Bolt Ultimate Strength Calculator

This section calculates the ultimate (beaking) load of ISO metric bolts with fine and coarse pitches using the same method as most published data. This uses the equations from ANSI B1 1-1974 and B18.3.1-1978 which calculates the area carrying the tensile load from the mean of the pitch diameter and the minor diameter.

The minor diameter is given by: dr = d - 1.226869p and the pitch diameter by dm = d - 0.649519p. A slightly different value will be obtained if the 'core' diameter values are used to calculate the tensile stress area.

Unless it is specified that the thread is 'fine pitch' it should be assumed that it is 'coarse pitch'.

The table below shows the pitches corresponding to fine and coarse threads for a range of bolt sizes.

Nominal diameter, mm Fine pitch, mm Coarse pitch, mm
3 n/a 0.5
3.5 n/a 0.6
4 n/a 0.7
5 n/a 0.8
6 n/a 1
7 n/a 1
8 1 1.25
10 1 1.5
12 1.5 1.75
14 1.5 2
16 1.5 2
18 1.5 2.5
20 1.5 2.5
22 1.5 2.5
24 2 3
27 2 3
30 2 3.5
33 2 3.5
36 3 4
39 3 4

Valid bolt grades to be entered in the box below are:

12.9 (alloy steel, quenched and tempered with a minimum UTS of 1220 MPa) or,

8.8 (carbon steel quenched and tempered or carbon steel with additions of boron or chromium or manganese, quenched and tempered) with a minimum UTS of 800 MPa (up to 16 mm diameter - the figures used in the calculations, although above 16 mm diameter 830 MPa UTS can be assumed).

Bolt diameter:
Thread pitch:
Bolt grade:

Answer (Bolt ultimate strength in N):

Return to Page on Bolted Joints.

Return to Module Introduction.

David J Grieve, 26th August 1999.