How to calculate bolt force

Written by doug leenhouts
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How to calculate bolt force
Bolt torque is a function of the bolt's diameter. (Getty Images)

Calculating bolt torque, the force required to turn a bolt such that it delivers a known amount of tension, can be accomplished using a simple equation. This equation assumes the most rudimentary of cases; it does not account for any lubrication used on the bolt or nut, any locking mechanisms used or non-standard threads used. It is critical to apply the correct amount of torque to a bolt; under-torquing can lead to uneven loads delivered to the rest of the assembly, while over-torquing can result in failure at the secured area.

Skill level:

Things you need

  • Calculator
  • Ruler
  • Desired axial tension

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  1. 1

    Measure the diameter of the bolt using your ruler. This measurement includes the outside edge of the threads.

  2. 2

    Find your desired bolt load in kg (or pounds). This number is dependent on the material of the bolt and thread design. It is most commonly a given value and is considered the "clamping force," according to EngineersEdge.

  3. 3

    Multiply the diameter by the axial load, then multiply that product by 0.2. This calculation will give you the bolt torque.

Tips and warnings

  • If your diameter was measured in inches, divide the bolt force by 12 to convert it to foot-lbs.

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