How this instrument works
An inch-pound is a torque built from two older definitions: one pound-force acting one inch out from a pivot. Since 1959 the avoirdupois pound has been exactly 0.45359237 kg and the inch exactly 25.4 mm, and standard gravity has been pinned at 9.80665 m/s² since the third General Conference on Weights and Measures met in 1901. Multiply those three fixed quantities together and a single pound-force inch resolves to 0.1129848290276167 N·m — a terminating decimal with no experimental uncertainty attached to it anywhere.
A newton-metre is the SI quantity on the other side: one newton applied one metre from the axis. It reduces to the same kg·m²/s² as a joule, yet torque is never quoted in joules, and the SI keeps that distinction deliberately. Work is a scalar accumulated by force along a displacement; torque is a moment about an axis, with a direction. Since the 2019 redefinition the newton traces through the kilogram to the Planck constant, so both ends of this conversion now rest on fixed numbers rather than artefacts.
Inch-pounds survive wherever North American drawings meet small fasteners — airframe and avionics work following AC 43.13, scope rings torqued to 15 or 25 in·lb, instrument and laptop assembly, valve covers and oil-pan bolts. Anything larger gets quoted in pound-feet, twelve inch-pounds apiece. Screwdriver dials from Japan and Taiwan still show kilogram-force centimetres, and small motors are rated in ounce-inches, sixteen of which make one inch-pound.
- Type your figure into the Inch-pounds (inlb) field; 100 in·lb is loaded as a starting point.
- Read the Newton-metres (Nm) line below it — recomputed on every keystroke, shown to six decimals.
- Working from a pound-foot spec? Multiply by twelve before typing, because one pound-foot is twelve inch-pounds.
- Reversing direction: divide newton-metres by 0.112984829028, or multiply by 8.850745791327 for the same answer.
- Round to what your tool can resolve — hand wrenches certified to ISO 6789 are allowed roughly four percent error.
Worked example — a 100 in·lb cover screw, metric wrench
An American assembly drawing calls for 100 in·lb on the four screws holding a gearbox cover, and the only tool on the bench is a digital wrench that reads newton-metres. Put 100 into the Inch-pounds (inlb) field and the Newton-metres (Nm) line returns 11.2984829028, displayed as 11.298483. Dial in 11.3 and you sit 0.014 percent above the drawing figure — far inside anything a hand tool can resolve.
That same 100 in·lb is 8.33 pound-feet, which is precisely why the spec was written in inch-pounds. A pound-foot wrench graduated every 5 lbf·ft cannot place it convincingly, whereas a quarter-inch-drive inch-pound wrench spanning 20 to 200 in·lb puts it mid-scale, the region where click mechanisms behave best.
Questions
Is 0.112984829028 an exact conversion factor?
Almost — it is the exact value rounded at the twelfth decimal place. Running the definitions gives 0.45359237 kg × 9.80665 m/s² × 0.0254 m = 0.1129848290276167 N·m per pound-force inch, a decimal that terminates and carries no uncertainty. Cutting it short here shifts a result by about three parts in a trillion, smaller than the rounding you see on screen and unmeasurably smaller than any wrench error.
What is the difference between inch-pounds and foot-pounds?
A factor of twelve, and confusing them is the costliest slip in this corner of a workshop. 100 in·lb equals 8.33 lbf·ft; someone who reads 100 and sets a pound-foot wrench applies twelve times the intended load and strips the threads out of an aluminium casting. Check which scale is stamped on the handle before the first click, especially with dual-scale beam tools.
Should it be written in·lb, lb-in, or lbf·in?
NIST prefers lbf·in — force first, lever arm second, with the f making clear that a pound is acting as force rather than mass. Trade practice reverses the order and drops the f, and nearly every torque wrench sold in North America is marked in-lb or inch-pounds. All three spellings name one unit, and this converter takes the trade form as its input label.
Torque and energy share dimensions, so is a newton-metre a joule?
Not in practice, and the SI asks you not to treat them as interchangeable. Joules are reserved for energy and work; torque keeps the newton-metre even though both collapse to kg·m²/s². The reason is physical rather than typographic: work is a scalar produced by force along a path, torque a moment about an axis. Inch-pounds lead the same double life on the imperial side, where 1 lbf·in of work is 0.113 joules.
Do imperial and US customary specs give different answers here?
No — the pound-force inch is identical in both. The 1959 international yard and pound agreement fixed the pound at 0.45359237 kg and the inch at 25.4 mm simultaneously for the United States, United Kingdom, Canada, Australia, New Zealand and South Africa, so a British and an American torque spec convert with one factor. The deprecated US survey foot applied only to geodetic surveying, never to fastener work.
How many decimals do I actually need?
Two or three significant figures is plenty for turning fasteners. Hand torque wrenches certified to ISO 6789 are permitted around four percent error across most of their range, and operator technique adds more on top, so 11.3 N·m and 11.2984829028 N·m are the same instruction to a tool. Full precision earns its keep elsewhere: calibration certificates, test records, and spreadsheets that convert back again without drifting.