Torque units explained: N·m, lb-ft, lb-in and kgf·m

Torque is a turning force: the force applied multiplied by its distance from the pivot. The SI unit is the newton-metre (N·m). In the US and on many older manuals you will see pound-feet (lb-ft), where 1 lb-ft = 1.3558 N·m; small fasteners use pound-inches (lb-in), and some Asian and older European manuals use kilogram-force metres (kgf·m), where 1 kgf·m = 9.80665 N·m. The torque converter converts between all of them and shows how hard you need to push on a wrench.

What torque is

Push on a door near the hinges and it barely moves; push at the handle with the same force and it swings open. The difference is the lever arm. Torque, τ = F × r, is the force multiplied by the perpendicular distance from the axis. A force of 100 newtons at the end of a 0.3 m wrench gives 30 N·m. The same force at 0.6 m gives 60 N·m. That is why a longer breaker bar makes stubborn nuts easier to shift: you need less force for the same torque. If you are working out the force itself from a mass, the force calculator handles F = m × a.

The common units

• Newton-metre (N·m): the SI unit, used in most car manuals, bicycle component markings and engine specifications outside the US.
• Pound-foot (lb-ft, also written ft-lb or lbf·ft): one pound-force at one foot, 1.3558 N·m. Standard in US automotive work.
• Pound-inch (lb-in or in-lb): one twelfth of a lb-ft, 0.1130 N·m, used for small bolts and torque screwdrivers.
• Kilogram-force metre (kgf·m): one kilogram of weight at one metre, 9.80665 N·m. Kgf·cm (0.0981 N·m) is common for hobby servo motors.
• Ounce-inch (oz-in): used for small electric motors, about 0.00706 N·m.

Mixing up lb-ft and lb-in is a classic mistake: a figure of 25 meant in lb-in but applied in lb-ft is twelve times too tight and can strip a thread or crack a part. Carbon bicycle parts are a good example of where this matters, because the specified torques printed on them are often only a few newton-metres, well below what a hand on a long wrench produces without thinking.

Converting between them

Some worked conversions, all checked against the US National Institute of Standards and Technology factors:

• 100 N·m = 73.76 lb-ft = 885 lb-in = 10.2 kgf·m;
• 25 lb-ft = 33.9 N·m;
• 80 lb-ft = 108.5 N·m;
• 10 kgf·m = 98.1 N·m.

Quick mental shortcuts: multiply lb-ft by about 1.36 to get N·m, or multiply N·m by about 0.74 to get lb-ft. For other unit families the general unit converter covers length, mass and more, and Metric vs imperial: how we got two systems explains why both systems persist.

Using a torque specification

A torque spec tells you how tightly to fasten something so it clamps properly without stretching or breaking the bolt. Use a torque wrench set in the units the manual gives, or convert carefully. If you do not have one, the converter's wrench helper shows the push needed: a 110 N·m wheel-nut spec on a 45 cm bar needs about 244 N at the end, roughly the weight of 25 kg. Always follow the manufacturer's figure and conditions; whether threads are dry, oiled or treated with threadlocker changes how much clamping force a given torque produces, which is why specs often state it. When I built the converter I made each result tap-to-copy, because retyping a converted number is an easy place to slip.

Torque is not energy

A newton-metre of torque has the same base units as a joule of energy, but they are different quantities. Torque is a force acting at a distance from an axis; energy is force acting through a distance moved. To keep them apart, the SI uses N·m for torque and the joule for energy. The same applies in imperial units, where "ft-lb" is sometimes reserved for energy and "lb-ft" for torque, though in practice the two spellings are used interchangeably for torque. Torque becomes power when it turns something: power = torque × rotational speed, which is how engine torque and horsepower are linked. The horsepower calculator does that sum, and Horsepower explained: hp, PS, kW and how torque fits in covers it in full.