How this instrument works
Torque is twist — force multiplied by perpendicular distance from the pivot. Push with one newton at your spanner's end, one metre out from the bolt centre, and you have applied one newton-metre. That newton was named by the 9th General Conference on Weights and Measures in 1948 and amounts to one kilogram accelerated at one metre per second squared, so N·m is coherent SI, assembled from base units and nothing else.
A pound-force foot rests instead on three fixed numbers, each settled at a different moment in history. Standard gravity was pinned at 9.80665 m/s² by the 3rd General Conference in 1901. The avoirdupois pound became 0.45359237 kg under the 1959 international yard and pound agreement, which also made a foot 0.3048 m. Multiply pound by gravity and one pound-force is 4.4482216152605 N precisely; multiply again by 0.3048 m of lever arm and a pound-force foot lands on 1.3558179483314004 N·m — long, but terminating, and free of any uncertainty.
Run that chain backwards and digits stop terminating: 1 ÷ 1.3558179483314004 = 0.7375621492772654…, truncated here at twelve places. Which direction you need depends on whose paperwork sits open in front of you. European and Japanese assembly data, ISO fastener tables, and every bicycle component maker publish N·m. American automotive service manuals, aircraft maintenance documents, oilfield make-up torque charts, and structural bolting procedures still print pound-force feet, so anyone servicing an import with a domestic wrench crosses this boundary daily.
- Type your figure into the Newton-metres (Nm) field; it opens at 20 Nm as a reference.
- Read the Foot-pounds (ftlb) line beneath, which recomputes on every keystroke.
- Round to whatever your wrench can genuinely set — usually a half or whole pound-force foot.
- Going the other way? Multiply a pound-force foot figure by 1.3558179483314004 to recover newton-metres.
- For small fasteners specified in inch-pounds, multiply this converter's output by 12.
Worked example — a 20 Nm bolt, an American wrench
A workshop manual printed in Japan calls for 20 Nm, and every click wrench on your bench is graduated in pound-force feet. Type 20 into Newton-metres (Nm); Foot-pounds (ftlb) answers 14.7512429855. Set 14.75 if your wrench resolves quarters, 14.7 if it clicks in tenths.
Manuals carrying all three columns tend to round hard: beside that 20 Nm you will often find 2.0 kgf·m and a flat 14 lbf·ft. Fourteen is five percent light. Since ISO 6789 already allows a hand wrench four percent of error on its reading, chasing 14.7512429855 beyond two decimals buys nothing — yet starting from a truncated 14 stacks a known error atop an unknown one.
Questions
Is 0.737562149277 an exact factor?
No, it is a rounded reciprocal. Exactness lives in reverse: one pound-force foot equals precisely 1.3558179483314004 N·m, because 1 lbf = 0.45359237 kg × 9.80665 m/s² = 4.4482216152605 N, and a foot is precisely 0.3048 m. Dividing 1 by 1.3558179483314004 yields 0.7375621492772654…, a decimal with no end. Twelve places is far beyond what any wrench, transducer, or fastener specification will ever use.
What is the difference between ft-lb and lb-ft?
By strict American engineering convention: nothing in magnitude, everything in intent. Writing lb-ft (pound-feet) signals torque, while ft-lb (foot-pounds) signals work or energy. Both amount to 1.3558179483314004 SI units, since torque and energy share dimensions. Outside that convention nobody is consistent — wrenches, catalogues, and service literature are stamped FT-LB constantly. Read from context: a fastener specification means torque.
Why is torque written in newton-metres rather than joules?
Because a joule describes energy, and torque is not energy, even though both reduce to identical base units. NIST SP 811 and the SI Brochure ask that joules be reserved for work and heat, with torque expressed as N·m. Physically they differ by geometry: energy comes from force acting along a displacement, torque from force acting across a lever arm. Hold a wrench motionless at 20 Nm and no energy moves anywhere.
My specification is in inch-pounds — what then?
Multiply this converter's output by 12, since a pound-force foot contains twelve pound-force inches. Small fasteners in American aviation, instrumentation, and electronics work are nearly always specified in inch-pounds, and confusing one scale for the other produces a twelvefold error, which is a reliable way to shear a screw. As a sanity check, 20 Nm is 14.75 lbf·ft, or about 177 lbf·in.
How do I handle kgf·m on an older manual?
Kilogram-force metres fill Japanese and European manuals printed before roughly 1990, and they convert through standard gravity alone: 1 kgf·m = 9.80665 N·m, exactly. So 20 Nm works out at 2.039 kgf·m, which explains why such manuals round it to 2.0. To reach pound-force feet straight from kgf·m, multiply by 7.2330138512.
How many decimal places actually matter on a wrench?
Two at most, frequently none. ISO 6789 permits a hand torque wrench four percent of deviation on its reading, so a setting of 15 lbf·ft genuinely sits somewhere between about 14.4 and 15.6. Full precision exists here so repeated conversions do not drift, not because a bolt can perceive it. Convert once at full precision, then round to whatever your tool can be set to.