Power to Weight Ratio Calculator (hp/tonne)
How do you calculate power to weight ratio? Enter engine power and the vehicle weight to see how much power each tonne carries, or how many kilograms each horsepower has to move. Add the driver weight to see the figure as you actually drive it. It is the ratio behind quick acceleration; to turn it into a quarter-mile estimate, use the quarter mile calculator.
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Formula
hp per tonne = hp / (weight in kg / 1000)
kg per hp = weight in kg / hp
kW per tonne = kW / (weight in kg / 1000)
lb per hp = weight in lb / hp
"With driver" repeats the same formulas using vehicle weight plus driver weight. Constants: 1 hp = 745.69987 W, 1 PS = 735.49875 W, 1 lb = 0.45359237 kg. A tonne is 1000 kg.
Worked example
300 hp, 1500 kg, 80 kg driver:
- Without driver: 300 / 1.5 = 200 hp per tonne; 1500 / 300 = 5 kg per hp; 300 hp = 223.7 kW, so 149.1 kW per tonne; 3306.9 lb / 300 = 11.02 lb per hp
- With driver (1580 kg): 300 / 1.58 = 189.9 hp per tonne; 1580 / 300 = 5.27 kg per hp
Use this when
You are comparing cars or bikes of different sizes. Raw horsepower favours big, heavy cars; the ratio shows which one will feel quicker. A light 150 hp car can out-accelerate a heavier 200 hp car.
Typical values
As typical bands: small and family cars roughly 70 to 120 hp per tonne, performance cars roughly 150 to 250, supercars 300 or more. Motorbikes often exceed 400 hp per tonne.
How it differs from nearby pages
The horsepower calculator tells you the power; this page divides it by mass. The quarter mile calculator goes one step further and turns weight per horsepower into an estimated time and trap speed.
Common mistakes
- Using dry weight when the car you drive is heavier with fuel, fluids and passengers.
- Mixing hp, PS and kW. 220 kW is 295.0 hp.
- Confusing tonnes with 2000 lb short tons; a tonne here is 1000 kg.
- Comparing crank power on one car with wheel power on another.
Frequently asked questions
How do you calculate power to weight ratio?
Divide power by weight. A 300 hp car weighing 1500 kg has 300 / 1.5 = 200 hp per tonne, or 1500 / 300 = 5 kg per hp.
What counts as a tonne here?
A metric tonne of 1000 kg, not the 2000 lb short ton or the 2240 lb long ton. Pounds you enter are converted at 0.45359237 kg per lb, so 3000 lb is 1360.8 kg.
Why include the driver?
Manufacturers quote kerb or dry weight, but the car moves with you in it. Adding an 80 kg driver to the 1500 kg example lowers 200 hp per tonne to 189.9 and raises 5 kg per hp to 5.27.
Are hp, PS and kW the same?
No. Mechanical horsepower is 745.7 W, metric horsepower (PS) is 735.5 W and 1 kW is 1000 W, so 220 kW is 295.0 hp. The result also uses the quoted crank power; power at the wheels is lower.
What is a good power to weight ratio for a car?
As typical bands, roughly 70 to 120 hp per tonne is ordinary for small and family cars, 150 to 250 is warm to fast, and 300 or more is supercar territory. A 110 hp, 1200 kg car with an 80 kg driver is 85.9 hp per tonne.
How do I convert kg per hp to hp per tonne?
Divide 1000 by the kg per hp figure. A car at 5 kg per hp is 1000 / 5 = 200 hp per tonne, and 2.96 kg per hp is 337.8 hp per tonne. They are the same number written in opposite directions.
What is the power to weight ratio of a 200 hp 2.0L car?
A 200 hp car weighing 1300 kg has 153.8 hp per tonne (114.7 kW per tonne) or 6.5 kg per hp. Engine size alone does not set it: a 500 hp, 1400 kg car with an 80 kg driver reaches 337.8 hp per tonne.