Bolt Torque Calculator
Work out the tightening torque for a bolted joint using T = K·F·d. Pick a thread size (ISO metric coarse/fine or UNC) and a property class (8.8, 10.9, 12.9, A2-70, SAE 5/8) and the preload is derived from proof load automatically, or enter a target clamp load directly. Results in N·m, lbf·ft, lbf·in and kgf·m, with the resulting bolt stress. Runs entirely in your browser.
Input
Most people know the bolt and the assembly rule, not the clamp load.
65–75% is the usual range for a reusable structural joint.
Output
| Unit | Value |
|---|---|
| No data yet | |
Guides
Work out how hard to tighten a bolt. Pick the thread, pick the grade, pick how greasy the joint is, and read the torque in N·m, lbf·ft, lbf·in and kgf·m.
The calculator uses the short-form torque–tension relationship that every fastener handbook opens with:
T = K · F · d
where T is tightening torque, K the nut factor, F the target preload (clamp load) and d the nominal thread diameter. Everything runs in your browser — no uploads, no sign-up.
How to use it
- Preload source — leave it on From bolt grade & % of proof load if you know the bolt but not the clamp load. Switch to Enter target preload directly if a drawing or spec already gives you a figure in newtons.
- Thread size — ISO metric coarse (M3–M36), the common fine pitches (M8×1, M10×1.25, M12×1.25, M16×1.5, M20×1.5) and UNC sizes from 1/4"-20 to 3/4"-10. Each carries its tabulated tensile stress area Aₛ. Pick Custom to type a diameter and stress area yourself.
- Bolt grade — ISO property classes 4.6 through 12.9, A2/A4 stainless, and SAE Grades 5 and 8. Each maps to a proof strength.
- Preload target — the slider is the fraction of proof load you're aiming for. 65–75% is the normal window for a reusable structural joint.
- Joint condition — this is the nut factor in disguise. Dry as-received steel is around K = 0.30; lightly oiled plain steel 0.20; zinc plated 0.22; moly grease 0.16; anti-seize as low as 0.10. Choose Custom if your fastener supplier publishes a K value.
The output shows the torque, the preload it corresponds to, and the resulting tensile stress in the bolt as a percentage of proof — so you can see immediately whether you are asking too much of the fastener.
How the preload is calculated
Preload comes from the bolt's proof strength and its tensile stress area:
F = (percentage / 100) × Sₚ × Aₛ
Aₛ is the standard tabulated tensile stress area, computed from the pitch diameter and minor diameter (Aₛ = π/4 · (d − 0.9382·p)² for ISO metric threads), not from the nominal diameter — a common source of 15–20% errors when people do this by hand.
Why does the same bolt have different torque figures in different tables?
Almost always because the tables assume different nut factors or a different percentage of proof load. A published "M10 8.8 = 47 N·m" is a K and a percentage someone chose for you. Changing K from 0.20 to 0.30 raises the torque by half for exactly the same clamp force.
How accurate is T = K·F·d?
Good enough to set a torque wrench for a general assembly, and not good enough for a critical joint. K absorbs thread friction, under-head friction, surface finish, plating, washer condition and whether the fastener has been used before. Torque control alone typically scatters the achieved preload by ±25–30%. Where the clamp load actually matters — head bolts, flanges under pressure, anything safety-critical — use angle control, bolt elongation measurement, or a tensioner, and validate against torque-tension testing on your real hardware.
What percentage of proof load should I target?
65–75% of proof load is the usual guidance for a reusable steel joint: high enough that the joint stays clamped under working load, low enough that the bolt does not yield during tightening. Below about 50% the joint risks separating or self-loosening under vibration. Above 90% you are effectively tightening into yield, which is a legitimate technique (torque-to-yield bolts) but not one to do with a click wrench.
Does this cover torque-to-yield or lubricated-vs-dry differences?
Lubrication is handled through the nut factor — that is exactly what K is for. Torque-to-yield bolts are single-use and tightened by angle after a snug torque, so a K·F·d figure is only the snug step; follow the manufacturer's angle sequence for the rest.
Related tools
Need a unit this tool doesn't print? The Torque Converter covers the full set of torque units, and the Force Unit Converter turns the preload figure between newtons, kN, lbf and kgf. If you're working the joint from the pressure side instead, the Pressure Converter handles MPa, bar and psi.