Definitions

The ISO metric screw threads are the world-wide most commonly used type of general-purpose screw thread. They were one of the first international standards agreed when the International Organization for Standardization was set up in 1947.

## Basic profile

The design principles of ISO general-purpose metric screw threads ("M" series threads) are defined in international standard ISO 68-1. Each thread is characterized by its major diameter D and its pitch P. ISO metric threads consist of a symmetric V-shaped thread. In the plane of the thread axis, the flanks of the V have an angle of 60° to each other. The outermost 1/8 and the innermost 1/4 of the height H of the V-shape are cut off from the profile.

The height H and the pitch P are related through the 60° angle via

$H = sin\left(60^circ\right)cdot P = frac\left\{sqrt 3\right\}2cdot P = 0.866025404 times P$

In an external (male) thread (e.g., on a bolt), the major diameter Dmaj and the minor diameter Dmin define maximum dimensions of the thread. This means that the external thread must end flat at Dmaj, but can be rounded out below the minor diameter Dmin. Conversely, in an internal (female) thread (e.g., in a nut), the major and minor diameters are minimum dimensions, therefore the thread profile must end flat at Dmin but may be rounded out beyond Dmaj.

The minor diameter Dmin and effective pitch diameter Dp are derived from the major diameter and pitch as

$D_mathrm\left\{min\right\} = D_mathrm\left\{maj\right\} - 2cdotfrac58cdot H = D_mathrm\left\{maj\right\} - frac\left\{ 5 \left\{sqrt 3\right\}\right\}\left\{8\right\}cdot P = D_mathrm\left\{maj\right\} - 1.082532 times P$

$D_mathrm\left\{p\right\} = D_mathrm\left\{maj\right\} - 2cdotfrac38cdot H = D_mathrm\left\{maj\right\} - frac\left\{ 3 \left\{sqrt 3\right\}\right\}\left\{8\right\}cdot P = D_mathrm\left\{maj\right\} - 0.649519 times P$

## Designation

A metric ISO screw thread is designated by the letter M followed by the value of the nominal diameter D (Dmaj in the diagram above) and the pitch P, both expressed in millimetres and separated by the multiplication sign “×”.

Example: M8×1.25

If the pitch is the normally used "coarse" pitch listed in ISO 261 or ISO 262, it is omitted.

Example: M8

Tolerance classes defined in ISO 965-1 can be appended to these designations, if required.

Example: M10 – 6g in external threads.

## Preferred sizes

International standard ISO 261 defines a detailed list of preferred combinations of outer diameter D and pitch P for ISO metric screw threads. ISO 262 defines the following shorter list of thread dimensions – a subset of the combinations listed in ISO 261 – which are selected sizes for screws, bolts and nuts:

Nominal diameter
D (mm)
Pitch
P (mm)
1st
choice
2nd
choice
coarse fine
1 0.25
1.2 0.25
1.4 0.3
1.6 0.35
1.8 0.35
2 0.4
2.5 0.45
3 0.5
3.5 0.6
4 0.7
5 0.8
6 1
7 1
8 1.25 1
10 1.5 1.25 or 1
12 1.75 1.5 or 1.25
14 2 1.5
16 2 1.5
18 2.5 2 or 1.5
20 2.5 2 or 1.5
22 2.5 2 or 1.5
24 3 2
27 3 2
30 3.5 2
33 3.5 2
36 4 3
39 4 3
42 4.5 3
45 4.5 3
48 5 3
52 5 4
56 5.5 4
60 5.5 4
64 6 4

The "coarse" pitch is the commonly-used default pitch for a given diameter. In addition, one or two smaller "fine" pitches are defined, for use in applications where the height of the normal "coarse" pitch would be unsuitable (e.g., threads in thin-walled pipes). The terms "coarse" and "fine" have in this context no relation to the manufacturing quality of the thread.

## References

International standards:

• ISO 68-1: ISO general purpose screw threads — Basic profile — Metric screw threads.
• ISO 261: ISO general purpose metric screw threads — General plan.
• ISO 262: ISO general purpose metric screw threads — Selected sizes for screws, bolts and nuts.
• ISO 965-1: ISO general purpose metric screw threads — Tolerances — Part 1: Principles and basic data.

Equivalent national standards: