Quick Answer: How Do You Convert Between Motor Torque, HP and RPM?
Motor torque, horsepower and speed are linked by a single relationship. In US units, HP = torque (lb-ft) × RPM ÷ 5252, so torque (lb-ft) = HP × 5252 ÷ RPM. In metric, kW = torque (N·m) × RPM ÷ 9549. The constants 5252 and 9549 come from unit math, not magic, which is why the HP and torque curves always cross at exactly 5252 RPM on a dyno chart.
This calculator converts between HP, kW, torque (lb-ft or N·m) and RPM. Enter any two values and it solves for the rest, with motor efficiency built in. It also applies service-factor and gearbox-ratio multipliers so you can size drives, spindles and gearmotors accurately.
Key conversions: 1 HP = 0.746 kW | 1 kW = 1.341 HP | 1 lb-ft = 1.356 N·m | 5 HP @ 1800 RPM → 14.6 lb-ft | AC induction motor efficiency 85–92% | power factor 0.80–0.90 at full load.
How Motor Torque and HP Work Together
Horsepower and torque measure two different things. Torque is how hard the shaft twists. HP is how much work the motor can do over time. You cannot have HP without torque, and you cannot use torque at zero RPM to do work. The bridge between them is RPM.
The Key Formulas
In US units: HP = Torque (lb-ft) × RPM ÷ 5252. In metric: kW = Torque (N·m) × RPM ÷ 9549. These two formulas are the same equation, just in different units. One HP equals 0.746 kW. One lb-ft equals 1.356 N·m.
Why 5252?
The number 5252 comes from the definition of one horsepower as 33,000 foot-pounds per minute. Dividing by 2π gets you 5252. It is not magic, just unit math. It also explains why HP and torque curves always cross at exactly 5252 RPM on a dyno chart.
Pro tip: For CNC spindle sizing, look at the torque curve at low RPM. High HP at 10,000 RPM does not help if you are taking heavy cuts at 500 RPM. A motor that makes constant torque from 100 to 3000 RPM is much more versatile for machining.
Motor Efficiency Matters
AC induction motors are 85 to 92 percent efficient at full load. The rest is heat. If you need 10 HP of real shaft power and the motor is 88 percent efficient, the motor actually pulls 11.4 HP worth of electrical power. Premium efficiency motors save energy and run cooler, which extends bearing life.
3-Phase Motor Current
For a 3-phase AC motor, input current (amps) equals kW times 1000 divided by voltage times square root of 3 times power factor. A 5 HP motor (3.73 kW mechanical) at 460 V, 88% PF, 90% efficiency pulls about 5.9 amps per phase at full load. Use this for sizing fuses, contactors, and wire gauge.