Quick answer
Efficiency = useful output ÷ input × 100. If 1,000 units go in and 850 do useful work, efficiency is 85% and the losses are 150 units.
Enter the useful output and the input, in the same unit. The calculator returns the efficiency as a percentage and the losses.
Related tools: Tip Calculator: Tip, total and per-person share. Discount Calculator: Final price after one or two successive discounts. Depreciation Calculator: Straight-line depreciation schedule.
How to use the Efficiency Calculator
- Enter useful output (above 0).
- Enter input (above 0).
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Useful output | A number | > 0 |
| Input | A number | > 0 |
Formula and method
Formula
efficiency = output ÷ input × 100
IEEE-754 double precision; shown to the decimals indicated.
Use the same unit for both (energy or power).
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Useful output = 850; Input = 1000.
| Result | Value |
|---|---|
| Efficiency | 85% |
| Losses | 150 |
Example 2
Inputs: Useful output = 45; Input = 60.
| Result | Value |
|---|---|
| Efficiency | 75% |
| Losses | 15 |
Example 3
Inputs: Useful output = 1; Input = 3.
| Result | Value |
|---|---|
| Efficiency | 33.33% |
| Losses | 2 |
The first line is the headline result (efficiency). Below it the efficiency Calculator lists losses, which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
What counts as output
Define the output you want: for a motor, mechanical shaft power; for a heater, heat delivered to the room; for a power supply, DC watts out. The remainder is lost, usually as heat. The units can be watts, joules or any energy or power unit, as long as input and output use the same one.
Efficiencies above 100%
A value above 100% means something is wrong with the definitions, or the device is a heat pump or similar, where a 'coefficient of performance' compares heat moved with work put in. This calculator rejects an output larger than the input.
How the result changes with input
Holding every other input at the first worked example, this table changes only input. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Input | Efficiency | Losses |
|---|---|---|
| 1000 | 85% | 150 |
| 2000 | 42.5% | 1,150 |
| 4000 | 21.25% | 3,150 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Useful output: entering "0" shows "Useful output must be greater than 0." beside that field and no result.
Choosing between the Efficiency Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Tip Calculator](/finance/tip-calculator/) | bill amount, tip and split between | Tip |
| [Discount Calculator](/finance/discount-calculator/) | original price and discount | Final price |
| [Depreciation Calculator](/finance/depreciation/) | asset cost, salvage value and useful life | Annual depreciation |
| [Decimal to Hex Converter](/technology/decimal-to-hex-calculator/) | input base and value | Hexadecimal |
| [Pixels Per Inch (PPI) Calculator](/technology/pixels-per-inch/) | horizontal pixels, vertical pixels and diagonal size | Pixel density |
Assumptions and limitations
- Efficiency cannot exceed 100% for ordinary machines
Use results as engineering estimates and check them against the governing standards, safety factors and a qualified engineer's judgement before relying on them.
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for useful output to confirm this.
Frequently asked questions
What is a good efficiency?
It depends on the technology: large electric motors can exceed 90%, while petrol engines are around 25 to 35%.
Is efficiency the same as the coefficient of performance?
No; COP can exceed 1.
Can I use it for money?
Yes, it is just a ratio, though 'return on investment' calculators handle that better.
What does the Efficiency Calculator calculate?
Efficiency as output divided by input. It returns Efficiency and Losses.
What formula does the Efficiency Calculator use?
efficiency = output ÷ input × 100. Use the same unit for both (energy or power).
What do I need to enter in the efficiency Calculator?
You enter Useful output and Input.
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. Efficiency cannot exceed 100% for ordinary machines.
What is an example of a efficiency result?
With useful output 850 and input 1000, the efficiency is 85%.
Can I rely on this calculator for a decision?
Use results as engineering estimates and check them against the governing standards, safety factors and a qualified engineer's judgement before relying on them. For this calculator in particular: Efficiency cannot exceed 100% for ordinary machines.
Related calculators
- Tip Calculator — Tip, total and per-person share.
- Discount Calculator — Final price after one or two successive discounts.
- Depreciation Calculator — Straight-line depreciation schedule.
- Decimal to Hex Converter — Convert integers between binary, octal, decimal and hexadecimal.
- Pixels Per Inch (PPI) Calculator — Pixel density of a display.
- Ohm's Law Calculator — Solve voltage, current or resistance.
Sources
- OpenStax, Prealgebra 2e (Rice University): fractions, decimals, percents, ratios and rounding
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.