Torque Units Explained: Newton-Meters vs Pound-Feet

Torque is the twisting force a wrench applies, a motor delivers, or a bolt resists — and it is measured in newton-meters on SI drawings, pound-feet and pound-inches in US manuals, and kilogram-force meters on older plates. The factors between them are exact, which makes every torque conversion a multiplication, not a measurement.

The newton-meter and its exact relatives

Torque is force times lever arm: push one newton on the end of a one-meter wrench and you apply one newton-meter (N·m). Anchoring every other unit to the N·m gives two exact definition chains:

1 kgf·m  = 9.80665 N·m    (standard gravity, exact)
1 lb·ft  = 1.3558179483314 N·m
1 lb·in  = 0.11298482902762 N·m
1 ozf·in = 0.0070615518 N·m

The kilogram-force meter is exact because standard gravity is a defined constant: 9.80665 m/s², agreed internationally, so one kilogram-force meter is the twist of a 1 kg mass on a 1 m lever. The pound-foot inherits its exactness from the pound-force (4.4482216152605 N) times the international foot (0.3048 m). Nothing here is measured or approximate — a 1960s plate reading "12 kgf·m" and a modern spec reading "117.68 N·m" describe precisely the same twist.

Reading a torque wrench across the pond

The everyday conversion is newton-meters to pound-feet, and the factor to remember is 0.7375621493 — or roughly "multiply by 0.74":

100 N·m = 73.76 lb·ft     80 lb·ft = 108.5 N·m
300 N·m = 221.27 lb·ft    25 N·m   = 221.27 lb·in

A typical car wheel-nut spec of 120 N·m is 88.5 lb·ft; the classic 25 lb·ft shop-manual setting is 33.9 N·m. The mistake that ruins fasteners is setting an imperial wrench by the metric number without converting — overtightening by a factor of 1.3558 can stretch or snap the bolt, and undertightening lets it loosen. The Torque Converter moves between all nine units, including the small scales, in one panel.

Small fasteners live three scales down

Torque scales with lever length, which is why small work uses small units: 1 lb·ft is 12 lb·in, and 1 lb·in is 16 ozf·in. Electronics assembly, instrument screws and rifle scope rings are tightened to single-digit pound-inches or a few ounce-inches — numbers like "0.5 lb·ft" are awkward, but "9.6 lb·in" or "52 ozf·in" read naturally. The metric equivalents run the same ladder: 1 N·m is 100 N·cm and 1,000 N·mm, so a laptop screw spec of 0.4 N·m is 40 N·cm or 35.4 ozf·in. Using the wrong scale is not just ugly — the fine increments on a torque screwdriver are graduated in exactly these units.

Pound-feet or foot-pounds?

Both spellings circulate, and they mean the same arithmetic — force times distance. Engineers prefer pound-feet (lb·ft) for torque to keep it distinct from the foot-pound (ft·lb), which is the standard unit of energy: 1 ft·lb = 1.35582 joules. The number is identical because torque and energy differ only by rotation: energy is force times distance, torque is force times lever, and the angle in between (in radians) converts one into the other. The overlap is why a torque converter and an energy converter share a factor table even though they convert different quantities — the energy units guide covers the other side of that coin, and the whole parallel-unit-systems story is told in Why Two Measurement Systems.

How torque becomes power

Twist a shaft and you have torque; twist it while it spins and you deliver power. In SI units the link is one line: watts = newton-meters × radians per second. That is why engine brochures list peak torque and peak power at different rpm — peak torque is the strongest twist, while peak power (which multiplies torque by speed) usually comes later. A motor making 20 N·m at 3,000 rpm is delivering 2π × 3,000/60 × 20 ≈ 6,283 W, or 8.4 hp. The RPM Converter handles the speed side and the Power Converter the wattage side; the force underneath it all is covered by the Force Converter, since torque is just force with a lever attached.

Try it yourself

Three checks with the Torque Converter. First, your car: enter the wheel-nut spec from the door sticker — 120 N·m should read 88.51 lb·ft, the number a US torque wrench needs. Second, a drill or motor plate: enter 5 kgf·m and confirm 49.03 N·m. Third, a hobby scale: enter 1 N·m and read 8.85 lb·in and 141.6 ozf·in, the range where instrument screws live. If those three agree with your wrench, every fastener spec in the shop is now bilingual.