# What is the power to weight ratio?

Finds the power-to-weight ratio of a car, motorcycle, bike or rider from power and weight, in W/kg, hp per lb, hp per ton and lb per hp, or the power or weight from the ratio.

- Page: https://www.acalculator.org/physics/power-to-weight-ratio-calculator
- JSON spec: https://www.acalculator.org/physics/power-to-weight-ratio-calculator.json
- Version: b9aec05fdd40

## Default answer

Example with the default inputs (Power 300 hp, Weight 3,500 lb): The power to weight ratio is 140.91 W/kg, or 11.667 lb per hp.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| p | Power | The engine, motor or rider power output. |
| w | Weight | The weight (mass) of the vehicle or person, with the driver or rider if you want them counted. |
| r | Power to weight ratio (W/kg) | Power divided by mass, in watts per kilogram. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| power | Power | The engine, motor or rider power output. |
| mass | Weight | The weight (mass) of the vehicle or person, with the driver or rider if you want them counted. |
| ratio | Power to weight ratio (W/kg) | Watts per kilogram. |
| hpPerTon | hp per ton | Horsepower per US short ton (2,000 lb). |
| hpPerLb | hp per lb | Horsepower per pound. |
| lbPerHp | Weight to power (lb per hp) | Pounds per horsepower, the inverse ratio; lower is quicker. |
| kgPerKw | Weight to power (kg per kW) | Kilograms per kilowatt, the inverse ratio in metric units. |

## Method

ratio = power ÷ mass in W/kg; power = ratio × mass; mass = power ÷ ratio. 1 hp = 550 ft·lbf/s = 745.69987 W, 1 lb = 0.45359237 kg, 1 short ton = 2,000 lb.

## Assumptions

- Weight means mass: the ratio divides power by mass, as is usual for cars, bikes and cyclists.
- Horsepower (hp) is mechanical horsepower, 550 ft·lbf/s; metric horsepower (PS) is 75 kgf·m/s = 735.49875 W.
- Values run in watts and kilograms, in double precision.

## Worked examples

1. p = 100,000, w = 1,000 gives r = 100, kgPerKw = 10, hpPerLb = 0.060828, lbPerHp = 16.439868. Source: OpenStax, University Physics Volume 1, §7.4 Power (power is the rate of doing work, in watts), https://openstax.org/books/university-physics-volume-1/pages/7-4-power (retrieved 2026-10-02); NIST Special Publication 811, Appendix B (1 hp = 550 ft·lbf/s = 745.699 872 W; 1 lb = 0.453 592 37 kg), https://www.nist.gov/pml/special-publication-811 (retrieved 2026-10-02).
2. p = 223,709.961475, w = 1,587.573295 gives lbPerHp = 11.666667, hpPerTon = 171.428571, r = 140.913155. Source: NIST Special Publication 811, Appendix B (1 hp = 550 ft·lbf/s = 745.699 872 W; 1 lb = 0.453 592 37 kg), https://www.nist.gov/pml/special-publication-811 (retrieved 2026-10-02).
3. r = 4, w = 70 gives p = 280. Source: OpenStax, University Physics Volume 1, §7.4 Power (power is the rate of doing work, in watts), https://openstax.org/books/university-physics-volume-1/pages/7-4-power (retrieved 2026-10-02).

## FAQ

### How do I calculate the power to weight ratio?

Divide the power by the weight (mass): ratio = power ÷ weight. A 100 kW car that weighs 1,000 kg has 100,000 W ÷ 1,000 kg = 100 W/kg.

### What is the power to weight ratio of a 300 hp, 3,500 lb car?

300 hp ÷ 3,500 lb = 0.0857 hp per lb, or 171.4 hp per ton (3,500 lb is 1.75 short tons). Turned the other way, 3,500 ÷ 300 = 11.67 lb per hp. In metric units it is 140.9 W/kg.

### Is a higher or lower number better?

A higher power to weight ratio (more W/kg or hp per ton) means more power for each unit of weight, so quicker acceleration and climbing, other things equal. For the inverse, weight to power (lb per hp), lower is quicker.

### How is W/kg used in cycling?

Cyclists divide the power they can hold, in watts, by their body weight in kilograms. A 70 kg rider who holds 280 W rides at 4 W/kg. Type 4 as the ratio and 70 kg as the weight to get 280 W.

### Which horsepower does the page use?

Horsepower (hp) is mechanical horsepower: 550 foot-pounds per second, or 745.69987 W (NIST SP 811). OpenStax rounds it to 746 W. Metric horsepower (PS) is 735.49875 W and can be picked as the power unit.

### Should I include the driver?

Either way works if you compare like with like. Makers often quote curb weight without a driver; adding the driver gives a ratio closer to what you feel on the road.

## Sources

- OpenStax, University Physics Volume 1, §7.4 Power (power is the rate of doing work, P = dW/dt, in watts; 1 hp = 746 W). https://openstax.org/books/university-physics-volume-1/pages/7-4-power (retrieved 2026-10-02)
- NIST Special Publication 811, Guide for the Use of the International System of Units, Appendix B (horsepower, 550 ft·lbf/s = 745.699 872 W; 1 lb = 0.453 592 37 kg). https://www.nist.gov/pml/special-publication-811 (retrieved 2026-10-02)