What this calculator does
Power-to-weight ratio is how much power is available for every unit of mass that power has to move. It is the single number that best predicts acceleration and hill-climbing ability, because a heavier vehicle or body needs proportionally more power just to match the acceleration of a lighter one producing the same output.
This calculator converts power and weight from whichever units they are given in, horsepower, kilowatts or watts, and kilograms, pounds or metric tonnes, and reports the ratio both in watts per kilogram and in horsepower per tonne, since different fields default to different conventions.
The formula
Power-to-weight is power divided by weight, once both are converted to a common unit system. Watts per kilogram is the SI form used in cycling and physiology; horsepower per tonne is the traditional form used for cars and motorcycles. The calculator converts internally so either input unit works.
| Term | Meaning |
|---|---|
| Power-to-weight | Power divided by weight: W/kg or hp per metric tonne. |
| Horsepower (hp) | A traditional power unit equal to 745.7 watts. |
| Metric tonne | 1,000 kilograms. |
The inputs explained
| Field | What to enter |
|---|---|
| Power | The power output, in whichever unit the source quotes it. |
| Power unit | The unit that power figure is in. |
| Weight | The total weight being moved, in whichever unit is available. |
| Weight unit | The unit that weight figure is in. |
When to use it
Comparing cars or motorcycles
A lighter car with less power can out-accelerate a heavier car with more power. Power-to-weight, expressed as hp per tonne, is the figure that actually predicts which one is quicker off the line.
Rating an engine for its application
Aircraft, generators and industrial engines are often specified by power-to-weight, since a lighter engine for the same output leaves more payload capacity elsewhere.
Benchmarking athletic performance
In cycling and rowing, power-to-weight in watts per kilogram is used to compare athletes of different body sizes on a level footing, since raw power alone favours larger athletes.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How power-to-weight changes as power increases at a fixed vehicle weight
A fixed 1,200 kg weight, across a range of power outputs.
| Power (hp) | Power-to-weight | In horsepower per tonne |
|---|---|---|
| 100 hp | 62.14 W/kg | 83.3 hp/t |
| 150 hp | 93.21 W/kg | 125.0 hp/t |
| 200 hp | 124.28 W/kg | 166.7 hp/t |
| 300 hp | 186.42 W/kg | 250.0 hp/t |
| 400 hp | 248.57 W/kg | 333.3 hp/t |
| 600 hp | 372.85 W/kg | 500.0 hp/t |
How power-to-weight changes as weight increases at a fixed power
A fixed 200 hp output, across a range of vehicle weights.
| Weight (kg) | Power-to-weight | In horsepower per tonne |
|---|---|---|
| 800 kg | 186.42 W/kg | 250.0 hp/t |
| 1000 kg | 149.14 W/kg | 200.0 hp/t |
| 1200 kg | 124.28 W/kg | 166.7 hp/t |
| 1500 kg | 99.43 W/kg | 133.3 hp/t |
| 1800 kg | 82.86 W/kg | 111.1 hp/t |
| 2200 kg | 67.79 W/kg | 90.9 hp/t |
Questions
What is a good power-to-weight ratio for a car?
Ordinary road cars typically sit around 60 to 100 hp per tonne. Performance cars reach 150 to 250 hp per tonne, and dedicated track or race cars can exceed 400 hp per tonne. Context matters more than any single benchmark.
Why do cyclists use watts per kilogram instead of hp per tonne?
Because the numbers involved, a few hundred watts and a body weight in tens of kilograms, land in a convenient range in that unit. Hp per tonne is simply a different scale of the same ratio.
Does power-to-weight account for aerodynamics or friction?
No, it is a simple ratio of the two quantities. Real-world acceleration and top speed are also shaped by drag, rolling resistance, gearing and traction, which this figure does not capture.
Is a higher power-to-weight ratio always better?
For acceleration and hill-climbing, yes, more power per unit of weight is an advantage. It says nothing about handling, efficiency or comfort, which depend on other design choices entirely.
For a cycling-specific version with training categories, see the cycling power-to-weight calculator.