The units of torque
Torque is a turning force: a force applied at a distance from a pivot. The SI unit is the newton metre, and it looks like a joule dimensionally but means something entirely different, which is why it is never written as a joule. American tools and specifications use pound-feet, and smaller fasteners are specified in pound-inches, twelve of which make a pound-foot.
| Unit | Symbol | Equals |
|---|---|---|
| Newton metres | N·m | 1 N·m |
| Kilogram-force metres | kgf·m | 9.80665 N·m |
| Pound-feet | lb·ft | 1.35581795 N·m |
| Pound-inches | lb·in | 0.11298483 N·m |
How to convert by hand
Every torque conversion follows the same procedure: take the amount to the base unit, then from there to the target unit. In practice it is a single multiplication, because the two steps combine into one factor.
One newton metre is 0.7376 pound-feet, and one pound-foot is 1.3558 newton metres. For pound-inches, divide the pound-feet figure by 12.
The equivalents people look up most
| Conversion | Result |
|---|---|
| A car wheel nut | 110 N·m · 81 lb·ft |
| A bicycle stem bolt | 5 N·m · 44 lb·in |
| A family car's engine | 250 N·m · 184 lb·ft |
| 1 N·m | 0.738 lb·ft · 8.85 lb·in |
| 1 kgf·m | 9.807 N·m |
Common mistakes
- Writing torque in joules. The units cancel the same way but the quantities are unrelated, and the convention exists to keep them apart.
- Confusing pound-feet with foot-pounds. They mean the same torque, but foot-pounds is also used for energy, which is where the ambiguity starts.
- Mixing pound-feet and pound-inches. The factor of 12 turns a delicate bolt specification into a stripped thread.
- Ignoring the tolerance on a torque figure. Manufacturer specifications assume clean, dry threads unless they say otherwise.