How the ratio is worked out
Power is converted to horsepower and kilowatts, and weight to kilograms and pounds. One mechanical horsepower is 0.7457 kilowatts and one pound is 0.4536 kilograms, so the two units stay consistent whichever you typed.
Horsepower per tonne = horsepower divided by weight in tonnes, where a tonne is 1,000 kilograms. Pounds per horsepower = weight in pounds divided by horsepower, so a lower figure means more power for the weight.
Worked example
A car with 200 hp and a kerb weight of 1,500 kg has 133.3 hp per tonne and 99.4 kW per tonne. The same figures are 3,306.9 lb and 16.53 lb per horsepower.
These numbers describe the ratio only. Two cars with the same figure can feel very different because of gearing, traction, torque delivery and mass distribution.
Which weight to use
Kerb weight excludes passengers, luggage and often most of the fuel. Adding a driver and fuel 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 cars whenever you compare them. A quoted dry weight, a kerb weight and a gross vehicle weight are three different measurements and should not be mixed.
Horsepower and kilowatt confusion
A metric horsepower (PS) is 0.7355 kW, slightly smaller than the mechanical horsepower of 0.7457 kW used here. A figure quoted in PS will therefore come out a little higher than the same engine quoted in hp.
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, top speed or lap time. 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 UK gear ratio calculator.
For pounds per horsepower as the headline figure, use the US power to weight ratio calculator.
Inputs for the power to weight ratio calculator
Engine power
Use the figure from the handbook, a dyno run or the registration document, at the crank rather than at the wheels.
Power unit
Choose horsepower or kilowatts to match your figure. The result shows both units.
Kerb weight
Use the kerb weight in kilograms, or add fuel and driver if you want a real-world figure.
Weight unit
Choose kilograms or pounds to match your figure. The result converts to the other unit.
CALCULATOR QUESTIONS
Power to weight ratio calculator UK questions
Is horsepower per tonne the same as bhp per tonne?
Yes in ordinary use. Brake horsepower is measured at the crank, which is the figure this calculator expects.
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 transmission losses.
Why does my result differ from the handbook?
The handbook figure usually excludes fuel and driver. Adding those weights lowers the ratio.
Does a higher ratio always mean a faster car?
No. Gearing, traction and aerodynamics matter as much, especially above about 100 mph.