Bolt Torque Calculator
The torque that gives a bolt its intended clamping force — from the thread size and pitch, the property grade's proof stress, the share of proof load to preload to, and the nut factor for the lubrication — with the preload, the tensile stress area and the torque in both units.
Tightening torque is mostly spent on thread and head friction, and only a fraction stretches the bolt to create clamping force.
How the bolt torque calculator works
Tightening torque is mostly spent on thread and head friction, and only a fraction stretches the bolt to create clamping force. The nut factor K bundles the friction: about 0.2 for dry plated steel, 0.15 to 0.17 lubricated, under 0.12 with anti-seize. Torque is K times the preload times the diameter, and the preload is set as a share of the bolt's proof load — the tensile stress area times the grade's proof stress — usually 65 to 80%.
Formula: A_s = (π/4)(d − 0.9382 p)²; F = proof stress × A_s × share; T = K × F × d
Worked examples
| Inputs | Tightening torque (N·m) | Note |
|---|---|---|
| M12 × 1.75, grade 8.8, dry | 91.01 | about 91 N·m |
| Grade 10.9 | 125.89 | a third more |
| Lubricated | 68.26 | a quarter less torque for the same clamp |
FAQFrequently asked questions
How much torque should a bolt get?
Enough to preload it to 65 to 80% of its proof load — for an M12 grade 8.8 bolt dry, about 90 N·m. Manufacturer tables give the same numbers; this page shows where they come from.
What is the nut factor?
A friction coefficient bundling thread and bearing friction into one number: about 0.2 for dry zinc-plated steel, 0.15 to 0.17 with oil, 0.10 to 0.12 with anti-seize or wax. It is the biggest uncertainty in the whole calculation.
Why does lubrication change the torque?
Because most torque is spent on friction. A lubricated bolt reaches the same preload at a quarter less torque — and the same torque over-tightens it, which is how lubricated bolts get snapped.
What is proof load?
The load a bolt can carry without permanent stretch, roughly 90% of yield. Grade 8.8 has a proof stress of about 600 MPa, 10.9 about 830, 12.9 about 970.
Is torque a reliable measure of preload?
Only to about ±25% because of friction scatter. Where the clamp matters, engineers measure bolt stretch or use torque-plus-angle.
Where these figures come from
- The Engineering ToolBox — material densities, coverage and unit data — bulk densities, concrete at 2,400 kg/m³, board and sheet sizes
- Brick Development Association — brick sizes and quantities — 215 × 102.5 × 65 mm, 60 bricks per m²
- UK Government — Approved Documents (Building Regulations) — Building Regulations guidance
Last checked: September 2026. Coverage rates and material sizes are typical manufacturer figures — check the product you buy.