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Power to weight ratio calculator

Enter the power and the weight of the car, truck or motorcycle. See every common way of writing the ratio, in US and metric units, so you can compare vehicles quoted either way.

Power and weight

Power

Enter a power figure above zero.

Vehicle weight

Enter a weight above zero.

Driver, fuel and cargo

Added to the vehicle weight, for example the driver. Enter a weight of 0 or more, or leave it empty.

Hp here is mechanical horsepower (745.7 W). PS is metric horsepower (735.5 W), which European brochures often quote. Use the same kind of power figure (crank or wheel) for every vehicle you compare.

What the ratio tells you

Power to weight ratio compares how much power a vehicle has with how much mass it has to move. Two cars with the same power but different weights will not accelerate the same way, and this number puts them on one scale. It is written two ways round: weight per unit of power (lb/hp, kg/kW, lower is quicker) or power per unit of weight (hp per ton, kW per tonne, higher is quicker).

What the inputs mean

Power: horsepower in US units, kilowatts in metric. Use crank power from the maker, or wheel power from a dyno, but do not mix the two when comparing.
Vehicle weight: curb weight or a weight you measured on a scale.
Driver, fuel and cargo: optional. Add it to compare the vehicle as it is really driven.

The formulas

lb/hp = weight (lb) ÷ power (hp)hp per ton = power (hp) ÷ (weight (lb) ÷ 2,000)kW per tonne = power (kW) ÷ (weight (kg) ÷ 1,000)

The conversions are exact definitions: 1 lb = 0.45359237 kg and 1 hp = 0.7457 kW. A US ton is 2,000 lb; a metric tonne is 1,000 kg, about 2,205 lb.

Worked example

A 3,500 lb car with 300 hp:

3,500 ÷ 300 = 11.67 lb/hp
300 ÷ 1.75 tons = 171.4 hp per ton
3,500 lb is 1,587.6 kg and 300 hp is 223.7 kW, so 1,587.6 ÷ 223.7 = 7.10 kg/kW, or 140.9 kW per tonne (189.0 hp per tonne, 191.6 PS per tonne)

Add a 180 lb driver and the car weighs 3,680 lb, so the ratio drops to 12.27 lb/hp.

In metric: a 1,400 kg car with 150 kW is 1,400 ÷ 150 = 9.33 kg/kW, or 107.1 kW per tonne.

The ratio is a quick way to compare. Actual acceleration also depends on gearing, traction, drag and how the power is delivered across the rev range.

Questions

How do you calculate power to weight ratio?

Divide the weight by the power for lb/hp or kg/kW, or divide the power by the weight for hp per ton or kW per tonne. A 3,500 lb car with 300 hp is 11.67 lb/hp.

Is a lower or higher power to weight ratio better?

For acceleration, you want less weight per unit of power (a lower lb/hp or kg/kW) or more power per unit of weight (a higher hp per ton or kW per tonne). They are the same ratio written the other way round.

How do I convert lb/hp to kg/kW?

Multiply lb/hp by 0.6083 (0.45359 ÷ 0.7457). So 11.67 lb/hp is 7.10 kg/kW.

Should I include the driver's weight?

If you want the ratio as the vehicle is really driven, yes. A 180 lb driver in a 3,500 lb, 300 hp car moves it from 11.67 to 12.27 lb/hp.