Torque Converter
Torque is twisting force: a force applied at a distance from a pivot. Workshop manuals quote it in newton-metres or foot-pounds depending on where they were written, and getting the unit wrong is how fasteners get stripped.
Formula
newton-metres = value × the factor for its unit
- 1 ft-lbf = 1.35582 N·m
- 1 in-lbf = one twelfth of a foot-pound
- Inch-pounds against foot-pounds is a factor of twelve, and strips threads
Wheel nuts at 110 N·m are 81 ft-lbf
The Conversion Factors
Torque is force multiplied by the perpendicular distance to the pivot, so every unit pairs a force with a length:
- 1 ft-lbf = 1.35582 N·m
- 1 N·m = 0.737562 ft-lbf
- 1 in-lbf = 0.112985 N·m, one twelfth of a foot-pound
- 1 kgf·m = 9.80665 N·m, the factor being standard gravity
A rough field conversion: newton-metres are about three quarters of foot-pounds, and foot-pounds about a third more than newton-metres.
Inch-Pounds Are Where Bolts Get Stripped
The dangerous confusion is not metric against imperial. It is inch-pounds against foot-pounds, which differ by a factor of twelve and are abbreviated almost identically.
A small fastener specified at 25 in-lbf (2.8 N·m) set to 25 ft-lbf (33.9 N·m) receives twelve times its intended torque. The thread strips, or the casting cracks.
Small fasteners, electronics and instrument work are specified in inch-pounds; wheel nuts and engine work in foot-pounds. If a figure looks implausibly small for the job, check which unit the manual means before reaching for the wrench.
Why Torque Is Written N·m and Never Joules
A newton-metre and a joule have identical dimensions, and they are not interchangeable. Energy is force applied along a distance; torque is force applied perpendicular to a lever arm. The two describe different physical situations that happen to share units.
Convention keeps them apart: torque is written N·m and energy J, and a torque figure is never expressed in joules. The same caution applies in imperial, where ft-lbf for torque sits alongside ft·lb used for work.
For reference, typical values: wheel nuts around 110 N·m (81 ft-lbf), spark plugs around 25 N·m (18 ft-lbf), and a bicycle stem bolt as little as 5 N·m.
Quick Reference
| Fastener | N·m | ft-lbf | in-lbf |
|---|---|---|---|
| Bicycle stem bolt | 5 | 3.7 | 44 |
| Spark plug | 25 | 18.4 | 221 |
| Cylinder head bolt | 50 | 36.9 | 443 |
| Car wheel nut | 110 | 81.1 | 974 |
| Heavy suspension bolt | 200 | 147.5 | 1,770 |
Frequently Asked Questions
- How many newton-metres are in a foot-pound?
- 1.35582 newton-metres. In reverse, a newton-metre is 0.737562 foot-pounds.
- What is the difference between inch-pounds and foot-pounds?
- A factor of twelve. One foot-pound is twelve inch-pounds, and confusing them over-torques a fastener twelvefold.
- Why is torque measured in newton-metres and not joules?
- They share dimensions but describe different things. Energy is force along a distance; torque is force perpendicular to a lever arm. Convention reserves joules for energy.
- What torque should wheel nuts be?
- Commonly around 110 N·m, or 81 foot-pounds, but the figure varies by vehicle and the handbook value is the one to use.