How this instrument works
Torque in this system is a pound-force acting through a lever arm measured in inches or feet. One inch-pound turns a bolt with a single pound-force pushing on a one-inch handle; one foot-pound does it with a twelve-inch handle. Nothing about that ratio was ever measured. A foot contains twelve inches by definition — a survival of Roman arithmetic, where uncia meant a twelfth part of a pes and gave English both inch and ounce — so twelve lb-in make a lb-ft, and no experiment can revise it.
Absolute size comes from two fixed numbers rather than from any ratio. Avoirdupois pound mass became 0.45359237 kg exactly under a 1959 accord among six English-speaking standards bodies, and standard gravity had already been pinned at 9.80665 m/s² by CGPM in 1901. Multiply those and pound-force is 4.4482216152605 N exactly, making lb-in worth 0.112984829027617 N·m and lb-ft worth 1.3558179483314 N·m. Divide one by another and twelve reappears, untouched.
Which unit a trade reaches for depends almost entirely on how small its fasteners are. Inch-pounds rule aircraft AN hardware, electrical terminal lugs and breaker screws, carbon bicycle stems, rifle scope rings, valve covers and oil pans. Foot-pounds take over above roughly fifteen: lug nuts near 90, cylinder heads, suspension pivots, crank pulleys. Tool makers split them by drive size, which is why a quarter-inch click wrench usually carries lb-in graduations and a half-inch one does not.
- Type your figure into the Inch-pounds (inlb) field — 120 sits there already as a starting point.
- Read Foot-pounds (ftlb) directly below; it recalculates on every keystroke and carries six decimals.
- Confirm your wrench's own scale is graduated in lb-ft before dialling that number in.
- Reversing? Multiply by twelve instead: 45 lb-ft becomes 540 lb-in, exactly.
Worked example — a 120 lb-in spec, a lb-ft wrench
An intake manifold sequence calls for 120 lb-in, but every click wrench in your drawer is graduated in lb-ft. Type 120 into Inch-pounds (inlb) and 10.0 appears beside Foot-pounds (ftlb). Dial 10 onto your wrench and you are on spec, not near it.
That whole number is arithmetic rather than luck: 120 is ten twelves, and shop manuals favour round lb-in figures precisely so they divide cleanly. Should a metric wrench be nearer to hand, 10 lb-ft is 13.56 N·m. What ruins threads is dialling 120 onto a lb-ft scale — twelvefold overload, roughly 163 N·m, enough to shear an 8 mm bolt long before anything clicks.
Questions
Is this factor exact, or rounded?
Exactly one twelfth, and definitionally so. A foot holds twelve inches by definition, and both units use an identical pound-force, so nothing in that ratio rests on measurement. This sheet applies 0.0833333333333, a thirteen-digit truncation of a repeating twelfth, biasing any result by roughly four parts in a trillion. A good click wrench is accurate to perhaps four percent — eleven orders of magnitude coarser than that shortfall.
Should I write lb-in or in-lb, lb-ft or ft-lb?
Either order reads as torque in practice, and shop manuals use both freely. Metrologists prefer force first — lbf·in and lbf·ft — since torque is force times distance, reserving ft·lbf for energy or work. Automotive writing leans toward lb-ft for engine output, while ballistics almost always prints ft-lb for muzzle energy. Same symbols, opposite conventions, which is why context settles more than punctuation ever will.
Do imperial and US customary figures differ here?
No. Britain and America have shared an identical inch, foot and avoirdupois pound since 1959, so a lb-in specified in Coventry equals one specified in Detroit. Divergence certainly exists elsewhere in these systems — gallons and fluid ounces differ substantially, and America kept a separate survey foot for geodesy until 2022 — but none of that reaches torque. A twelfth is a twelfth on either side of the Atlantic.
Why does my torque wrench read in inch-pounds when my manual says foot-pounds?
Because tool makers match graduation to drive size. Quarter-inch drives cover roughly 20 to 200 lb-in, a span where foot-pound markings would be uselessly cramped, while half-inch drives run 30 to 250 lb-ft. Small-fastener work therefore arrives in lb-in and heavy work in lb-ft, so almost any mixed job needs translating. Confusing them is a twelvefold error whichever way you go: stripped threads, or lug nuts loose enough to walk off.
Is a foot-pound of torque the same as a foot-pound of energy?
Dimensionally identical, physically different. Torque is force applied at right angles to a lever arm and does no work at all unless something turns; energy is force acting through distance along its own line. SI keeps them visually apart, writing newton-metres for torque and joules for energy, even though a joule is a newton-metre. Imperial extends no such courtesy, so 10 lb-ft on a wrench and 10 ft-lb from a bullet share a name and little else.
What is 120 inch-pounds in newton-metres?
13.558 N·m. Route it through foot-pounds first: 120 lb-in is 10 lb-ft, and one lb-ft is 1.3558179483314 N·m exactly, giving 13.558179483314 N·m. Working straight from lb-in, multiply by 0.112984829027617 N·m and you land on an identical figure. Both constants descend from an exact pound of 0.45359237 kg and standard gravity of 9.80665 m/s², so neither carries measurement uncertainty.