About the Efficiency Calculator
Efficiency tells you how much of what you put into a machine comes out as something useful. It is the useful output divided by the total input, written as a percentage. The rest is lost, usually as heat. This calculator works with any unit of energy or power, as long as the output and the input use the same one, and it shows the loss as well. Type the useful output and the total input, in the same unit and the efficiency appears at once, with a few related figures underneath, and you can copy the result with one tap. The sums run in your browser, so nothing you enter is uploaded.
Updated October 2026
How to use the Efficiency Calculator
- 1Enter the useful output and the total input, in the same unit.
- 2Pick the units or options from the menus if the efficiency tool offers them.
- 3Read the efficiency in the large result box.
- 4Look at the extra rows for related figures.
- 5Press Copy result to keep the answer.
When you need the efficiency
Engineers compare motors, pumps, heaters and boilers by their efficiency. Home owners use it to understand the label on a furnace, and students use it in energy questions. A small improvement in efficiency saves a lot of fuel over years of use.
Efficiencies multiply when machines work in a chain. A motor that is 90 percent efficient driving a gearbox that is 95 percent efficient gives 85.5 percent overall. This is why every extra step costs something.
How the efficiency is worked out
Efficiency = useful output ÷ total input × 100%. Loss = input − useful output. A machine that takes in 1,000 joules and delivers 750 joules of useful work is 75 percent efficient, and 250 joules, or 25 percent, are lost. A motor taking 1,000 W of electricity and giving 880 W at the shaft would be 88 percent efficient.
Every answer on this page comes from that one rule, so you can check any result by hand with a calculator. The figures shown underneath, such as other units, are conversions of the same answer.
- Any unit
- Shows loss and its share
- Clear error above 100%
Mistakes to avoid with efficiency
The output cannot be more than the input; the tool reports an error if it is, because no machine is more than 100 percent efficient. Make sure that both numbers measure the same thing in the same unit, such as watts in and watts out, and that the output is the useful part only.
On phone and computer
On a phone, type into the boxes one after another and read the efficiency underneath; Copy result puts it on the clipboard for a message or a note. On a computer the inputs sit on the left and the answer on the right, so you can change one figure and watch the efficiency move. The efficiency calculator works the same on both. Convert energy with Energy Converter and power with Power Converter, find hours of work with Work and Power Calculator, and see what losses cost with Electricity Cost Calculator.
Features
- Same unit for both
- Chain efficiencies multiply
- Heat is the usual loss
- Clear messages when the efficiency inputs make no sense
- Copy the result with one tap
- Private: nothing is uploaded
Frequently asked questions
How do I calculate efficiency?
Efficiency = useful output ÷ total input × 100%. Loss = input − useful output. A machine that takes in 1,000 joules and delivers 750 joules of useful work is 75 percent efficient, and 250 joules, or 25 percent, are lost. A motor taking 1,000 W of electricity and giving 880 W at the shaft would be 88 percent efficient.
What is the formula for efficiency?
Efficiency = useful output ÷ total input × 100%. Loss = input − useful output. The efficiency calculator applies it exactly and rounds only the displayed answer to six significant figures.
Can efficiency exceed 100 percent?
Not for a machine on its own. Heat pumps are rated by a coefficient of performance above 1 because they move heat rather than make it.
What is a typical motor efficiency?
Electric motors are often 85 to 95 percent efficient; petrol engines in cars convert roughly 25 to 35 percent of the fuel energy into motion.
Where does the lost energy go?
Mostly into heat, plus some sound and vibration.
Is this efficiency calculator free and private?
Yes. It is free, needs no account and runs in your browser, so the numbers you type are not uploaded or stored.