Find efficiency as a percentage — how much of the energy or work put into a process comes out as useful output.
How it works
Efficiency is useful output divided by total input, as a percentage: efficiency = (output ÷ input) × 100. A machine producing 80 J of useful work from 100 J of input energy is 80% efficient.
What this does not include
This is a general-purpose efficiency ratio. It doesn’t calculate a specific physical process’s theoretical maximum efficiency (like the Carnot limit for heat engines), which depends on the physics of that particular system.
How to use this calculator
- Enter the useful output.
- Enter the total input.
A worked example
80 units of useful output from 100 units of input: efficiency = 80 ÷ 100 × 100 = 80%.
450 units of output from 600 units of input: efficiency = 75%.
What the variables mean
| Variable | Meaning |
|---|---|
| Output | Useful energy or work produced |
| Input | Total energy or work supplied |
Edge cases worth knowing
No real machine or process reaches 100% efficiency. Some energy is always lost, typically as heat or friction — this formula simply measures how much of the input made it to useful output, not whether that’s a good or bad result for the specific system.
Zero input makes efficiency undefined — there’s no baseline to measure output against, so the calculator declines to show a result.
Why is no real machine 100% efficient?
Some input energy is always lost to friction, heat, or other forms that don’t contribute to the intended output — the second law of thermodynamics guarantees some loss in any real energy conversion.
Can efficiency exceed 100%?
Not for a simple energy conversion — output can never exceed input without violating conservation of energy, though some devices (like heat pumps) are described with a coefficient of performance that can appear to exceed 1 because they move existing heat rather than generate new energy.
Does the input and output need to be in the same units?
Yes — both need to be measured in the same energy or work units for the ratio to mean anything (both in joules, or both in watts, for example).
