Torque Units Explained, N·m, lbf·ft and More

Torque is the turning effect of a force. It equals the force multiplied by the perpendicular distance from the axis of rotation to the line of the force, often called the lever arm. A 100 N push at the end of a 0.5 m wrench gives the same torque as a 50 N push at 1 m.

Because torque is a force times a length, every torque unit is a force unit combined with a length unit.

• Newton meter (N·m): one newton acting at one meter. The SI unit, used in engineering worldwide and in car and tool specifications outside the US.
• Pound-force foot (lbf·ft): one pound-force acting at one foot, about 1.355818 N·m. The usual US unit for engine and bolt torque, often called "foot-pounds".
• Pound-force inch (lbf·in): one pound-force at one inch, exactly one twelfth of a pound-force foot, about 0.1129848 N·m. Used for small fasteners and torque screwdrivers.
• Kilogram-force meter (kgf·m): one kilogram-force at one meter, exactly 9.80665 N·m. An older metric unit still seen in older manuals and some spec sheets.
• Kilonewton meter (kN·m): 1000 N·m, used for bending moments in structural engineering.

Torque and energy both have the dimension of force times length, so both can be written in N·m. They are still different quantities. In energy, the force acts along the distance moved. In torque, the force acts at right angles to the lever arm and nothing has to move at all.

For that reason the joule is used for energy and work, and the newton meter for torque. The SI Brochure notes that although torque has the same dimension as energy, the joule is not used to express torque.

US usage has the same overlap: "foot-pound" can mean work (ft·lbf) or torque (lbf·ft). Some writers put the force first for torque and the length first for energy to keep them apart. That is a convention, not a rule, so read the context.

• Wheel lug nuts: suppose a service manual lists 110 N·m. A US torque wrench set in lbf·ft needs 81.13 lbf·ft. Always use the value for your own vehicle, not an example.
• A small bolt rated at 60 lbf·in is 6.779 N·m, or 5 lbf·ft.
• An older spec of 12 kgf·m is 117.68 N·m: multiply by 9.80665.
• An engine rated at 300 lbf·ft produces 406.7 N·m.

• Mixing up lbf·in and lbf·ft. They differ by a factor of 12, and setting a wrench in the wrong one can strip or snap a fastener.
• Converting N·m to joules. The numbers match, but a torque is not an amount of energy.
• Using 10 as a factor for kgf·m to N·m. The exact factor is 9.80665, so 10 kgf·m is 98.0665 N·m, not 100 N·m.