Torque Converter (N·m, kgf·m, lb·ft, lb·in)

Understanding the torque converter — concept and principle

Torque is the turning effect of a force, and it appears in two everyday places: engine and motor specifications, and tightening specs for bolts and wheel nuts. Manuals from different countries use newton-metres (N·m), kilogram-force metres (kgf·m), pound-feet (lbf·ft) or pound-inches (lbf·in). This converter changes a value between them and lists all units at once, so you can set a torque wrench correctly or compare two cars' specifications.

How it is calculated

Each unit is stored in N·m: result = value × N·m(from) ÷ N·m(to).

• 1 kgf·m = 9.80665 N·m (exact, from standard gravity)
• 1 kgf·cm = 0.0980665 N·m
• 1 lbf·ft = 1.355817948 N·m
• 1 lbf·in = 0.112984829 N·m (one twelfth of lbf·ft)

The principle

Torque equals force × lever arm: τ = F × r, with r measured perpendicular to the force. One newton pushing on a one-metre wrench gives 1 N·m. The other units change both rulers. A kilogram-force is the weight of 1 kg under standard gravity, 9.80665 N. A pound-force is the weight of one pound under the same gravity, 4.4482216152605 N, and one foot is 0.3048 m, so 1 lbf·ft = 4.4482216… × 0.3048 = 1.3558179… N·m. Although N·m has the same dimensions as the joule, torque is not energy, and the SI Brochure recommends writing N·m rather than J for torque.

Worked example

Default inputs: 100 N·m to pound-feet.

1. 1 lbf·ft = 1.355817948 N·m.
2. 100 ÷ 1.355817948 = 73.75621493 lbf·ft.
3. Ratio check: 1 N·m = 0.7375621493 lbf·ft.
4. Other units: 100 ÷ 9.80665 = 10.19716213 kgf·m, 1,019.716213 kgf·cm and 885.0745791 lbf·in.

How to read the result

For a feel of scale: small screws in electronics are often tightened to well under 1 N·m, passenger car wheel nuts commonly sit roughly in the 80–140 N·m range, and car engines produce from about 100 N·m to several hundred N·m. A higher engine torque figure means stronger pulling force at the wheels for a given gear, but the engine speed at which it occurs matters too.

Common mistakes

• Confusing lbf·ft with lbf·in; the difference is a factor of 12 and can strip a thread.
• Reading "kg·m" in old manuals as mass times length; it means kgf·m.
• Using torque values as energy or power. Power depends on torque and rotational speed.

Frequently asked questions

How many N·m is 1 lb·ft?

1.355817948 N·m.

How do I convert kgf·m to N·m?

Multiply by 9.80665. A spec of 10 kgf·m is about 98.1 N·m.

Is "lb-ft" the same as "ft-lb"?

For torque, people use both names for the same unit; strictly, "ft·lbf" is also used for energy, so check the context.

Sources: BIPM, The International System of Units (SI Brochure), 9th edition, 2019 · NIST Special Publication 811 (2008), Guide for the Use of the SI, Appendix B