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Power Calculator (Physics)

Find power as work divided by time.

How it is calculated

Power is the rate at which work is done: P = W / t, where W is work (energy transferred) in joules and t is time in seconds, so the answer is in watts (1 W = 1 J/s). The tool divides the two numbers and shows the result with two decimals followed by W. Time must be greater than zero, otherwise nothing is calculated. The calculation is the average power over the whole interval, not the peak at any instant. Use consistent units: if work is in kilojoules or time in minutes, convert to joules and seconds first.

Worked example

A motor performs 1,500 J of work in 20 s. Power = 1,500 / 20 = 75. The tool displays 75.00 W. That means the motor transfers on average 75 joules every second. If the same work were done in 10 s, the power would double to 150 W, which shows that doing the same work faster needs more power.

Try this example →

Common mistakes

  • Entering time in minutes or hours instead of seconds, which gives a wrong value in W.
  • Typing work in kilojoules or calories and still reading the result as watts.
  • Confusing power with energy: a kilowatt-hour is energy, a watt is power.
  • Dividing by zero or a very small time and expecting a meaningful figure; average power depends on the full time interval.

About this calculator

This calculator applies the physics formula P = W / t (power equals work done divided by time), measured in watts, to show how quickly energy is being transferred or converted rather than just how much total work is done.

Physics and engineering students use it for mechanics problems, while it also underlies practical questions like comparing two machines that do the same job — the one that finishes faster has higher power output even if total work is identical. The calculator also makes clear the distinction between energy/work (a fixed quantity, joules) and power (a rate, watts), a mix-up that's common when people first learn the concept.

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  • Editorial team

    A quick real-world check: lifting a 70 kg person up a 3 m flight of stairs takes about 70 × 9.81 × 3 = 2,060 J of work. Enter that with 10 s and you get roughly 206 W, a plausible figure for a brisk climb. Remember the result is an average over the whole time, so peak power in a sprint or a jump is higher. Tell us which machine or activity you are comparing against.

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