Work out bolt preload instantly with clear inputs, formula shown and shareable results.
Preload is the tensile stress area multiplied by the target stress. Tensile area uses the unified thread relation pi/4 x (D - 0.9743/n)², which sits between the pitch and minor diameters. Structural practice targets 65-75 percent of proof strength so the bolt stays elastic while clamping hard enough to keep the joint from slipping.
Tensile stress area
At = pi/4 x (D - 0.9743 / n)²
Preload
F = At x yield strength x target fraction
Preliminary estimate only. Bolted joint design, including preload targets and tightening method, must be verified by a qualified engineer against the governing design code.
At = pi/4 x (D - 0.9743 / n)². Preload is the tensile stress area multiplied by the target stress.
A highly preloaded bolt sees very little of the external load as stress variation, so its fatigue life is far better than a loosely tightened one. The clamp also prevents joint slip and thread loosening.
This calculator takes 4 inputs: Bolt nominal diameter, Threads per inch, Proof or yield strength, Target percentage of yield. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.