Torque Calculator
Calculate bolt tightening torque from clamp load with the T = K·D·F short-form equation.
Loading calculator…
What this tool does
Calculate bolt tightening torque from clamp load with the T = K·D·F short-form equation. The short-form T = KDF is an approximation: the nut factor K bundles thread and under-head friction, which is why the same bolt needs far less torque when lubricated. Critical joints follow the manufacturer's torque spec, not this formula alone.
How to use the Torque Calculator
- Enter or select nominal bolt diameter in in.
- Enter or select target clamp load (preload).
- Enter or select nut factor k (friction condition).
- Read the calculated result; change any measurement to compare alternatives.
Formula
T = K × D × F (in-lb); ft-lb = in-lb / 12; N·m = in-lb × 0.112985
- diameter
- Nominal bolt diameter (in, before metric conversion)
- clamp
- Target clamp load (preload)
- k
- Nut factor K (friction condition)
The short-form T = KDF is an approximation: the nut factor K bundles thread and under-head friction, which is why the same bolt needs far less torque when lubricated. Critical joints follow the manufacturer's torque spec, not this formula alone.
Worked example
For torque calculator, the following measurements illustrate the exact method: Nominal bolt diameter: 0.5; Target clamp load (preload): 10000; Nut factor K (friction condition): 0.20.
Inputs
- Nominal bolt diameter0.5 in
- Target clamp load (preload)10000
- Nut factor K (friction condition)Plain / dry steel — K 0.20
Result
- Tightening torque1,000
- Torque (in-lb)1,000 in-lb
- Torque (ft-lb)83.33 ft-lb
- Torque (N·m)112.99 N·m
Nut factor K by condition (typical)
| Condition | K |
|---|---|
| Plain, dry steel | 0.2 |
| Lightly oiled | 0.15 |
| Waxed / anti-seize | 0.11 |
| Hot-dip galvanized, dry | 0.25 |
Results explained
- Tightening torque
- Tightening torque from the formula above. The short-form T = KDF is an approximation: the nut factor K bundles thread and under-head friction, which is why the same bolt needs far less torque when lubricated. Critical joints follow the manufacturer's torque spec, not this formula alone.
- Torque (in-lb)
- Torque (in-lb) from the formula above. The short-form T = KDF is an approximation: the nut factor K bundles thread and under-head friction, which is why the same bolt needs far less torque when lubricated. Critical joints follow the manufacturer's torque spec, not this formula alone.
- Torque (ft-lb)
- Torque (ft-lb) from the formula above. The short-form T = KDF is an approximation: the nut factor K bundles thread and under-head friction, which is why the same bolt needs far less torque when lubricated. Critical joints follow the manufacturer's torque spec, not this formula alone.
- Torque (N·m)
- Torque (N·m) from the formula above. The short-form T = KDF is an approximation: the nut factor K bundles thread and under-head friction, which is why the same bolt needs far less torque when lubricated. Critical joints follow the manufacturer's torque spec, not this formula alone.
Frequently asked questions
T = K × D × F: nut factor × nominal bolt diameter (in) × target clamp load (lb) gives torque in in-lb. A ½ in bolt at 10,000 lb clamp load, dry (K 0.20), needs 1,000 in-lb ≈ 83 ft-lb.
An empirical friction factor: about 0.20 for plain dry steel, 0.15 lightly oiled, 0.11 with wax or anti-seize. Most of your torque is spent overcoming friction, not stretching the bolt.
With less friction, more of the applied torque converts to bolt stretch (clamp load). Torquing a lubricated bolt to a dry spec can over-tighten it past yield — always match the spec to the condition.
Common design practice targets about 75% of the bolt's proof load for reusable joints. Proof loads come from the bolt grade's published table (e.g. ASTM/SAE) — this tool takes your target as an input rather than guessing a grade.
Roughly ±25–35% on achieved clamp load in real joints — fine for general work, not for cylinder heads, wheels with printed specs, or pressure joints, where the manufacturer's procedure rules.