How this instrument works
Torque is force acting through a lever arm, so every torque unit is a force unit multiplied by a length unit. Newton-metres pair the SI newton with the metre. Inch-pounds pair pound-force with an inch, which is why you also see them written lbf·in or lb-in — same quantity, different word order. Neither name says anything about how a fastener behaves; both describe twist about an axis.
Three legislated numbers fix this factor, and not one of them was measured. A pound is 0.45359237 kg exactly and an inch is 25.4 mm exactly, both settled by an International Yard and Pound Agreement signed in 1959 by six English-speaking standards bodies. Standard gravity, g₀ = 9.80665 m/s², was fixed by decree at CGPM in 1901 so that weight units would rest on something unchanging. Multiply: 0.45359237 × 9.80665 = 4.4482216152605 N per pound-force, and 4.4482216152605 × 0.0254 = 0.1129848290276167 N·m per inch-pound. That decimal terminates, and it is exact.
Run it backwards and exactness ends: 1 ÷ 0.1129848290276167 = 8.850745791327… with digits that never settle, so twelve significant figures are carried here as 8.85074579133. Which direction you need usually depends on paperwork rather than preference. Airframe manuals from US manufacturers, electrical lug tables behind UL 486A, gunsmithing specs and small American automotive fasteners still speak inch-pounds; ISO and DIN fastener data, bicycle components and nearly every service manual written elsewhere speak newton-metres. Shops handling both keep converters within reach.
- Type your figure into Newton-metres (Nm); it opens at 20 Nm, a common small-fastener spec.
- Read Inch-pounds (inlb) below it — recomputed on every keystroke, no button to press.
- Round to whatever your wrench can be set to: whole inch-pounds on click types, half units on dial or beam gauges.
- Want foot-pounds? Divide that inch-pound result by 12. Want ounce-inches for small motors or optics? Multiply by 16.
- Going backwards, from inch-pounds to newton-metres, divide your figure by 8.85074579133.
Worked example — a 20 Nm bolt on an inch-pound wrench
A workshop is rebuilding a European twin whose manual calls for 20 N·m on its clutch cover bolts, and their smallest wrench is a quarter-inch drive click type marked 20 to 200 inch-pounds. Enter 20 into Newton-metres (Nm); Inch-pounds (inlb) returns 177.014915827. Dial 177 in-lb and those bolts land on spec, comfortably inside that wrench's working range.
Two checks before pulling. Divide by 12 and you get 14.75 ft-lb, a sane figure for an aluminium cover — anyone dialling 177 foot-pounds would apply twelve times spec and pull threads clean out of a casting. Second, a click wrench certified to ISO 6789 is allowed roughly ±4% of reading, near seven inch-pounds at this setting, so every digit past 177 is arithmetic rather than engineering.
Questions
Is 8.85074579133 an exact factor?
Not quite, though it is exact in reverse. One inch-pound equals 0.1129848290276167 N·m with no error at all, since a pound, an inch and standard gravity are defined quantities rather than measured ones. Inverting that terminating decimal yields 8.850745791327…, which never resolves, so twelve significant figures are kept. Residual error runs near 3 parts in 10^13, which on a 177 in-lb setting amounts to well under a billionth of one inch-pound — far beneath anything a calibrated wrench could reveal.
What happens if I confuse inch-pounds with foot-pounds?
You miss by a factor of twelve, which is how most torque accidents happen. 177 in-lb is 14.75 ft-lb; dialling 177 ft-lb onto that same fastener delivers 2124 in-lb and will strip aluminium threads or snap a small bolt outright. Quick sanity check: for identical torque, inch-pound numbers look large and foot-pound numbers look small, so if specs for a valve cover or battery terminal read in triple digits, they are almost certainly inch-pounds.
Are pound-inches and inch-pounds different units?
No — identical, merely written in reversed order. Standards writing favours force first, lbf·in, because a unit product should name force then distance; workshop habit says inch-pound. Either way, pound here means pound-force, never pound-mass. That distinction is precisely what standard gravity, 9.80665 m/s², is doing inside this factor.
Why give torque in newton-metres and not joules, when both are N·m?
Because a joule signals energy, while torque is a moment about an axis, built from perpendicular vectors rather than aligned ones. SI leaves both units algebraically identical yet advises reporting torque as N·m and energy as J, so a reader can tell which quantity is meant. Turning a fastener through one radian at 20 N·m does deliver 20 J of work — but only after multiplying by an angle.
Which trades still specify inch-pounds?
US aviation maintenance leads: airframe and engine manuals list fastener values in inch-pounds, and hangar wrenches are marked accordingly. Electrical work follows, since terminal and lug torques under UL 486A and equipment installation instructions get published in lb-in. Optics mounts, firearm scope rings, small automotive fasteners and model machinery use them too. Outside America, and on nearly anything designed to ISO or DIN, newton-metres rule.
How many decimals should I keep?
One at most, usually none. A click wrench certified to ISO 6789 is permitted around ±4% of its setting, and its scale steps in whole inch-pounds anyway, so 177.014915827 is honestly 177 ± 7. Full precision appears here because rounding early, then converting again, compounds error. Round once, at your final step, to whatever your tool can actually be set to.