Formula
Pounds per horsepower is total weight divided by mechanical horsepower. Horsepower per US ton divides that power by the weight in tons of 2,000 lb. A tonne is 1,000 kg. Watts per kilogram uses kilowatts. One mechanical horsepower is 0.74569987 kW. One PS is 0.73549875 kW. One pound is 0.45359237 kg.
lb/hp = (curb + added) ÷ hp
hp/US ton = hp ÷ (lb ÷ 2,000)
W/kg = kW × 1,000 ÷ kg
hp needed = weight ÷ target lb/hp
rider W/kg = watts ÷ rider kg
A 300 hp, 3,000 lb car is 10.00 lb/hp and 200.0 hp per US ton. The weight is 1,360.78 kg. That is 220.5 hp per tonne, 223.71 kW, and 164.4 W/kg. Acceleration follows this ratio more closely than horsepower alone, until traction, gearing, the torque curve, and aero take over.
Curb weight leaves the driver out. Race weight includes the driver and the fuel aboard. A published net figure under SAE J1349 is crank power, with the accessories the standard requires, corrected to a standard atmosphere. A chassis dyno reads wheel power. A drivetrain often absorbs something like 10 to 20 percent. That range is an illustration. This page does not apply it. The crank-or-wheel choice is a label. Cycling watts per kilogram is the same kind of ratio for a climb. The page does not say what the number means for health.
Worked examples
300 hp, 3,000 lb. 10.00 lb/hp. 200.0 hp per US ton. About 1,360.78 kg, 220.5 hp per tonne, 223.71 kW, 164.4 W/kg.
150 kW, 1,500 kg. 100.0 kW per tonne, which is 100 W/kg. About 201.2 hp, 134.1 hp per tonne, 7.46 kg/hp.
200 hp, 450 lb plus a 180 lb rider. 630 lb, 3.15 lb/hp, 634.9 hp per US ton.
250 W, 70 kg. 3.57 W/kg. A target of 4.0 W/kg at 72 kg is 288 W. A 3,000 lb car at 8.0 lb/hp needs 375 hp.
Reference
The bands are approximate and unnamed. Use them only to see which neighborhood a ratio sits in.
Questions
How do you calculate power-to-weight ratio?
Divide the weight by the horsepower. A 3,000 lb car with 300 hp is 10.00 lb/hp. The same pair is 200.0 hp per US ton. Metric figures use the kilogram and the kilowatt on the same weight and power.
What is a good power-to-weight ratio for a car?
This page does not call a ratio good. The strip shows approximate bands: an economy car is often near 15–25 lb/hp, a sports car near 8–12, a supercar near 4–7, and a superbike with a rider near 2–4. Those are planning ranges, not a test of a model.
What is the difference between lb/hp and hp/ton?
Pounds per horsepower is weight divided by power. Horsepower per ton is power divided by weight in US tons of 2,000 lb. Ten lb/hp is 200 hp per ton. They are the same comparison written two ways.
How do you convert hp per tonne to watts per kilogram?
One mechanical horsepower is 0.74569987 kW, and a tonne is 1,000 kg. Watts per kilogram is kilowatts times 1,000, divided by the kilograms. On the 300 hp, 3,000 lb example that is about 164.4 W/kg.
What is a good watts per kilogram for cycling?
This page does not rank it. It divides watts by kilograms and stops. There is no heart-rate zone, no weight-loss claim, and no training category.
Does adding a driver change the ratio?
Yes. A 450 lb motorcycle at 200 hp is 2.25 lb/hp. Add a 180 lb rider and the total is 630 lb, which is 3.15 lb/hp. Curb weight leaves the driver out. Race weight includes the driver and the fuel you actually carry.
Is it better to cut weight or add power?
A percent off the weight and the same percent onto the power move lb/hp by the same amount. Ten percent less weight on a 10 lb/hp car is 9 lb/hp. Ten percent more power is also 9 lb/hp. Traction, gearing, and aero still decide what the car does with that ratio.
Sources
- NIST Special Publication 811, Guide for the Use of the International System of Units. One pound-mass is 0.45359237 kg. One mechanical horsepower is 0.74569987 kW. One metric horsepower (PS) is 0.73549875 kW.
- SAE J1349, Engine Power Test Code. Cited for the idea of a net crankshaft rating corrected to a standard condition. This page does not apply the correction, and it does not turn a wheel figure into a crank figure.
- The lb/hp bands are approximate planning ranges written for this page. They are not a road test. The quarter-mile calculator estimates ET and trap speed from weight and horsepower. The torque calculator converts torque and RPM.