Example 1
P_out = 100, P_in = 200 → η = 0.5 (50%).
Efficiency Calculator
Type output power P_out [W] and input power P_in [W]. The calculator computes efficiency η = P_out/P_in and also shows the percent. 750 W from 1000 W is 0.75, or 75%. Zero on the input does not divide.
Power from work: P = W/t. Joule loss: P = I²·R.
Result
Useful power P_out (W) and Input power P_in (W). The result shows up here.
How it works
Efficiency is useful power over input power: η = P_out / P_in. At 750 W and 1000 W you get 0.75, or 75%. At 100 W and 200 W you get 0.5. At 8 and 10 you get 0.8. The result is the fraction and the percent side by side.
Both fields take power in the same units: watts with watts, kilowatts with kilowatts. The fraction has no unit. A comma and a period in 750.5 are the same P_out.
P_in must be positive. Zero does not divide. Both fields need a number before 0.75 can appear. If P_out comes out larger than P_in, the calculator still reports η > 1: that flags bad data, not new physics.
Power from work P = W/t is on the neighbouring page when you have joules and seconds instead of two powers. Joule loss P = I² R says how many watts sit in a resistor. Heat Q = c m ΔT is yet another calculator.
At 400 W and 500 W, η = 0.8. At 20 W and 100 W, η = 0.2. Equal powers, for example 50 and 50, give η = 1. At 1 W and 4 W, η = 0.25.
Type 750 and 1000, click Calculate, and match 0.75 plus 75%. Then compare 100 and 200. The header symbol does not convert a tariff or a currency.
How to use
- In the first field enter useful power P_out, for example 750 W.
- In the second field enter input power P_in in the same unit, for example 1000 W.
- Click Calculate. The calculator computes P_out / P_in. 750 and 1000 give 0.75, with 75% beside it.
- P_in must be positive. Zero does not divide. Both fields need a number.
- Have work and time instead of two powers? Open P = W/t. Here it stays a ratio.
Formula
η = Pout / Pin
Often also reported as η·100%. P_in > 0.
η, P_out, and P_in
Efficiency is a ratio of two powers: η = P_out / P_in. 750 W from 1000 W is 0.75 (75%). 100 W from 200 W is 0.5.
- η
- A unitless fraction. 750/1000 = 0.75. Beside it the result is η·100%, which is 75%.
- P_out
- Useful power [W], first field. 750 W or 100 W. Larger than P_in yields η > 1: bad data.
- P_in
- Input power [W]. 1000 W. Must be positive, because you divide by it.
Real-life examples
Example 2
P_out = 750 W, P_in = 1000 W.
Example 3
P_out = 8, P_in = 10.
Example 4
P_out = 400, P_in = 500.
Example 5
η = 0.2 (20%).
Example 6
η = 0.9 (90%).
Example 7
η = 1 (100%).
Example 8
η = 0.25.
Example 9
P_out = 5000, P_in = 8000.
Example 10
η = 1.5: result with a warning.
Ways to use this calculator
- You compute 0.75 from 750 W out and 1000 W in.
- You compare a 50% motor (100/200) with 80% (8/10).
Frequently asked questions
What is η at 750 W and 1000 W?
Efficiency is 0.75. Beside the fraction the result is 75%, because η·100% is the same ratio written as a percent.
What about 100 W and 200 W?
You get 0.5, or 50%. At 8 W from 10 W the result is 0.8, or 80%.
Which units do I type?
Both powers in the same units: P_out [W] and P_in [W], or both in kW. Result η is unitless, plus a percent.
What if P_in = 0?
Zero does not divide. Input power must be positive, and both fields need a number before η appears.
Does the calculator show a percent?
Yes. Beside the fraction sits η·100%. 0.75 is 75%, 0.5 is 50%.
How is this different from P = W/t?
The calculator divides two powers, output by input. There you compute one power from work and time, with no efficiency.
Is η > 1 a data error?
The calculator will still divide. In physics η greater than 1 means you swapped input and output, not that you built a new machine.
Does a comma in 750,5 work?
Yes. 750.5 and 750,5 are the same output power. A comma and a period mean the same value.
What about 400 W and 500 W?
Efficiency is 0.8. At 20 and 100 you get 0.2, and at 50 and 50 you get 1, or 100%.
Where is heat Q = c m ΔT?
On the heat page. Here there is only the ratio of two powers, with no temperature and no mass.
Knowledge sources
The formula is the school one. Units follow SI; NIST SP 330 and BIPM define the measures, not your result.
Page updated in 2026.