Power Calculator: P = W / t

Find the power, the energy used or the time taken.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Choose which quantity you want to find.

Enter the other values in the units shown.

The calculator gives the answer with the formula, the substituted numbers, and the answer in other units.

Formulas used

  • P = \frac{W}{t}
  • W = P\,t
  • t = \frac{W}{P}

Worked example

With the default numbers in the calculator above:

  1. The formula, arranged to find the power: power = (work or energy) / (time)
  2. Put the values in (every value in its SI unit): power = (5000) / (20)
  3. The answer: power: 250 W
  4. The same answer in other units: 0.25 kW; 0.335256 horsepower; 853.035 BTU/h

Change the numbers in the calculator to see the full working, every step, for your own case.

Common questions

What is the formula for power?

Power equals work divided by time. 5000 J of work in 20 s is 250 W.

How do I convert watts to horsepower?

Divide by 745.7. A 750 W motor is about 1 horsepower.

How do I find the energy used by an appliance?

Multiply the power by the time. A 2 kW heater on for 3 hours uses 6 kWh.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: Any school physics text (power).

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