Weight to Power Ratio Calculator
How much weight each horsepower has to move. Lower is quicker — the inverse of power-to-weight, and the way most racers quote it.
Weight-to-power and power-to-weight describe the same physics upside down. Power-to-weight rises as a car gets faster; weight-to-power falls. Racers tend to use the second because the number stays small and readable — 8 lb/hp is easier to picture than 0.125 hp/lb.
Which Way Round Should You Use?
Both are correct; the convention depends on the discipline.
- lb/hp or kg/hp — drag racing and street performance. Lower is quicker. Most bracket-racing rules of thumb are built on this figure.
- hp/ton or bhp/tonne — road-car journalism, especially in the UK and Europe. Higher is better.
- kW/kg or W/kg — motorsport engineering and cycling. Higher is better.
The mistake to avoid is comparing figures without checking the direction. A car at 8 lb/hp and a car at 250 hp/ton may be identical machines described two ways. Our power-to-weight ratio calculator handles the other orientation, and horsepower per ton covers the journalism convention.
Benchmark Weight-to-Power Figures
Approximate figures using kerb weight and quoted crank power. Add driver weight and fuel for a realistic race-day number.
| Category | lb/hp | kg/hp | Feel |
|---|---|---|---|
| Economy hatchback | 25–30 | 11–14 | Adequate |
| Family saloon | 15–20 | 7–9 | Comfortable |
| Hot hatch | 10–13 | 4.5–6 | Quick |
| Sports car | 7–9 | 3.2–4.1 | Fast |
| Supercar | 4–6 | 1.8–2.7 | Very fast |
| Superbike | 2–3 | 0.9–1.4 | Extreme |
| Top Fuel dragster | 0.05 | 0.02 | Another category entirely |
What Weight-to-Power Actually Predicts
It is the single best simple predictor of straight-line acceleration, because it captures the physics directly: force divided by mass gives acceleration. Two cars at the same lb/hp will run broadly similar quarter-mile times regardless of how they got there.
What it does not capture is everything else that decides a real race. Traction determines whether the power reaches the road at all. Gearing decides whether the engine stays in its powerband. Aerodynamics dominate above roughly 100 mph. And torque delivery shapes how the car feels even when the peak figures match.
This is why a 400 HP rear-drive car on street tyres can lose to a 350 HP all-wheel-drive car despite a better ratio on paper — the second one converts more of its power into forward motion off the line.
Use the Right Weight
Manufacturer kerb weight typically excludes driver and sometimes fuel. For a figure that matches what you experience, add roughly 180 lb for a driver and the weight of fuel actually in the tank — petrol runs about 6.1 lb per US gallon.
A 3,200 lb car at 400 HP shows 8.0 lb/hp on paper, but with a driver and a half tank it is closer to 8.6 — a 7% difference that matters when comparing against a published figure. Race classes that police weight-to-power always specify weight with driver aboard for exactly this reason.
Related Calculators
To estimate quarter-mile performance from these figures, use our quarter mile calculator or 0-60 time calculator. For the torque equivalent, see torque to weight ratio.
Frequently Asked Questions
Under 10 lb/hp feels genuinely quick on a road car. Sports cars sit at 7–9, supercars 4–6, superbikes 2–3. Lower is better.
Divide weight by horsepower. A 3,200 lb car with 400 HP gives 8.0 lb/hp.
They are inverses. Weight-to-power (lower is better) is weight ÷ power; power-to-weight (higher is better) is power ÷ weight. Check the direction before comparing.
Multiply lb/hp by 0.4536. A car at 8 lb/hp is 3.63 kg/hp.
It is the strongest simple predictor, since acceleration is force ÷ mass. But it ignores traction, gearing and aerodynamics, so identical ratios can run different times.
Use weight with driver and fuel. Kerb weight often excludes both — adding a 180 lb driver to a 3,200 lb car worsens the ratio by around 6%.
Because traction and drivetrain layout decide how much power reaches the road. An AWD car with less power often out-launches a RWD car with a better ratio.