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Horsepower Calculator

Horsepower from torque and RPM, or from work over time.

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Power across the rev range

What your torque figure produces at each engine speed. Note where the two columns cross.

RPMTorque (lb-ft)Horsepower

Why 5,252?

One horsepower is 33,000 foot-pounds of work per minute. A rotating shaft covers 2π radians per revolution, so 33,000 ÷ 2π = 5,252. At exactly 5,252 RPM the multiplier in the formula is 1, which means torque and horsepower must be numerically equal — and every imperial power curve crosses there, whatever the engine.

Horsepower

—

Every definition

Mechanical (hp)—
Metric (PS)—
Electrical—
Kilowatts—

Torque

lb-ft—
Newton-metres—

—

Where this sits against the crossover point

Horsepower is torque × RPM ÷ 5,252 in imperial units, or kW = Nm × RPM ÷ 9,549 in metric. Torque is the twisting force available; horsepower is how quickly that force can do work. European figures quoted in PS read about 1.4 percent higher than the same engine expressed in mechanical horsepower.

Quick Tool Overview

The Horsepower Calculator works out power from torque and engine speed, or from work done over time, and converts between the three definitions still in everyday use — mechanical, metric and electrical horsepower.

What it does
Calculates horsepower from torque and RPM, or from work
You give it
Torque and engine speed, or work and time
You get back
Horsepower in three definitions, plus kilowatts
Best for
Engine tuning, coursework and comparing quoted figures
Calculators Free, unlimited use No sign-up Runs in your browser

About the Horsepower Calculator

Calculate horsepower from torque and engine speed, or from work done over a period of time, and convert between the three definitions of horsepower that are still in everyday use: mechanical, metric and electrical.

The torque relationship is the one worth understanding, because it explains a great deal about how engines behave. Horsepower is torque multiplied by RPM divided by 5,252 — and that constant is why every torque and power curve, when plotted in imperial units, crosses at exactly 5,252 RPM. Torque is the twisting force available; horsepower is how quickly that force can do work. An engine can make enormous torque and modest power if it will not rev.

Key features of the Horsepower Calculator

Two calculation routes

From torque and RPM, or from work done over a period of time.

Three horsepower definitions

Mechanical, metric (PS) and electrical, which differ by enough to matter when comparing quoted figures.

Explains the 5,252 crossover

Torque and power curves always cross there in imperial units, because that is the constant in the formula.

Power across the rev range

A table showing what your torque figure produces at each engine speed.

Metric and imperial torque

lb-ft and Nm both accepted, with the right constant applied to each.

How to use the Horsepower Calculator

  1. 1

    Choose a method

    From torque and RPM, or from work done over a time period.

  2. 2

    Enter your figures

    Torque in lb-ft or Nm, and engine speed in RPM.

  3. 3

    Read the horsepower

    In mechanical, metric and electrical horsepower, plus kilowatts.

  4. 4

    Check the curve

    The table shows power across the rev range for your torque figure.

Frequently asked questions

Horsepower equals torque in lb-ft multiplied by RPM, divided by 5,252. In metric, kilowatts equal torque in Nm times RPM divided by 9,549.

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