Answer first
What this calculator tells you
Calculate horsepower and kilowatts from torque and engine speed. Compare an engine's output at a given RPM with the figure quoted on its spec sheet. Formula: Horsepower = torque (lb-ft) × RPM ÷ 5,252; kilowatts = horsepower × 0.7457. At the worked-example inputs, the horsepower is 285.6. Holding every other input steady, moving torque (lb-ft) from 240 to 360 moves the result from 228.5 to 342.7.
Transparent method
The formula
Compare an engine's output at a given RPM with the figure quoted on its spec sheet.
Worked example
Example inputs
How to interpret the result
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.
At the worked-example inputs the horsepower is 285.6. It rises with torque (lb-ft) and engine speed (RPM).
These are textbook formulas for ideal conditions: no air resistance, no friction, gravity of 9.81 meters per second squared and gases that behave ideally. Real results differ, so treat them as first estimates and use the units the formulas expect (meters, kilograms, seconds).
Before you rely on it
What to check
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.
The common error
Where people go wrong with horsepower calculator
Mixing torque units. Newton-meters need a different constant than pound-feet, and using the wrong pair is off by about a third.
Sensitivity evidence
How torque (lb-ft) changes the horsepower
Holding every other input at the worked-example value, moving torque (lb-ft) from 240 to 360 moves the horsepower from 228.5 to 342.7: a spread of 114.2, or 40% of the worked-example result.
| Torque (lb-ft) | Horsepower | Kilowatts |
|---|---|---|
| 240 | 228.5 | 170.4 |
| 270 | 257 | 191.7 |
| 300worked example | 285.6 | 213 |
| 330 | 314.2 | 234.3 |
| 360 | 342.7 | 255.6 |
Every input, tested
Which input moves the horsepower most
Of the 2 inputs, torque (lb-ft) moves the horsepower most (57.1 across the range tested) and engine speed (RPM) moves it least (57.1).
| Input | Tested from | To | Horsepower at each end | Swing |
|---|---|---|---|---|
| Torque (lb-ft) | 270 | 330 | 257 to 314.2 | 57.1 (20%) |
| Engine speed (RPM) | 4,500 | 5,500 | 257 to 314.2 | 57.1 (20%) |
Two variables at once
Horsepower by torque (lb-ft) and engine speed (RPM)
Across the grid the horsepower runs from 182.8 to 411.3. Moving torque (lb-ft) from 240 to 360 shifts it by 114.2 at the middle column, and moving engine speed (RPM) from 4,000 to 6,000 shifts it by 114.2 at the middle row, so neither is the bigger lever here.
| 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 |
The highlighted cell is the worked example: 285.6.
Step by step
The worked example, input by input
| Input | Value used | What it means |
|---|---|---|
| Torque (lb-ft) | 300 | Enter the torque (lb-ft) used in this calculation. |
| Engine speed (RPM) | 5,000 | Enter the engine speed (RPM) used in this calculation. |
| Horsepower | 285.6 | |
| Kilowatts | 213 | |
Inputs, definitions and assumptions
Torque (lb-ft)
Enter the torque (lb-ft) used in this calculation. The prefilled worked-example value is 300.
Engine speed (RPM)
Enter the engine speed (RPM) used in this calculation. The prefilled worked-example value is 5,000.
How to use this calculator
- 1Verify the inputs. Gather torque (lb-ft) and engine speed (RPM) from your own documents; the prefilled values are examples.
- 2Save a baseline. The worked example puts the horsepower at 285.6. Store your own version of it as Scenario A.
- 3Test one change. Start with torque (lb-ft), the input with the biggest effect here: moving torque (lb-ft) from 270 to 330 takes the horsepower from 257 to 314.2, a swing of 20% of the worked-example figure.
- 4Check the extremes. At half the example torque (lb-ft) (150) the horsepower is 142.8; at double (600) it is 571.2.
People also ask
Frequently asked questions
How do you calculate horsepower?
Horsepower = torque (lb-ft) × RPM ÷ 5,252; kilowatts = horsepower × 0.7457. At the worked-example inputs the horsepower is 285.6.
What does the horsepower result mean?
Compare an engine's output at a given RPM with the figure quoted on its spec sheet. At the worked-example inputs the horsepower is 285.6. It rises with torque (lb-ft) and engine speed (RPM).
How much does torque (lb-ft) change the horsepower?
Holding every other input at the worked-example value, moving torque (lb-ft) from 240 to 360 moves the horsepower from 228.5 to 342.7, a spread of 114.2.
What are the limits of this horsepower calculator?
These are textbook formulas for ideal conditions: no air resistance, no friction, gravity of 9.81 meters per second squared and gases that behave ideally. Real results differ, so treat them as first estimates and use the units the formulas expect (meters, kilograms, seconds). The tables on this page test torque (lb-ft) only from 240 to 360; a value outside that range is not tabulated here.
Which input moves the horsepower most in the horsepower calculator?
Ranked by how far each moves the horsepower across the range tested: torque (lb-ft) (57.1, 20%) and engine speed (RPM) (57.1, 20%).
If I double torque (lb-ft) in the horsepower calculator, does the horsepower double?
Doubling it from 300 to 600 takes the horsepower from 285.6 to 571.2, which is 2.00 times the worked-example figure. So the result scales almost exactly in proportion. Halving it to 150 gives 142.8.
How much does engine speed (RPM) matter in the horsepower calculator?
The worked example uses 5,000. Holding every other input at its worked-example value, moving engine speed (RPM) from 4,500 to 5,500 takes the horsepower from 257 to 314.2, a swing of 20% of the worked-example figure.
Which inputs change the kilowatts in the horsepower calculator?
At the worked-example inputs it is 213. Torque (lb-ft) takes it from 191.7 to 234.3 and engine speed (RPM) takes it from 191.7 to 234.3.
Why does kinetic energy grow with the square of speed?
Energy of motion is half the mass times the speed squared, so doubling the speed quadruples it. A car at 60 mph carries four times the energy of the same car at 30, which is why stopping distances lengthen so fast.
Sources and evidence
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