TorqueMath

Bolt Torque Calculator

Pick the bolt size, strength class and how the thread is lubricated. Get the tightening torque, the clamping force it creates, how far the bolt stretches, and the hand force on your wrench.

Thread size
Strength class (head marking)
Thread condition
K factor
% of proof load
75% is the usual choice for reusable joints, 90% for permanent structural joints.
mm
mm
Tightening the nut stretches the bolt and clamps the plates

Torque table

How it is calculated

Short-form torque equation with a nut factor K, which lumps together thread and under-head friction. d is the nominal diameter, Aₛ the tensile stress area, σₚ the proof stress of the strength class.

Proof load Fp = σp · As Preload F = (target %) · Fp Tightening torque T = K · F · d Bolt stretch ΔL = F · L / (E · As) E = 205 GPa steel, L = grip length Hand force P = T / wrench length Where torque goes ≈ 50% under the head or nut, ≈ 40% in the thread, ≈ 10% stretches the bolt (dry steel)

Proof stress (ISO 898-1): 4.6 = 225, 5.8 = 380, 8.8 = 600 (640 above M16), 10.9 = 830, 12.9 = 970 MPa. SAE J429: Grade 2 = 55 ksi (up to 3/4"), Grade 5 = 85 ksi, Grade 8 = 120 ksi. Tensile stress areas from ISO 898-1 and ASME B1.1. Friction varies a lot, so a torque setting gives preload within about ±25%. For critical joints use the maker's value, angle tightening or a calibrated method.

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