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Power-to-Mass Ratio — kW/kg & hp/lb

Compare power per unit mass and solve for power or mass using metric and customary units.

  • Formula & worked example
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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

Private by default

Inputs and results stay in this browser tab. Bookify does not upload or store the values you enter.

Formula and method

100 kW divided by 1,000 kg is 0.1 kW/kg, or 100 W/kg.

Power-to-mass ratio = power ÷ mass

Worked example

Enter these known values and leave the other values blank.

Power
100 kW
Mass
1000 kg
Power per unit mass
0.1 kW/kg

Assumptions and limitations

  • The weight field represents mass, not gravitational force. Use a positive mass and nonnegative power.
  • Compare power ratings measured on the same basis, such as engine output or wheel output. This ratio alone does not predict acceleration, traction or efficiency.

Common questions

Is this the same as power divided by weight in newtons?

No. This tool divides by mass in kilograms or pounds. A ratio per unit force has different dimensions.

Can I recover the required power?

Yes. Enter a positive mass and target ratio. Power is their product; mass cannot be recovered uniquely when both power and ratio are zero.

References

Bookify allows zero power and zero power-to-mass ratio while retaining positive mass. Mechanical horsepower per pound uses exactly 550 foot-pounds-force per second divided by one international pound, consistently with the power selector. Mechanical horsepower uses exactly 550 foot-pounds-force per second instead of a rounded factor.

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