Bolt Torque Calculator

How to use this calculator

1. Enter or choose find.
2. Enter or choose nominal bolt diameter d.
3. Enter or choose nut factor K.
4. Enter or choose tensile stress area A_s.
5. Enter or choose proof strength.
6. The tightening torque updates as you type. Use Copy result or Share to keep it.

Method

T = K × F × d; F = T ÷ (K × d); σ = F ÷ A_s; proof load = A_s × proof strength

T

tightening torque

K

nut factor (dimensionless)

F

bolt preload

d

nominal diameter

A_s

tensile stress area

Worked example

1. T = 0.2 × 20,000 N × 0.010 m = 40 N·m.
2. σ = 20,000 N ÷ 58 mm² = 344.8 MPa.
3. Proof load = 58 × 580 = 33,640 N; 20 kN is 59.5% of it.

Clamping an M10 bolt to 20 kN with K = 0.2 needs about 40 N·m, loading it to about 60% of a 580 MPa proof strength.

What the result means

Preloads of roughly 70–90% of proof load are common for reusable and permanent joints respectively, but follow your standard or the fastener maker. Because K varies widely with lubrication and finish, torque control alone typically gives preload scatter of ±25% or more.

Assumptions and limits

• Short-form torque–preload relation with a single nut factor; standard threads; preload carried by the tensile stress area.
• Does not model joint stiffness, external load sharing, embedding, relaxation or prevailing-torque nuts.
• For critical joints, use the fastener manufacturer’s data or measure preload directly.