Convert torque

About the units

• Newton metres (Nm) – The newton metre is the SI coherent unit of moment of force: one newton on a lever arm of one metre. As a combination of two legal units it is legal throughout the EU, and EU type-approval papers require an engine’s maximum net torque in newton metres at a speed in min⁻¹. The trap is the joule. Newton metre and joule share a dimension, kilogram times square metre per second squared, but they are different quantities. Write Nm for torque and J for energy.
• Kilogram-force metres (kgf·m) – A kilogram-force metre is the moment of a kilogram-force on a lever arm of one metre, exactly 9.80665 newton metres – the same figure as the kilogram-force itself, because the arm is one metre. The unit is absent from the EU units directive and is not legal in trade, but it appears in older European workshop literature and older engine specifications. Ten kilogram-force metres is 98.07 newton metres. The trap is the spelling: older texts write “kgm” or “mkg”, which looks like a mass.
• Pound-feet (lb-ft) – A pound-foot is the moment of a pound-force on a lever arm of one foot, exactly 1.3558179483314 newton metres. Since both the pound and the foot are exact figures, so is this one. The unit belongs to American automotive and workshop engineering: engine specifications and torque wrenches. A wrench marked 100 lb-ft is 135.6 newton metres. The trap is the word order. “lb-ft” is torque and “ft-lb” energy – the same number, different quantities, and English writes them almost alike.
• Pound-inches (lb-in) – A pound-inch is the moment of a pound-force on a lever arm of one inch, a pound-foot divided by twelve: exactly 0.1129848290276167 newton metres. The unit is used for small bolted joints in electronics, optics, precision mechanics and aircraft maintenance, and low-range torque wrenches are marked in it. One newton metre is 8.85 pound-inches. It is absent from the EU units directive. Two traps: the factor against the pound-foot is twelve, not ten, and it is written both “lb-in” and “in-lb”.
• Ounce-inches (oz-in) – An ounce-inch is the moment of one ounce-force on a one-inch lever arm, 7.06 millinewton metres and one sixteenth of a pound-inch. It is absent from the EU units directive but used for small motors and servos: Hitec’s servo chart gives torque both in kilogram-force centimetres and in ounce-inches. The conversion between them is 13.89 ounce-inches per kilogram-force centimetre, and one newton metre is 141.6 ounce-inches. The trap is the word ounce: here it is a force, neither mass nor volume, and both “oz-in” and “in-oz” occur.