Bolt torque and preload

Pick a thread family, size, grade, and lubrication, and the preload as a percentage of the proof load (75% is the usual target for joints you will take apart; up to 90% for permanent ones).

Stress area — metric A_s = π/4·(d − 0.9382·P)²; unified A_t = π/4·(D − 0.9743/n)².
Preload — F = % × proof strength × stress area. Proof strengths follow ISO 898-1 (4.6 to 12.9), ISO 3506 (A2-70, A4-80), SAE J429 (grades 2, 5, 8), ASTM A574 (socket head cap screws), and ASTM F593 (18-8 stainless), stepping with diameter where the standards do.
Nut factor — T = K·F·d. K is about 0.20 dry, 0.18 lightly oiled, 0.14 with anti-seize, 0.10–0.12 with moly paste or wax. Override K if you have a measured value.
Thread + head friction — T = F·(P/2π + 0.577·d₂·μ_G + D_km/2·μ_K), the form used by VDI 2230 and ISO 16047. It shows how little of the torque actually stretches the bolt (often about 10%) — the rest goes into friction, which is why lubrication changes preload so much.
Preload from a torque — enter a torque from a manual or your wrench setting to see the clamp force and how close it takes the bolt to its proof load.

Starting points — always follow the manufacturer’s torque specification for engines, vehicles, structures, and pressure equipment. Thread engagement matters too: in aluminium allow about 2 × d of thread, in cast iron 1.5 × d, in steel 1 × d.

Share link copies a URL that opens this tool exactly as you have it set up. It carries the fastener, grade, lubrication and method. Opening a link replaces your own saved settings for the tool; a Restore my settings button brings them back. To write a link by hand, use fam, size, grade, cond, pct — for example /tools/bolt-torque/?fam=metric&size=M12&grade=10.9&cond=oiled.

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