Horsepower Calculator

Estimate engine horsepower and kilowatts from torque and RPM, or from quarter-mile trap speed and vehicle weight.

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Method

Enter engine details Enter torque and RPM to estimate horsepower.

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Horsepower calculator: torque, RPM, and quarter-mile estimation

A horsepower calculator converts between torque and power or estimates engine output from quarter-mile performance figures. Power is the rate of doing work — mathematically it is torque multiplied by rotational speed, scaled by a unit-conversion constant.

Torque and RPM method

The relationship between torque, RPM, and horsepower is a direct consequence of the physics of rotating systems. One horsepower equals 550 foot-pounds per second, and that constant (along with 2π for circular motion) produces the familiar conversion factor of 5252.

Dyno figures are quoted at a specific RPM because both torque and power vary across the rev range. Peak torque typically occurs at a lower RPM than peak power, which is why the power band and the torque curve are separate lines on a dyno chart.

HP = (Torque lb·ft × RPM) / 5252

Standard SAE horsepower formula. The constant 5252 equals 33,000/2π.

kW = HP × 0.7457

Convert mechanical horsepower to kilowatts.

Quarter-mile estimation (Hale's formula)

When dyno figures are not available, a vehicle's trap speed at the end of a quarter-mile run can be used to estimate power. Hale's formula relates trap speed and vehicle weight to horsepower through an empirical cubic relationship.

This method is an approximation. It assumes typical drivetrain efficiency and tyre grip. Lightweight vehicles with high grip tyres or vehicles with significant wheel-to-crank drivetrain losses may produce estimates that differ noticeably from actual dyno readings.

HP ≈ Vehicle weight (lbs) × (Trap speed (mph) / 234)³

Hale's approximation formula for quarter-mile power estimation.

Frequently asked questions

Why does horsepower cross torque at 5252 RPM?

The formula HP = (Torque × RPM) / 5252 means that when RPM = 5252, HP numerically equals torque in lb-ft. Below 5252 RPM torque is higher in absolute value; above it, horsepower is higher. This is a mathematical consequence of the unit conversion constants, not a physical phenomenon.

 What is the difference between brake horsepower and wheel horsepower?

Brake horsepower (BHP) is measured at the engine crankshaft. Wheel horsepower (WHP) is measured at the driven wheels and is lower due to drivetrain losses from the gearbox, differential, and axles — typically 10–20% for rear-wheel-drive cars.

 Is the quarter-mile formula accurate?

It is a useful estimate, not a precision measurement. Variables including tyre grip, launch technique, ambient conditions, and drivetrain efficiency all affect trap speed. For accurate power figures, a rolling-road dynamometer test is needed.

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