Engineering Tools

Bolt Torque Calculator

Estimate the recommended tightening torque and resulting clamp load for a fastener, using the nut-factor (K) method. Select your bolt size, grade or property class, and lubrication condition.

Bolt size

Grade / class

Condition (nut factor K)

Target preload (% of proof load)

Recommended torque

0lb-ft

Torque (alt)

0 N·m

Clamp load

0 lb

Nut factor K

0.20

How it works

The short-form torque equation is T = K · D · F, where D is the nominal bolt diameter, F is the target clamp load (preload), and K is the nut factor that lumps together thread and head friction. Clamp load is taken as a percentage (75% by default) of the bolt's proof load, which is the proof strength multiplied by the tensile stress area. Friction dominates the result — roughly 90% of applied torque is spent overcoming it — so the lubrication condition you pick matters as much as the bolt grade.

Typical nut factors (K)

Condition	K (approx.)
Plain / as-received steel, dry	0.20
Zinc-plated	0.22
Lightly oiled	0.15
Wax, moly, or anti-seize	0.10

These are planning estimates from the nut-factor method and standard proof strengths. Actual torque-tension behavior varies with the specific fastener, surface finish, gasket or joint material, and assembly method. For structural, safety-critical, or torque-to-yield joints, follow the manufacturer's or engineer's specified torque values.

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