About the Horsepower Calculator
This horsepower calculator connects the three numbers on every dyno sheet and motor nameplate: torque, rotational speed and power. Enter torque and RPM to get mechanical horsepower, kilowatts and metric horsepower (PS), or switch mode to find the torque an engine or motor must produce at a given RPM, or the RPM at which a torque figure delivers a given power.
Car enthusiasts use it to read dyno charts and compare engines, engineers use it to size electric motors, pumps and gearboxes, and students use it to check physics homework. Torque can be entered in pound-feet or newton-meters, and power in horsepower or kilowatts.
Power is simply torque multiplied by angular speed. In US units that reduces to hp = torque (lb-ft) × RPM ÷ 5,252, which is why torque and horsepower curves always cross at 5,252 RPM when plotted on the same scale. The calculator uses mechanical horsepower (550 ft·lbf/s = 745.7 W).
How to use the horsepower calculator
- 1Choose what to solve for: horsepower, torque or RPM.
- 2Enter the torque and choose lb-ft or N·m.
- 3Enter the RPM, or the power in hp or kW, depending on the mode.
- 4Read the result along with kW, PS and both torque units.
Formula and method
Power equals torque multiplied by angular velocity (P = τ × ω, with ω = 2π × RPM ÷ 60). One mechanical horsepower is defined as 550 foot-pounds per second, so combining the constants gives hp = torque × RPM ÷ 5,252.11 when torque is in pound-feet. In SI units, kW = torque (N·m) × RPM ÷ 9,549.3.
To solve for torque, rearrange to T = hp × 5,252 ÷ RPM; to solve for speed, RPM = hp × 5,252 ÷ T. One horsepower is 745.7 W, and metric horsepower (PS, CV) is slightly smaller at 735.5 W, so a figure in PS is about 1.4% higher than the same power in hp.
- hp
- Mechanical horsepower (745.7 W)
- T
- Torque
- RPM
- Rotational speed in revolutions per minute
- 5,252
- 33,000 ÷ 2π — the unit-conversion constant
Worked examples
300 lb-ft at 5,000 RPM
Horsepower = 300 × 5,000 ÷ 5,252.11 ≈ 285.6 hp. Multiplying by 0.7457 gives about 213 kW, and 300 lb-ft is about 406.7 N·m.
Torque of a 200 hp engine at 4,000 RPM
Rearranging the formula, torque = 200 × 5,252.11 ÷ 4,000 ≈ 262.6 lb-ft (356 N·m). The engine must make that much torque at 4,000 RPM to produce 200 hp.
Electric motor: 400 N·m at 3,000 RPM
In SI units, power = 400 × 3,000 × 2π ÷ 60 = 125,664 W, or 125.7 kW. Dividing by 745.7 W per hp gives about 168.5 hp.
Frequently asked questions
How do you convert torque to horsepower?+
Multiply torque in lb-ft by RPM and divide by 5,252. For example, 400 lb-ft at 4,500 RPM is 400 × 4,500 ÷ 5,252 ≈ 343 hp. With torque in N·m, kW = torque × RPM ÷ 9,549.
Why do torque and horsepower cross at 5,252 RPM?+
Because hp = torque × RPM ÷ 5,252, the two numbers are equal exactly when RPM equals 5,252 — as long as torque is in lb-ft and power in hp and both are plotted on the same scale.
How many kW is one horsepower?+
One mechanical horsepower equals 0.7457 kW (745.7 W). One kilowatt is about 1.341 hp. Metric horsepower (PS or CV) is 735.5 W, about 1.4% smaller.
Is horsepower or torque more important?+
Torque is the twisting force at a moment in time; horsepower is how quickly that force does work. Acceleration at the wheels depends on power and gearing, which is why a high-revving engine with less torque can still be fast.
Is this wheel horsepower or crank horsepower?+
It is whatever the torque figure represents. Torque measured at the flywheel gives crank (brake) horsepower; torque measured on a chassis dyno gives wheel horsepower, which is typically 10–20% lower because of drivetrain losses.