Torque Converter

How do you convert torque units?

To convert torque units, turn the value into newton-meters, then divide by the size of the target unit in newton-meters. This works because torque is a force multiplied by the distance from the pivot, so every torque unit is a force unit times a length unit with a fixed value in N·m.

The method

1. Multiply by the starting unit’s value in N·m (1 lbf·ft = 1.3558179483 N·m, 1 lbf·in = 0.1129848290 N·m, 1 kgf·m = 9.80665 N·m, 1 kgf·cm = 0.0980665 N·m).
2. Divide by the target unit’s value in N·m.

Worked example: a wheel spec of 90 lbf·ft is 90 × 1.3558179483 = 122.0236 N·m. A 250 N·m spec is 250 ÷ 1.3558179483 = 184.3905 lbf·ft. Small fasteners work the same way: 35 lbf·in × 0.1129848290 = 3.9545 N·m. Use the Nm to ft-lbs converter or the ft-lbs to Nm converter for any other figure.

Pound-feet vs foot-pounds

Both names describe a pound-force acting over a foot, and the conversion factor is identical. By engineering convention, pound-feet (lbf·ft) is used for torque, a twisting effort, while foot-pounds (ft·lbf) is used for work or energy, a force moving through a distance. In the workshop the terms are used interchangeably, and most torque wrenches are simply marked “ft-lb”.

The SI system makes the same distinction. A newton-meter of torque has the same dimensions as a joule of energy, but the BIPM SI Brochure says torque should be written in N·m, never in joules. For energy, use the energy converter.

Setting a torque wrench

Many wrenches carry dual scales, but if yours reads only one unit, convert the spec first and round to the wrench’s smallest graduation. Use an inch-pound wrench for small specs: the ft-lbs to in-lbs converter multiplies by 12, so a 7 lbf·ft spec becomes 84 lbf·in. Many click-type wrenches are rated to about ±4% of the reading, so extra decimal places in the conversion add nothing in practice.

What is torque, and what is its SI unit?

Torque is the turning effect of a force about an axis, equal to the force times its perpendicular distance from the axis, and its SI unit is the newton-meter (N·m). ISO 80000-4, the international standard for quantities in mechanics, lists torque and the closely related moment of force in N·m, and the BIPM SI Brochure asks for torque to be written in N·m rather than joules, even though the two share dimensions.

The lever arm matters as much as the force. Pushing with 100 N on the end of a 0.5 m wrench gives 50 N·m; halve the arm to 0.25 m and you need 200 N for the same torque. In US units, 50 lbf on the end of an 18-inch breaker bar gives 75 lbf·ft, which is 101.69 N·m. Only the part of the force at right angles to the handle counts: pull at 60° to the handle and you get sin 60°, or about 86.6%, of the full effect.

Extensions change what a torque wrench delivers

A crowfoot or adapter that extends the wrench in line with its handle lengthens the lever, so the fastener sees more torque than the dial shows. Set the wrench to the spec × L ÷ (L + E), where L is the wrench's effective length and E is the extension: for a 100 lbf·ft spec, an 18-inch wrench and a 2-inch crowfoot, set 90 lbf·ft. An adapter mounted at 90° to the handle adds no length and needs no correction. Torque comes from force, so the force converter and the newtons to pounds converter help when a spec gives the pull rather than the torque.

How do newton-meters, pound-feet and kg-meters compare?

One pound-foot is 1.355818 N·m, one newton-meter is 0.737562 lbf·ft, and one kilogram-force meter is exactly 9.80665 N·m. Every factor comes from exact definitions: the international pound (0.45359237 kg), the international foot (0.3048 m) and standard gravity (9.80665 m/s²), which turns a pound or kilogram of mass into a unit of force.

The pound-foot factor is 0.45359237 × 9.80665 × 0.3048 = 1.3558179483 N·m, which NIST SP 811 rounds to 1.355818 N·m. Pound-inches are one twelfth of that, and kgf·cm is one hundredth of kgf·m. Because the metric-to-customary factors are not round numbers, keep three or four significant figures for workshop specs and more only for calculations.

Where is each torque unit used?

Newton-meters are the world standard for torque specs, pound-feet and pound-inches dominate US workshops, and kilogram-force units linger in older Asian manuals and hobby electronics. A single repair can involve two or three of them.

• Newton-meters (N·m): the standard across Europe, Asia and most of the world for vehicle service data, engine output figures, bicycle components and industrial tools.
• Pound-feet (lbf·ft): US and, to a lesser extent, UK automotive work: lug nuts, cylinder heads, suspension bolts and advertised engine torque.
• Pound-inches (lbf·in): small fasteners in the US, such as valve covers, oil pan bolts, electrical terminals, firearm scope rings and some bicycle parts. The in-lbs to Nm converter is handy when a spec sheet mixes the two systems, and the Nm to in-lbs converter sets an inch-pound wrench from a metric spec.
• Kilogram-force meters (kgf·m): older Japanese and other Asian service manuals, and some motorcycle and car specifications from those markets.
• Kilogram-force centimeters (kgf·cm): hobby servos, small electric motors and gearboxes, often written as “kg·cm”. A 20 kgf·cm servo delivers about 1.96 N·m.

Torque and power

Power is torque multiplied by rotational speed. In SI units, power in watts equals torque in N·m times angular speed in rad/s, which works out to kW = N·m × rpm ÷ 9,549. In US units, horsepower = lbf·ft × rpm ÷ 5,252, which is why torque and horsepower curves cross at 5,252 rpm. See the power converter for watts and horsepower, and the frequency converter for rpm and rad/s; the rpm to Hz converter turns engine speed into revolutions per second.