High Strength Bolt Torque Calculator
Preload by size and class, with snug and final torque.
Inputs
Result
Formula
| Final torque | T = K × P × d |
| Snug torque | 0.5T |
| Inspection torque | 1.1T |
| Construction preload | 1.1P |
| Meaning of K | Share of torque lost to thread and bearing friction |
| Units | P in kN and d in mm gives N·m directly |
M20 10.9S at K = 0.13: 155 kN design preload, 171 kN construction preload, 443 N·m final torque, 222 N·m snug and 488 N·m inspection.
Preload and final torque for high strength hex bolts at K = 0.13
| Size | 8.8S preload kN | 8.8S torque N·m | 10.9S preload kN | 10.9S torque N·m |
|---|---|---|---|---|
| M16 | 80 | 166 | 100 | 208 |
| M20 | 125 | 325 | 155 | 403 |
| M22 | 150 | 429 | 190 | 543 |
| M24 | 175 | 546 | 225 | 702 |
| M27 | 230 | 807 | 290 | 1018 |
| M30 | 280 | 1092 | 355 | 1385 |
FAQ
Why not just tighten until it stops?
Because a high strength bolt works by clamping the plates so friction carries the load, not by shear in the shank. Under-tighten and the joint slips; once slipped the bolts bear in shear, capacity collapses and the movement is permanent. Codes require snug, re-snug and final passes so the faying surfaces close progressively — one pass to final torque loses about 20% of preload closing the gaps.
Why must K be measured?
Because K depends on thread accuracy, surface treatment and lubrication, and batches from different makers differ by up to 30%. Codes require eight torque coefficient tests per batch and the mean value used. A wrong K goes straight into preload: assuming 0.13 when the truth is 0.15 under-tightens by 13%, and the joint slips then loosens under fatigue. This is why bolt, nut and washer must come from the same batch.
Tension control or hex head?
Tension control bolts are run until the splined end shears off, at which point preload is reached — no torque control, little operator error, but single use and inspection is visual. Hex head bolts use the torque method, can be re-tightened, but need K testing, three passes and torque checks. Long-span and industrial work in China favours tension control; joints that get dismantled favour hex head.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.