# High Strength Bolt Torque Calculator

Preload by size and class, with snug and final torque.

- Page: https://jianzhujisuan.com/en/bolt-torque
- Category: Construction (https://jianzhujisuan.com/en/c/construction)
- Chinese version: https://jianzhujisuan.com/zh/bolt-torque.md
- Data checked: 2026-10-25

## Quick result

With the default inputs (Bolt size M20, Property class 10.9S (hex head), Torque coefficient K 0.13): **Final torque 443 N·m**

## Inputs

| Parameter | Type | Default | Unit | Note |
|---|---|---|---|---|
| Bolt size | select | M20 | — |  |
| Property class | select | 10.9S (hex head) | — |  |
| Torque coefficient K | number | 0.13 | — | 0.110-0.150 for hex head, use the batch test value |

## 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

## Worked example

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.

## Questions

### 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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