Bolt Ultimate Strength Calculator

This calculator works out the ultimate (breaking) tensile load of a bolt from its nominal diameter, thread pitch, and material tensile strength — useful for a quick check on whether a bolt has enough margin for an applied load, or for comparing the raw strength of different bolt sizes and grades before selecting one for a design. It calculates from the bolt's tensile stress area (the effective cross-sectional area used for thread strength calculations, which is smaller than the nominal diameter would suggest) multiplied by the material's ultimate tensile strength.

For the strength grade markings themselves, see our Bolt Grades and Markings chart; for recommended tightening torque at a given preload, see our Fastener Tightening Torque Information page.

The Calculators below can be used to calculate the ultimate (breaking) load of bolts.

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.

Results are in Newtons. To convert these you can use our unit conversion calculator.

BOLT ULTIMATE STRENGTH CALCULATOR (METRIC THREADS)

Select your Bolt Size and Grade, your result will be calculated.

RELATED PAGES

• Bolt Grades and Markings
• Fastener Tightening Torque Information
• Thread Designation Charts

FREQUENTLY ASKED QUESTIONS

What's the difference between ultimate strength and proof load?

Ultimate strength is the load at which the bolt actually fractures; proof load is a lower value the bolt must withstand without any permanent (plastic) deformation. In practice, a bolt is designed to operate well below both, with the proof load acting as the more relevant design limit for most static applications.

Why is the tensile stress area smaller than I'd expect from the bolt's diameter?

Because the thread root reduces the effective load-bearing cross-section compared to the nominal (major) diameter. The tensile stress area accounts for this, sitting roughly midway between the minor and major thread diameters, and is the correct value to use for strength calculations rather than the nominal diameter alone.

Does this calculator account for fatigue or dynamic loading?

No — this calculates static ultimate tensile strength only. Bolts under cyclic or dynamic loading should be assessed against fatigue limits separately, which are typically much lower than the static ultimate strength.