Newton-meters to Foot-pounds (N·m to ft·lb)

The newton-meter (N·m) is the SI-derived unit of torque (also called moment-of-force), defined as the torque produced by a one-newton force applied at one metre perpendicular distance from the rotation axis (1 N·m = 1 kg·m²/s² exactly). Dimensionally identical to the joule (1 J = 1 N·m), but the two are conceptually distinct: the joule expresses energy or work (force times distance moved in the direction of force), while the newton-meter expresses torque (force times perpendicular lever-arm distance from the axis of rotation). The 2019 SI redefinition fixed all SI-derived units including the newton-meter via the kilogram-metre-second chain anchored in the Planck constant, the speed of light, and the caesium-133 hyperfine transition. The newton-meter is the standard torque unit in physics-and-engineering work, with millinewton-meter (mN·m, 10⁻³ N·m), kilonewton-meter (kN·m, 10³ N·m), and meganewton-meter (MN·m, 10⁶ N·m) prefixed scales used at different application scales from precision-instrumentation through everyday-fastener-tightening to large-industrial-machinery torque ratings.

The newton-meter as a torque unit emerged with the codification of the SI unit system through the 1948 9th CGPM and the 1960 11th CGPM that formalised the SI. The name "newton-meter" combines the SI-derived force unit (newton, named in 1946 for Sir Isaac Newton) with the SI-base length unit (metre, defined exactly via the speed of light), giving a dimensionally consistent torque unit (1 N·m = 1 kg·m²/s²) that matches the dimensions of energy (1 J = 1 N·m) but represents a rotational rather than translational mechanical quantity. Before SI standardisation, continental-European engineering practice used kilogram-force-meters (kgf·m) for torque specifications, US-customary engineering used foot-pounds (ft·lb), and CGS-based scientific work used dyne-centimetres. The 1971 14th CGPM resolution that deprecated kilogram-force also formalised the newton-meter as the standard SI torque unit, with the modern engineering practice harmonising around N·m for international engineering documentation. The newton-meter has been the universal SI torque unit since, replacing kgf·m and ft·lb in scientific and most international engineering work, with the older units surviving in US-customary engineering (ft·lb) and legacy continental-European-and-Russian-and-Asian engineering documentation (kgf·m). The 2019 SI redefinition fixed the newton-meter via the kilogram-metre-second chain anchored in the Planck constant, the speed of light, and the caesium-133 hyperfine transition.

Modern automotive-engineering bolt-and-fastener torque specifications: typical engine cylinder-head bolts at 80-150 N·m, wheel-lug-nuts at 100-150 N·m, crankshaft-pulley bolts at 100-200 N·m, suspension bolts at 50-300 N·m. The N·m is the universal SI torque unit on European, Japanese, Korean, and Chinese vehicle service-and-maintenance documentation under ISO, JIS, KS, and GB conventions. Industrial-machinery torque-and-shaft specifications: typical electric-motor torque ratings (1-100 kN·m for industrial motors), gearbox-output-shaft ratings (10-10,000 kN·m), wind-turbine main-shaft torque ratings (1000-10,000 kN·m for large utility-scale turbines). Aerospace-and-launch-vehicle gimbal-torque specifications, spacecraft-attitude-control reaction-wheel torque ratings (typical 0.1-100 N·m for small-satellite and large-satellite attitude control). Bicycle component torque specifications under modern ISO conventions (typical bottom-bracket at 35-50 N·m, crank-arm at 35-45 N·m, brake-rotor at 6 N·m). Medical-device torque specifications under EU MDR and FDA conventions (typical orthopaedic-screw at 1-5 N·m, dental-implant at 0.2-0.5 N·m). The newton-meter is the universal scientific-and-international-engineering torque unit, with foot-pounds (ft·lb) surviving in US-customary engineering practice and kilogram-force-meters (kgf·m) in legacy industrial-equipment-and-automotive documentation.

