread diameter of 12mm.

How do I know if I have M6 or M8? ›

If the thread is specified simply as M6, it's a standard pitch. Standard pitch would be fully written as M6 x 1.0. Fine would be M6 x 0.8. Note, though, that the pitch varies with the thread diameter – so a standard M8 thread is M8 x 1.25 and a fine thread M8 x 1.0.

What does M4 mean in screws? ›

Standard metric screw dimensions are determined by the pitch and the major diameter, which as the illustration shows, is the largest diameter of the screw. Let's say it's 4mm, in which case it appears as M4 (the M means metric). Now let's say the pitch is 1mm. The size of your screw is M4 x 1mm.

How do you know if a thread is mm or inch? ›

First measure the diameter of the thread with a calliper. Using the conversion table below, you can determine the inch dimension as a metric dimension and convert it. By definition, one inch is 25.4 mm. The diameter therefore tells you whether the thread is metric or inch.

What does the M stand for in bolts? ›

For nuts the size dimensions used are simply diameter and pitch. Here's an example metric bolt specification: M8-1.0 x 20. This bolt specification breaks down like this: The letter 'M' indicates that this bolt uses a metric thread. The number '8' refers to the nominal diameter of the bolt shaft in millimeters.

What is the difference between M10 and M10x1 25? ›

So, a thread labeled “M10” implies a coarse pitch by default. Any added pitch callout, like M10x1. 25, indicates a non-coarse pitch. This ISO standardization dramatically aids in reducing confusion and errors in the manufacturing process.

What nut fits a M10 bolt? ›

Coarse Thread Bolts
27 more rows

Is M8 the same as 5 16? ›

M8 is the designation for a metric (M) sized bolt or screw that is 8mm (8) in diameter. This is roughly equivalent to a 5/16 bolt or screw.

What does M12 mean on a bolt? ›

The M12 in the name refers to 12 millimetres. That's not the diameter of the bolt's head, though. It's the diameter of the bolt's thread and thus what you should be keeping in mind when measuring your hole. Other M-labelled bolts all follow the same system – an M16 refers to a bolt with a 16-millimetre diameter thread.

What does M12 1.75 mean? ›

Coarse screws have a larger pitch than similarly-sized fine screws. A M12 x 1.75 x 85 screw, for instance, has threads every 1.75 mm (0.069 in). The second number, 1.75, indicates the pitch. The metric labeling system doesn't label coarse or fine screws, so you have to pay attention to the pitch.

How do you tell if a fastener is metric or standard? ›

Metric fasteners use thread pitch

It is expressed in millimetres. Metric fasteners are typically prefixed by an M, which makes them easily distinguishable from imperial fasteners. Thread patterns are classified as either coarse (longer) or fine (shorter) pitch.

What does 10.9 mean on a bolt? ›

Property Class 10.9 bolts and screws have a minimum tensile strength of 1040 MPa for all sizes. As you can see, they're slightly stronger than the nominal value in the property class designation. Property Class 10.9 bolts are considered high-strength parts. They're in the neighborhood of SAE J429 Grade 8 parts.

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