Bolt Torque Calculator

The bolt torque calculator helps you make sure you tighten your bolts correctly.

Tightening torque75.38 lb·ft904.6 lb·in · 102.21 N·m

Preload9,046 lb75% of the proof load, the usual target

Proof load12,061 lb85,000 psi over 0.1419 in²

Tensile stress area0.1419 in²π/4 × (d − 0.9743 ÷ TPI)²

K factor0.2the nut factor for this condition

Preload as a share of proof75%

Bolt stress63,750 psi

Torque in N·m102.21

If lubricated instead56.54 lb·ftthe same preload needs less torque

Thread pitch0.07692 in13 threads per inch

The formula

T = K × D × F, with K about 0.20 dry and 0.15 lubricated

Torque is a poor proxy for preload

What actually holds a joint together is the tension in the bolt. Torque is merely the easiest thing to measure, and the relationship between them runs through the K factor — which lumps together thread friction, the friction under the head, and the thread geometry.

K varies enormously with surface condition: from about 0.12 waxed to 0.22 zinc plated. Since the same torque then produces preloads differing by nearly two to one, torque-controlled tightening is typically accurate to only ±25%. Critical joints are tightened by angle, by bolt stretch, or with load-indicating washers.

Never reuse a bolt that has been tightened past yield, and never lubricate a joint whose torque figure assumed it dry — you will over-tension it.