The foot-pound (ft·lb, ft-lb, or ft-lbf) is the US-customary unit of torque, defined as the torque produced by a one-pound-force applied at one foot perpendicular distance from the rotation axis (1 ft·lb = 0.3048 m × 4.4482216152605 N = 1.35581794833 N·m exactly). The relationship is fixed by the underlying definitions: 1 ft = 0.3048 m exactly (1959 international yard-and-pound agreement) and 1 lbf = 4.4482216152605 N exactly (1 lb at exactly 0.45359237 kg multiplied by standard gravity exactly 9.80665 m/s²). The recognised symbols are "ft·lb" or "ft-lb" or "ft-lbf" — the various spellings refer to the same US-customary torque unit, with "ft·lbf" being the most explicit since it disambiguates pound-force from pound-mass. The inch-pound (in·lb, 1/12 ft·lb = 0.11298 N·m) is used for smaller-fastener torque-and-tightening specifications. The foot-pound is the standard US-customary torque unit, with kilopound-foot or kip-feet (k·ft, equal to 1000 ft·lb = 1356 N·m) used at structural-engineering moment-rating scale.

The foot-pound torque unit (ft·lb, also written ft-lb or ft-lbf) emerged in nineteenth-century US-and-British engineering practice as the natural torque unit combining the foot (12 inches, equal to exactly 0.3048 metres since the 1959 international yard-and-pound agreement) and the pound-force (the gravitational force on a one-pound mass at standard gravity, equal to exactly 4.4482216152605 N). The unit appeared in steam-engine torque-and-power specifications, fastener-tightening documentation, automotive-engineering torque ratings, and structural-engineering moment specifications throughout the late nineteenth and early-to-mid twentieth centuries. The 1959 international yard-and-pound agreement standardised both the foot and the pound exactly, fixing 1 ft·lb = 1.35581794833 N·m exactly via the underlying 1 ft = 0.3048 m exactly and 1 lbf = 4.4482216152605 N exactly definitions. The foot-pound survives in US-customary engineering practice for automotive, mechanical, structural, and aerospace work, where US engineering documentation, ASME-and-AISC-and-ASTM standards, and SAE-and-FAA-and-DoD-and-MIL-STD specifications express torque in ft·lb. The inch-pound (in·lb, equal to 1/12 ft·lb) appears in smaller-fastener-and-precision torque work, while the foot-pound dominates everyday US-customary engineering torque documentation. Modern US-customary practice is increasingly bilingual, with both ft·lb and N·m appearing on US-bound vehicle-service-manual and equipment-installation documentation.

US-customary engineering practice across automotive, mechanical, structural, aerospace, and industrial work. Automotive-engineering bolt-and-fastener torque specifications: typical engine cylinder-head bolts at 60-110 ft·lb, wheel-lug-nuts at 80-100 ft·lb, crankshaft-pulley bolts at 100-150 ft·lb, suspension bolts at 35-220 ft·lb. The ft·lb is the universal US-customary torque unit on Ford, GM, Chrysler/Stellantis, and US-bound vehicle service-and-maintenance documentation under SAE J-standard conventions. Mechanical-engineering: industrial-actuator torque ratings, fastener-tightening specifications under ASME and ASTM conventions, aerospace-fastener torque under MIL-STD and AS-standards in ft·lb. Structural-engineering: moment-and-torque specifications in kip-feet (1000 ft·lb) under AISC-360 and ACI-318 conventions, with typical W-shape steel-beam moment-capacity-ratings in kip-feet. Aerospace-and-defense: gimbal-torque, control-surface-actuation-torque, and propulsion-system-torque specifications in ft·lb under MIL-STD and AS-standards. Bicycle component torque specifications in older US-customary documentation (typical bottom-bracket at 25-37 ft·lb, brake-caliper at 6-9 ft·lb). The foot-pound appears in US engineering practice even when SI is used elsewhere on the same project — many international aerospace and automotive engineering programs maintain dual-unit documentation with N·m primary and ft·lb secondary for US-bound applications.