Percentage Error Calculator
Calculate the percentage error between a measured value and an expected value.
How to use the percentage error calculator
- Enter the measured (or experimental) value.
- Enter the expected (or theoretical) value.
- The percentage error updates automatically.
What is percentage error?
Percentage error (also called percent error) shows how large the difference between a measured value and an expected value is, expressed as a percentage of the expected value. It's often used in science and lab work to assess how accurate an experimental result is compared to a known or theoretical value. The formula is |measured − expected| ÷ expected × 100.
Worked example: a measured value of 105 against an expected value of 100 gives |105 − 100| ÷ 100 × 100 = 5 % error. Note that it is the expected value that goes in the denominator, not the measured one. Divide by 105 instead and you get 4.76 % — a different number answering a different question. Error is always measured relative to the reference, not relative to what you observed.
Frequently asked questions
What's the difference between percentage error and percent change?
Percentage error measures how far a measured value is from an expected/theoretical value (always positive). Percent change measures how much a value has changed from an old to a new value (can be positive or negative). See our percent change calculator for the latter.
When is percentage error used in practice?
Often in science and engineering, to assess the precision of a measuring instrument or an experimental method — how close does the measured result come to the known or expected value?
What counts as a "good" percentage error?
It depends entirely on the context — in some settings (e.g. precision instruments) even a 1% error is significant, while in others (e.g. rough estimates) a 10-20% error can be perfectly acceptable.