Power-to-Weight Ratio Calculator
Power-to-weight in W/kg, hp per tonne and lb per hp for a cyclist, runner, car or motorbike, or the power you need to reach a target ratio. Metric and imperial.
Result…
How it's calculated
W/kg = power (W) ÷ mass (kg) hp per tonne = power (hp) ÷ (mass (kg) ÷ 1000) lb per hp = mass (lb) ÷ power (hp) 1 hp = 745.69987 W; 1 PS = 735.49875 W; 1 lb = 0.45359237 kg Power for a target = target W/kg × mass
Power-to-weight ratio is the power something can produce divided by the mass it has to move. It is the number that decides how fast a cyclist climbs and how quickly a car or motorbike accelerates, more than power alone. Cyclists quote watts per kilogram of body weight, often using their functional threshold power. Car makers and magazines quote horsepower per tonne, and in the US the inverse, pounds per horsepower, where a smaller number is quicker. Horsepower has more than one definition: this calculator uses mechanical horsepower (550 foot-pounds per second) and also gives metric horsepower (PS), so check which one a spec sheet means.
Your numbers, step by step
About the Power-to-Weight Ratio calculator
This power-to-weight calculator divides power by mass and gives the answer every way people quote it: watts per kilogram, watts per pound, horsepower per tonne, metric horsepower (PS) per tonne, pounds per horsepower and kilograms per horsepower. Cyclists use it to turn an FTP into W/kg; drivers and riders use it to compare cars and motorbikes on more than their engine output. It also works backwards to the power you need to reach a target ratio at your weight. Power can be in watts, kilowatts, mechanical horsepower or PS, and weight in kg or lb. Horsepower here is mechanical horsepower (550 ft·lbf/s, 745.69987 W) and PS is 735.49875 W, the factors in the NIST Guide to the SI. Every conversion is shown in the working.
How to use it
- Choose what to “Find”: the ratio from power and weight, or the power for a target ratio.
- Enter the “Power” and pick the “Power unit”: W, kW, hp (mechanical) or PS (metric hp).
- Enter the “Weight (mass)” in kg or lb. For a rider that is body weight; for a vehicle, add the driver and load if you want the real-world figure.
- For a target, enter the “Target ratio” in W/kg, hp per tonne or lb per hp.
- Read W/kg, hp per tonne and lb per hp, and use the chart to see how the ratio changes if the weight goes up or down.
Worked example
A 1500 kg car with 300 hp: 300 × 745.69987 = 223,710 W, so 223,710 ÷ 1500 = 149.14 W/kg. That is 300 ÷ 1.5 tonnes = 200 hp per tonne (202.77 PS per tonne), or 3306.9 lb ÷ 300 hp = 11.02 lb per hp. For a cyclist weighing 165 lb (74.84 kg) who wants 4 W/kg, the power needed is 4 × 74.84 = 299.4 W.
Updated
Questions people ask
How do I calculate power-to-weight ratio?
Divide the power by the mass. In watts and kilograms, 250 W for a 70 kg rider is 250 ÷ 70 = 3.57 W/kg. For cars, divide horsepower by the mass in tonnes to get hp per tonne.
What is the difference between hp per tonne and lb per hp?
They are the same idea turned upside down. hp per tonne is power over mass, so higher is quicker. Pounds per horsepower is mass over power, so lower is quicker: 200 hp per tonne is about 11 lb per hp.
Which horsepower does this use?
hp is mechanical horsepower, 550 foot-pounds per second or 745.69987 W. PS (metric horsepower, also written CV or ch) is 735.49875 W, about 1.4% smaller, so a car rated 300 PS has about 296 hp. Check which one a spec sheet uses.
Should cyclists include the bike in the weight?
W/kg is usually quoted on body weight alone, which is what race data and training plans use. On a climb the bike and kit are lifted too, so enter rider plus bike to see the ratio the hill actually feels. To set training intensities from your FTP, use the FTP zones calculator.
Why does a car with less power sometimes feel quicker?
Acceleration depends on force over mass, so a lighter car with less power can match a heavier one with more. Gearing, traction, aerodynamics and the power curve matter too, so the ratio is a guide rather than a lap time. The horsepower and torque calculator links power to torque and rpm.
How accurate is a power-to-weight figure?
The arithmetic is exact, but it is only as good as the numbers you enter. Makers quote engine power at the crankshaft, and power at the wheels is lower. Two power meters on the same ride can read slightly differently.