# Bolt Torque Calculator

> Convert between bolt tightening torque and clamp preload with T = K × F × d, and check the bolt stress as a percentage of its proof load.

Category: Science & engineering · Applies to: Worldwide · Last updated: 2026-10-29
Interactive calculator: https://calculators.geekmidst.com/calculators/bolted-joint-torque-calculator

The short-form torque equation T = K × F × d links tightening torque to bolt preload. The nut factor K lumps together thread and under-head friction, so it dominates the accuracy.

Also called: Bolt preload calculator, Torque–tension calculator.

## Inputs
- Find (options: Torque, Preload; default torque)
- Nominal bolt diameter d (unit mm; default 10)
- Nut factor K (default 0.2)
- Target preload F (unit kN; default 20)
- Tightening torque T (unit N·m; default 40)
- Tensile stress area A_s (unit mm²; default 58)
- Proof strength (unit MPa; default 580)

## Results
- Tightening torque
- Preload (clamp force)
- Bolt tensile stress
- Share of proof load
- Proof load A_s × proof strength

## Formula

```
T = K × F × d;  F = T ÷ (K × d);  σ = F ÷ A_s;  proof load = A_s × proof strength
```

Where:
- T: tightening torque
- K: nut factor (dimensionless)
- F: bolt preload
- d: nominal diameter
- A_s: tensile stress area

## Worked example
1. T = 0.2 × 20,000 N × 0.010 m = 40 N·m.
2. σ = 20,000 N ÷ 58 mm² = 344.8 MPa.
3. Proof load = 58 × 580 = 33,640 N; 20 kN is 59.5% of it.

**Result:** Clamping an M10 bolt to 20 kN with K = 0.2 needs about 40 N·m, loading it to about 60% of a 580 MPa proof strength.

## What the result means

Preloads of roughly 70–90% of proof load are common for reusable and permanent joints respectively, but follow your standard or the fastener maker. Because K varies widely with lubrication and finish, torque control alone typically gives preload scatter of ±25% or more.

## Assumptions and limits
- Short-form torque–preload relation with a single nut factor; standard threads; preload carried by the tensile stress area.
- Does not model joint stiffness, external load sharing, embedding, relaxation or prevailing-torque nuts.
- For critical joints, use the fastener manufacturer’s data or measure preload directly.

## Questions

### Why does lubrication matter so much?

Most of the torque (often around 90%) is used overcoming friction. Lowering friction lowers K, so the same torque produces much more preload and can overload the bolt.

### Which diameter is d?

The nominal (major) thread diameter, in metres in the formula.

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_Quantities are estimates. Confirm requirements with your engineer or supplier before ordering materials._

Source: GM¹X CalcVerse — https://calculators.geekmidst.com/calculators/bolted-joint-torque-calculator