NumberTally Percent Error Calculator
Calculate percent error between an experimental and theoretical value instantly. Shows absolute error and step-by-step working — free, no sign-up.
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What the percentage means
Percent error = |experimental − theoretical| / |theoretical| × 100%. Always divide by the accepted value — that is the reference your measurement is judged against. Under 5% is excellent in most school labs; under 1% is publishable-grade in many fields.
Signed error vs percent error
Dropping the absolute value gives signed error: positive means you overshot, negative means you undershot. Percent error reports magnitude only — the comparison box shows both, since the sign tells you which direction to fix.
Frequently Asked Questions
What is the difference between percent error and absolute error?
Absolute error is the raw difference (|10 − 9| = 1); percent error expresses that difference relative to the accepted value (1/9 ≈ 11.11%).
Can percent error be negative?
By definition no — the absolute value makes it non-negative. A "signed" variant keeps the sign to show over- vs under-shooting.
What if the theoretical value is zero?
Percent error is undefined (division by zero). Report the absolute error instead, or use percent difference against the average.
What is a good percent error in a lab report?
Under 5% is generally strong for school labs; analytical chemistry often demands under 1%. Context matters more than any fixed cutoff.
Do I divide by the experimental or the theoretical value?
The theoretical (accepted) value — it is the standard your measurement is compared against.