Horsepower: formula
Enter the engine torque in pound-feet and the engine speed in RPM. Horsepower equals torque times RPM divided by 5,252, a constant that comes from converting rotational work to the horsepower unit. Take the numbers from a dynamometer chart at the same RPM, since torque and speed change across the range.
Kilowatts are horsepower times 0.7457.
Horsepower: worked example
| Torque (lb-ft) | 300 |
|---|---|
| Engine speed (RPM) | 5,000 |
| Horsepower | 285.6 |
| Kilowatts | 213 |
Crankshaft and wheel measurements refer to different points in the drivetrain. Loss depends on the vehicle, operating conditions and measurement method; this torque-to-power calculation does not assume a fixed loss percentage. Mechanical and metric horsepower also use different unit definitions.
Horsepower: changing Torque (lb-ft)
| Torque (lb-ft) | Horsepower | Kilowatts |
|---|---|---|
| 240 | 228.5 | 170.4 |
| 270 | 257 | 191.7 |
| 300 (example) | 285.6 | 213 |
| 330 | 314.2 | 234.3 |
| 360 | 342.7 | 255.6 |
Horsepower: compare input ranges
| Input | Tested from | To | Horsepower: low input | Horsepower: high input | Swing |
|---|---|---|---|---|---|
| Torque (lb-ft) | 270 | 330 | 257 | 314.2 | 57.1 (20%) |
| Engine speed (RPM) | 4,500 | 5,500 | 257 | 314.2 | 57.1 (20%) |
Horsepower: individual input comparisons
| Input | Value | Horsepower | Kilowatts |
|---|---|---|---|
| Torque (lb-ft) | 240 | 228.5 | 170.4 |
| 270 | 257 | 191.7 | |
| 300 | 285.6 | 213 | |
| 330 | 314.2 | 234.3 | |
| 360 | 342.7 | 255.6 | |
| Engine speed (RPM) | 4,000 | 228.5 | 170.4 |
| 4,500 | 257 | 191.7 | |
| 5,000 | 285.6 | 213 | |
| 5,500 | 314.2 | 234.3 | |
| 6,000 | 342.7 | 255.6 |
Horsepower: combined input changes
| Torque (lb-ft) / Engine speed (RPM) | 4,000 | 5,000 | 6,000 |
|---|---|---|---|
| 240 | 182.8 | 228.5 | 274.2 |
| 270 | 205.6 | 257 | 308.5 |
| 300 | 228.5 | 285.6 | 342.7 |
| 330 | 251.3 | 314.2 | 377 |
| 360 | 274.2 | 342.7 | 411.3 |
Horsepower: which assumptions move the result?
Horsepower: values and definitions
| Quantity and definition | Value used |
|---|---|
| Torque (lb-ft) | 300 |
| Engine speed (RPM) | 5,000 |
| Horsepower | 285.6 |
| Kilowatts | 213 |
Horsepower: half and double Torque (lb-ft)
| Torque (lb-ft) | Horsepower |
|---|---|
| 150 | 142.8 |
| 300 | 285.6 |
| 600 | 571.2 |
Common error: Mixing torque units. Newton-meters need a different constant than pound-feet, and using the wrong pair is off by about a third.
Physics & Mechanics calculators: Speed, Distance and Time Calculator, Kinetic Energy Calculator, Newton's Second Law Force Calculator, Uniform Acceleration Calculator, Hooke's Law Calculator, Ideal Gas Law Calculator.
Horsepower is torque multiplied by speed, then scaled by a constant, so an engine's peak power sits at a higher speed than its peak twist. An engine making 300 pound-feet at 5,000 RPM delivers about 286 horsepower, or 213 kilowatts. Quoted power without the RPM is incomplete, because the same engine makes far less at idle.
Match the RPM to the torque reading. A torque figure taken at one speed and an RPM from another gives a number the engine never produces.
See work and power on the work and power calculator, fuel economy on the MPG calculator and electric equivalents on the electricity cost calculator. The kinetic energy and force calculators cover related physics, and the power converter changes units.
What units does the 5,252 approximation require?
Torque in pound-feet, speed in revolutions per minute and mechanical horsepower. The exact conversion constant is about 5252.113.
Must the plotted curves cross?
Their numerical values are equal at about 5252 RPM in those units if data exist there. The engine need not operate at that speed, and independent graph scales can change a visual intersection.
Does this estimate wheel horsepower from engine horsepower?
No. That requires measured or separately assumed drivetrain behavior; no fixed loss percentage is applied. Power measured at the engine and power measured at the wheels have different measurement boundaries.
The torque-and-speed relationship does not establish transmission or drivetrain losses.