High Strength Bolt Torque Calculator
Preload by size and class, with snug and final torque.
Quick answer
With the default inputs: Bolt size M20 · Property class 10.9S · Torque coefficient K 0.13Final torque 443 N·m
Edit the inputs below to recalculate.Inputs
Result
Final torque
443N·m
| Design preload | 155kN |
| Construction preload (1.1P) | 171kN |
| Nominal diameter | 20mm |
| Torque coefficient | 0.130 |
| Snug torque (0.5T) | 222N·m |
| Final torque | 443N·m |
| Inspection torque (1.1T) | 488N·m |
| Estimated preload at snug | 85kN |
| Bolt shank mass (1.1d long) | 0.054kg |
Final torque is T = K × Pc × d with Pc = 1.1P, and K must be the batch test value, not a rule of thumb — moving K from 0.11 to 0.15 changes the preload by 27% at the same torque. Tension control bolts rely on shearing the splined end rather than a torque figure, but preload still applies. Slip factors for the faying surfaces must be tested batch by batch.
Reverse: known result, work out one input
Enter a target result; the other inputs stay fixed and this one is worked out.
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 |
How to work it out
- Look up the pretension P for the size and property class: an M20 10.9S bolt takes 155 kN and 8.8S takes 125 kN. These are code values, not calculations.
- Part-turn first: the initial pass is 50% of the final torque, to close the pack and remove gaps. For a hex bolt it is roughly 0.065 × P × d.
- Then final torque Tc = K × P × d, with K the torque coefficient (0.110-0.150, 0.13 nominal) and d the nominal diameter in metres.
- For M20 10.9S: 0.13 × 155 × 0.020 = 0.403 kN·m, that is 403 N·m.
- Verify by torque check on 10% of bolts within ±10%; for tension-control bolts the splined end must shear off instead and no torque check is made.
Where it goes wrong
- Torquing once to final value — high-strength bolts need two passes; going straight to final leaves the pack open, the pretension short and the joint slipping.
- Using K = 0.13 without testing — K depends on thread lubrication and ranges 0.11-0.15, giving 20% error in pretension; the code requires a coefficient test per batch.
- Treating pretension as the yield load — an M20 10.9S yields around 220 kN but is pretensioned to 155 kN, about 0.7 of it; torquing to yield over-tightens or breaks the bolt.
- Using one inspection method for both types — tension-control bolts are set by shearing the spline, and a torque wrench cannot verify them.
Material and cost
10.9S M20 × 60 assemblies 8-15 CNY each, 8.8S 5-10, tension-control 20-30% more. Installation 3-8 CNY per bolt, torque wrench hire 100-300 CNY per day.
On site
- High-strength bolts are single use — tension-control bolts are destroyed on installation and hex bolts lose pretension when undone, so rework needs new bolts, which is often missed in the cost.
- Test the slip factor of the prepared faying surface: design assumes 0.45 but in practice 0.35 occurs, directly cutting joint capacity.
- Keep bolt heads in one direction, usually down or out, for inspection and later replacement; mixed orientation is raised at acceptance.
Basis
- GB 50205-2020 Code for acceptance of steel structure construction, Chapter 6
- JGJ 82-2011 Technical specification for high-strength bolted connections in steel structures
- GB 50017-2017 Standard for design of steel structures, Chapter 11
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.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. High Strength Bolt Torque Calculator. 2026-10-25. https://jianzhujisuan.com/en/bolt-torque
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.