Motor Torque & Power Calculator
Enter shaft power and speed to get full-load torque in N·m and lb·ft, or solve power from torque.
Step-by-step
- Enter valid values to begin.
Metric: T = 9550 × kW / rpm. Imperial: T = 5252 × HP / rpm. Shear stress uses the plain torsion formula 16T / (πd³) and ignores the keyway stress concentration; 40 MPa (about 6,000 psi) is a common conservative guideline for keyed commercial steel shafts, not a substitute for a drive-train design check.
Motor Torque and Power: What the Shaft Actually Feels
Core Engineering Principles
Power tells you how fast work gets done. Torque tells you how hard the shaft is being twisted while it does it. A motor nameplate quotes kilowatts or horsepower because that’s what you buy, but couplings, keys, gearboxes and shafts fail on torque. The two are tied together by speed: power is torque times angular velocity. Put the units in kW and rpm and the constants collapse to 9550; use horsepower and lb·ft and you get 5252, which is just 33,000 divided by 2π.
The practical lesson is that a slow motor of the same power twists much harder. A 37 kW motor at 1480 rpm puts out about 239 N·m, but the same 37 kW at 2960 rpm is only 119 N·m. That’s why an eight-pole motor is physically bigger than a two-pole machine of identical rating. Then remember the nameplate torque is the rated point only. A NEMA design B motor can deliver roughly 150% of full-load torque at locked rotor and 200–250% at breakdown, and your shaft has to survive those moments, not just the quiet running number.
P (kW) = T × N / 9550 • τshaft = 16 T / (π d³)
NEC & Standard References
NEMA MG-1 Part 12 defines the torque characteristics of the standard designs: locked-rotor, pull-up and breakdown torque as a percentage of full-load torque. NEMA MG-1 Part 4 sets frame and shaft dimensions, and IEC 60072 does the same for metric frames. IEC 60034-1 covers rating and duty types. NEC Article 430 sizes the circuit from table current, not from torque, so a torque calculation never replaces the branch-circuit calculation; it feeds the mechanical design of the drive train.1. T = 9550 × 37 / 1480 = 238.8 N·m, which is 176.1 lb·ft. The HP check agrees: 49.6 HP × 5252 / 1480 = 176.0 lb·ft.
2. Input power = 37 / 0.93 = 39.8 kW.
3. Shaft stress = 16 × 238.8 / (π × 0.06³) = 5.6 MPa rated, about 14 MPa at 2.5× breakdown torque.
4. Behind a 10:1 gearbox at 98%, the output shaft sees 238.8 × 10 × 0.98 = 2,340 N·m at 148 rpm. That is the number the output coupling has to be chosen for.
- Rated torque is not peak torque. A jam, a locked rotor or a hard start on a direct-on-line motor can hit 2–2.5× rated. Size keys and couplings with a service factor, and fit a shear pin or torque limiter where a jam is plausible.
- Never mix the constants. 9550 is for kW and N·m; 5252 is for HP and lb·ft. Using HP with 9550 is a 34% error that goes straight into the gearbox selection.
- Torque multiplies through a gearbox. The motor shaft may be fine while the output shaft shears.
- On a VFD, rated torque only lasts to base speed. Above it you are in constant-power territory and available torque falls as 1/speed.