Power to Weight Calculator

How to use this calculator

1. Check which power unit the figure is inManufacturers quote PS in Germany and much of Europe, bhp in Britain, and kW in official documentation. PS is about 1.4 per cent smaller than bhp, so a 300 PS car is roughly 296 bhp.
2. Use kerb weight for comparisonsPublished power-to-weight figures almost always use kerb weight — the car with fluids but no occupants. Comparing your laden figure against a magazine's kerb figure is not a like-for-like comparison.
3. Add driver and fuel for the real-world numberA driver and a full tank typically add 120 kg or so, which costs about 8 per cent of the ratio on a 1,500 kg car. That is the figure that describes how the car actually feels.
4. Look at the added-weight lineThe breakdown shows what 100 kg costs and how much power you would need to offset it, which is the clearest illustration of why lightness matters.

How the calculation works

Power divided by mass. The only complication is which units, and Britain uses a mixture that requires conversion more often than not.

Brake horsepower and PS are close but not identical. One PS is about 0.9863 bhp, so a 300 PS engine produces roughly 296 bhp. The gap is small enough to ignore in conversation and large enough to matter when comparing two cars quoted in different units.

Kilowatts are the SI unit and appear on registration documents and official type approval. One bhp is 0.7457 kW, so 300 bhp is 224 kW.

The ratio matters because acceleration is force divided by mass, and force at the wheels comes from power. Two cars with the same power-to-weight ratio accelerate similarly regardless of their absolute size, which is why the ratio is a better predictor of feel than power alone.

It is not the whole story. Torque and where it arrives, gearing, traction, transmission losses and aerodynamic drag all shape real performance. Power-to-weight predicts the first few seconds well and top-end acceleration poorly, because drag rises with the square of speed and eventually dominates.

What the numbers mean

The bands are rough because so much else contributes. A 200 bhp per tonne rear-drive car with poor traction can be slower off the line than a 160 bhp per tonne four-wheel-drive one, and a heavy car with the same ratio will brake and change direction worse even if it accelerates identically.

Worked example

A 300 bhp car with a kerb weight of 1,500 kg.

300 ÷ 1.5 tonnes = 200 bhp per tonne. In metric that is 149 kW per tonne, or 149 watts per kilogram, and 5 kg per bhp.

Add an 85 kg driver and a 44 kg tank of fuel — 129 kg — and the mass becomes 1,629 kg. The ratio falls to 184 bhp per tonne, a drop of 8 per cent from simply getting in and filling up.

To recover that you would need 326 bhp rather than 300. Twenty-six extra horsepower to offset a driver and a tank of petrol.

That asymmetry is the whole argument for lightness. Removing 100 kg from this car raises the ratio to 214 bhp per tonne, which would otherwise require 21 more horsepower. Weight reduction also improves braking, cornering, tyre wear and fuel consumption, none of which more power does. It is the only modification that improves everything at once, and it is why serious track cars are stripped before they are tuned.

Where the ratio stops predicting anything

Power-to-weight describes potential rather than outcome, and several things intervene between the two.

Traction is the first limit. Below about 30 mph most performance cars are traction-limited rather than power-limited, so a four-wheel-drive car with a modest ratio will out-accelerate a rear-drive one with a better ratio from a standing start. That is why 0-60 times correlate less well with power-to-weight than people expect.

Drag is the second. Aerodynamic resistance rises with the square of speed, so power required rises with the cube. That is why doubling power does not double top speed, and why a slippery shape matters far more at 150 mph than at 50.

Gearing determines how much of the available power reaches the road at any given speed, and a car geared for economy will feel slower than its ratio suggests in the gears people actually use.

And the ratio says nothing about how a car changes direction or stops. A 250 bhp per tonne saloon weighing 1,900 kg will accelerate like a 250 bhp per tonne hatchback weighing 1,100 kg and will not corner or brake anything like it — because inertia scales with absolute mass, not with the ratio.

For a genuinely useful comparison, power-to-weight is the starting point rather than the conclusion.

Assumptions and limitations

• Uses the mass you enter — kerb weight and laden weight give very different answers
• Assumes the quoted power figure is accurate; manufacturer figures are measured at the flywheel, not the wheels
• Takes no account of transmission losses, typically 10 to 20 per cent to the wheels
• Predicts nothing about traction, gearing, torque delivery or aerodynamic drag
• Says nothing about braking or cornering, which depend on absolute mass rather than the ratio
• Comparisons are only meaningful when both figures use the same mass basis and power unit

Questions people ask about this in United Kingdom