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Power to Weight Ratio

Free Power to Weight Ratio calculator with clear step-by-step results.

About Power to Weight Ratio

Power to weight ratio is what actually predicts acceleration, because a given power has to accelerate a given mass. Two things are commonly fudged: the mass should include occupants and cargo rather than the bare kerb figure, and quoted power is measured at the crank, so 10-15% is lost through the gearbox and driveline on a conventional car. Both corrections push the honest ratio below the brochure number.

Formulas

Power at the wheels

Wheel power = engine power × (1 − drivetrain loss)

Ratio

kW/t = wheel power ÷ (total mass ÷ 1000); W/kg = wheel power × 1000 ÷ total mass

Worked example

With the default values below, Power to Weight Ratio returns Power to Weight Ratio of 82.50. Change any input above and every figure here is recalculated in your browser.

Inputs used

Engine or motor powerminimum 0 kW
150 kW
Kerb massminimum 1 kg
1,450 kg
Occupants and cargominimum 0 kg
150 kg
Drivetrain lossaccepts 0–40 %
12 %

Results

Power to Weight Ratio
82.50 kW/t
Specific Power
82.50 W/kg
Power to Weight Ratio
110.63 hp/t
Power at the Wheels
132.00 kW

Step by step

  1. 1Values used
  2. 2Power at the wheels
  3. 3Ratio
  4. 4Power to Weight Ratio= 82.50 kW/t
  5. 5Specific Power= 82.50 W/kg
  6. 6Power to Weight Ratio= 110.63 hp/t
  7. 7Power at the Wheels= 132.00 kW

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Quoted power and mass figures vary by measurement standard and specification. Do not use this to plan driving on public roads beyond legal limits.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Quoted power and mass figures vary by measurement standard and specification. Do not use this to plan driving on public roads beyond legal limits.

Frequently Asked Questions

Which unit should I quote?

kW per tonne is the metric convention and hp per tonne the imperial one; W/kg is the same quantity again and is standard in cycling and aviation. All three are shown so the figure can be compared with whatever source you are reading.

What drivetrain loss should I use?

Roughly 10-12% for a manual rear-wheel drive, 15-18% for an automatic all-wheel drive, and 5-8% for an electric vehicle with a single-speed reduction gear. Set it to zero if your power figure is already measured at the wheels.

Does a higher ratio always mean faster?

It correlates strongly with acceleration but is not the whole story. Traction, gearing, aerodynamic drag and where in the rev range the power arrives all decide the actual time to speed.

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Power to Weight Ratio – Calculate Power to Weight Ratio