Bolts work by stretching like stiff springs and clamping the joint. Most of the torque you apply is lost to friction under the nut and in the threads, which is why the nut factor K depends so much on the surface finish and lubricant. The short-form equation T = K·F·d is the standard way to estimate torque; critical joints use measured preload instead.
How to use the Bolt Torque Calculator
- Nominal bolt diameter
- Target preload (clamp force)
- Bolt condition (nut factor K)
- Extra lubrication reduction
- Every result field is editable: type into Tightening torque and Target preload (clamp force) plus every other field update to match.
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the nominal bolt diameter, the desired preload, the condition of the threads (which sets K) and any additional lubrication reduction. Type a torque value into the result to find the preload it produces.
Tips for an accurate estimate
- Keep units consistent – the calculator handles conversions, but check every field’s unit before reading a result.
- Use values from standards, data sheets or test pieces whenever you have them; typical values are only a starting point.
- Apply appropriate safety factors and have structural, electrical and pressure work checked by a qualified professional.
- Record the inputs you used so the calculation can be reviewed later.
Common mistakes to avoid
- Confusing diameter and radius, or nominal and actual sizes.
- Using allowable stresses and ultimate strengths interchangeably.
- Treating rule-of-thumb results as a substitute for code tables or an engineered design.
Inputs explained
| Field | What to enter |
|---|---|
| Nominal bolt diameter | Units: mm, in, cm |
| Target preload (clamp force) | Units: N, kN, lbf, kip, kgf |
| Bolt condition (nut factor K) | Choose one: Dry, as-received steel (K = 0.20), Zinc plated (K = 0.20), Black oxide / non-plated (K = 0.30), Lightly oiled (K = 0.18) … |
| Extra lubrication reduction | In % |
What each result means
- Tightening torque = [Bolt condition (nut factor K)]×[Target preload (clamp force)]×[Nominal bolt diameter]×(1 – [Extra lubrication reduction]÷100)
- Torque in lbf·ft = [Tightening torque]
Example values
How the main result changes when Target preload changes and everything else stays at the example values above:
| Target preload (clamp force) (N) | Tightening torque (N·m) |
|---|---|
| 15,000 | 36 |
| 22,500 | 54 |
| 30,000 | 72 |
| 37,500 | 90 |
| 45,000 | 108 |
| 60,000 | 144 |
Formula
T = K × F × d × (1 − l ÷ 100)T torque, K nut factor, F preload, d nominal diameter, l lubrication reduction %Worked example
Inputs — Nominal bolt diameter: 12 mm; Target preload (clamp force): 30,000 N; Bolt condition (nut factor K): Dry, as-received steel (K = 0.20); Extra lubrication reduction: 0 %.
Results — Tightening torque = 72 N·m; Torque in lbf·ft = 53.1 lbf·ft.
Frequently asked questions
What is the nut factor K?
An empirical coefficient that lumps together thread and under-head friction; about 0.2 for dry steel bolts and 0.1–0.18 when lubricated.
What preload should I use?
Commonly 75% of the proof load for reusable joints and 90% for permanent structural connections.
Why does lubrication reduce torque?
Less friction means more of the torque becomes bolt stretch – using dry-bolt torque on a lubricated bolt can overload it.
What formula does the Bolt Torque Calculator use?
T = K × F × d × (1 − l ÷ 100).