How the ratio is worked out
Power is converted to horsepower and kilowatts, and weight to pounds and kilograms. One mechanical horsepower is 0.7457 kilowatts and one kilogram is 2.2046 pounds, so the units stay consistent whichever you typed.
Pounds per horsepower = weight in pounds divided by horsepower, so a lower figure means more power for the weight. Horsepower per tonne = horsepower divided by weight in tonnes, where a tonne is 1,000 kilograms.
Worked example
A car with 450 hp and a weight of 3,800 lb has 8.44 lb per horsepower. That is 261.1 hp per tonne, 194.7 kW per tonne and 335.6 kW, with the same weight equal to 1,723.7 kg.
These numbers describe the ratio only. Two cars with the same figure can feel very different because of gearing, traction, torque delivery and weight distribution.
Which weight to use
Curb weight excludes passengers and cargo and usually includes a full tank. Adding a driver and passengers lowers the ratio, which is why a manufacturer's figure and a real-world figure can differ by several per cent.
Use the same definition for both vehicles whenever you compare them. Curb weight, gross vehicle weight rating and a drag-racing race weight are three different measurements and should not be mixed.
Horsepower and kilowatt confusion
SAE net horsepower is measured with the engine installed and all accessories driven, which is the figure manufacturers publish for US cars. An older gross rating measured without accessories will look much higher for the same engine.
Electric motors are usually rated in kW, and their available power can be limited by battery state. Treat a quoted peak figure as a peak, not as a constant output.
Limits of this calculator
It reports a ratio, not acceleration, quarter-mile time or top speed. It does not model gearing, traction, aerodynamic drag, driver weight or the difference between peak and average power.
For gearing and the speed at a given engine speed, use the gear ratio calculator. For the effect of a wheel change on the speedometer, use the wheel size calculator.
To turn engine speed into road speed, use the US gear ratio calculator.
For kilograms and horsepower per tonne as the headline figure, use the UK power to weight ratio calculator.
Inputs for the US power to weight ratio calculator
Engine power
Use the published SAE net figure, or a dyno result at the crank rather than at the wheels.
Power unit
Choose horsepower or kilowatts to match your figure. The result shows both units.
Vehicle weight
Use the weight in pounds, including fuel and driver if you want a real-world figure.
Weight unit
Choose pounds or kilograms to match your figure. The result converts to the other unit.
CALCULATOR QUESTIONS
Power to weight ratio calculator US questions
Is pounds per horsepower better than horsepower per tonne?
They describe the same thing. US comparisons usually quote lb/hp, where a lower number is better.
Should I use wheel horsepower?
Only if you compare it with another wheel figure. A wheel figure is lower than a crank figure by the drivetrain losses.
Why does my result differ from the spec sheet?
Published curb weight usually excludes passengers. Adding a driver and fuel lowers the ratio.
Does a lower lb/hp always mean a faster car?
No. Gearing, traction and aerodynamics matter as much, especially at higher speeds.