Car Power from Torque and RPM Calculator

Calculate engine power from torque and engine speed: enter torque and rpm to get horsepower and kilowatts. The result is the power at that particular engine speed, not a prediction of vehicle speed or a substitute for a measured dyno result.

Formula

For torque in pound-feet and rotational speed in revolutions per minute:

Horsepower = torque (lb-ft) × engine speed (rpm) ÷ 5,252

To express the same power in kilowatts, use kW = torque (lb-ft) × rpm × 0.000189985. This factor converts the product of lb-ft and rpm to kilowatts. The calculator’s inputs are t, torque in lb-ft, and r, engine speed in rpm. Its outputs are horsepower (hp) and kilowatts (kW). The constant 5,252 is a rounded conversion factor arising from the definitions of horsepower and rotational work; using it gives the customary automotive formula.

If your torque is given in newton-metres instead, the metric form is kW = torque (N·m) × rpm ÷ 9,548.8. Do not enter N·m into the lb-ft input: 1 lb-ft is approximately 1.35582 N·m. Convert units first, or use the metric form separately.

Worked Example

Using the calculator’s defaults, torque is 200 lb-ft and engine speed is 3,000 rpm.

• Multiply torque by rpm: 200 × 3,000 = 600,000.
• Divide by 5,252: 600,000 ÷ 5,252 = 114.24 hp, rounded to two decimal places.
• Convert to kilowatts: 200 × 3,000 × 0.000189985 = 113.99 kW, rounded to two decimal places.

Thus, 200 lb-ft at 3,000 rpm corresponds to about 114.24 hp, or 113.99 kW. Small discrepancies between the two displayed values can result from rounding the conversion constants.

What the Result Means

Torque describes a turning effect; engine speed describes how quickly the engine turns. Power combines both: the same torque produces more power at higher rpm, and the same rpm produces more power with greater torque. This is why a torque figure on its own does not tell you the engine’s power at every speed.

The calculator reports power at one operating point. An engine’s torque changes across its rev range, so its power changes too. To calculate power at several speeds, enter the torque measured at each corresponding speed. A peak torque number and a peak rpm number taken from different points cannot be combined to find peak power; the two values must describe the same operating point.

Engine power is commonly reported in horsepower and kilowatts. In car specifications, “hp” usually means mechanical horsepower, where 1 hp is about 0.7457 kW. Other horsepower conventions, including metric horsepower, differ slightly. Check which convention a specification uses before comparing close figures.

When to Use It and How to Compare Figures

This calculation is useful when a torque value and matching engine speed are known, for example from a technical specification, data log, or dyno chart. It can help check a listed power figure or convert between customary and metric power units. It does not tell you how quickly a car accelerates: vehicle mass, gearing, traction, aerodynamic drag, and other factors also matter.

• Use torque and rpm from the same point on a graph or measurement.
• Check whether torque is in lb-ft or N·m, and whether power is mechanical hp, metric horsepower, or kW.
• For a dyno comparison, check whether the reported figure is at the engine or wheels. Drivetrain losses mean wheel power and engine power are not interchangeable.
• Compare results measured under comparable conditions and methods; a formula cannot correct for differences in testing or reporting.

Common Mistakes and Limits

A frequent misconception is that an engine’s peak torque multiplied by its peak rpm gives peak horsepower. It does not, unless both peaks occur at the same speed—which is not generally the case. Use the torque at the rpm where you want the power calculation.

Unit errors can create a large mistake. The constant 5,252 applies to torque in lb-ft and speed in rpm. If torque is in N·m, use the metric equation or convert the torque to lb-ft first. Also distinguish rpm from revolutions per second: the formula expects revolutions per minute.

Finally, this is a mathematical conversion, not a measurement. It assumes the input torque is accurate and refers to the same shaft and operating condition as the rpm. It cannot establish whether a quoted torque figure is correct, estimate engine output from a vehicle’s speed, or determine wheel power without additional data.

Frequently Asked Questions

How do I calculate horsepower from torque and rpm?
Multiply torque in lb-ft by rpm, then divide by 5,252. For example, 300 lb-ft at 4,000 rpm is about 228.48 hp.

How do I calculate kilowatts from torque and rpm?
With torque in N·m, multiply it by rpm and divide by 9,548.8. With torque in lb-ft, multiply torque by rpm and by 0.000189985 to get kW.

Why are torque and horsepower equal at 5,252 rpm?
In the customary formula, horsepower equals torque in lb-ft at about 5,252 rpm because the conversion constant is approximately 5,252. This is a unit relationship, not a special engine characteristic.

Can I use peak torque and peak rpm to find peak horsepower?
Only if those figures refer to the same engine speed. Otherwise, use the torque value at the rpm where peak power occurs.

Is this engine horsepower or wheel horsepower?
The calculation gives power corresponding to the torque supplied. Whether that torque represents engine output or wheel output depends on how and where it was measured; the formula cannot determine that distinction.

Related

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in t | Torque | lb-ft | 200 | 0
in r | Engine speed | rpm | 3000 | 0
out hp | Power | hp | t * r / 5252 | 2
out kw | Power | kW | t * r * 0.000189985 | 2