Efficiency Calculator
Enter a useful output and a total input and get the efficiency as a percentage — the single number that tells you how much of your input does useful work and how much is wasted.
Output and input in, percentage out
Enter the useful output and the total input in the same unit and the calculator returns the efficiency, (output ÷ input) × 100, as a percentage.
Stay below 100 %
Useful output can never exceed total input — a result above 100 % means a typo or two different units.
What is an efficiency calculator?
Output over input, one clear number
An efficiency calculator turns two measurements — how much useful work or energy you get out (the output) and how much you put in (the input) — into a single number: the efficiency, the share of the input that becomes useful output. Efficiency is what separates a wasteful machine from a frugal one, and it explains why an LED bulb costs so much less to run than an incandescent one. That makes it the go-to figure for comparing engines, motors, and appliances on one scale, no matter how differently they work.
Enter the useful output and the total input in the same unit to get the efficiency as a percentage instantly.
One short formula: divide the useful output by the total input, then multiply by 100.
η = (output ÷ input) × 100The Greek letter η (eta) is the standard symbol for efficiency. Because output and input are in the same unit, they cancel and the result is a pure ratio; multiplying by 100 turns that ratio into a percentage. An efficiency of 75 % means three quarters of the input does useful work and the remaining quarter is lost.
Suppose a machine delivers 750 joules of useful output from 1000 joules of input.
Note the output and input
Useful output is 750 J and the total input is 1000 J — keep both in the same unit so they cancel cleanly.
Divide output by input, then × 100
(750 ÷ 1000) × 100 = 75 — the share of the input that becomes useful work.
Read the efficiency
75 % — three quarters of every joule put in comes back out as useful output.
The single efficiency figure tells a surprisingly rich story. The key insight is that it is bounded: a real machine always lands below 100 %, because some of the input is unavoidably lost to heat, friction, or sound rather than useful work. The gap between your result and 100 % is exactly that wasted share — at 75 %, a quarter of the input is lost. A higher percentage means less waste, so the number lets you compare wildly different devices on one scale: a car engine turns only about 20–35 % of its fuel energy into motion, an incandescent bulb wastes most of its electricity as heat at roughly 5 %, an LED reaches around 80–90 %, and a large electric motor can exceed 95 %. If a calculation ever returns more than 100 %, treat it as a red flag rather than a triumph — output cannot exceed input without breaking the conservation of energy, so check that you have not mistyped a value or mixed two different units. Read your result against these everyday anchors to judge whether a machine is wasteful or genuinely frugal.
The formula is exact, but a couple of practical points are worth keeping in mind.
Same units, useful output only, and the 100 % ceiling
The result is only meaningful if the useful output and the total input are measured in the same unit — joules and joules, watts and watts — never a mix. "Useful" output means the part that does the job you care about; energy lost as waste heat is not useful even though it still leaves the machine. And because output can never exceed input, efficiency is capped at 100 %; any higher value signals a measurement or unit error, not a perfect machine.