Percent Error Calculator
This free percent error calculator finds how far an experimental (measured) value is from a theoretical (accepted) value, expressed as a percentage. Commonly used to judge measurement accuracy in science classes and labs. Nothing you enter is sent anywhere or stored.
How to Use It
- Enter your experimental (measured) value.
- Enter the theoretical (accepted/expected) value.
- The percent error and direction update instantly, no submit button.
How It Works
Percent error measures how far a measured value is from an accepted or expected value, expressed as a percentage of that expected value. The difference between the two is divided by the theoretical value, then multiplied by 100, and the absolute value is taken so the result is always a positive percentage regardless of which direction the measurement missed by.
Formula
percent error = absolute value of (experimental − theoretical) ÷ theoretical × 100
A worked example: a measured value of 48 against a theoretical value of 50 gives |48 − 50| ÷ 50 × 100 = 4% error, with the experimental value coming in lower than expected. A measured value of 105 against a theoretical value of 100 gives 5% error, this time higher than expected. A more precise measurement, 9.8 against a theoretical 9.81, gives just 0.10% error.
Everything runs client-side in your browser. Nothing you enter is sent anywhere or stored.
Frequently Asked Questions
What counts as a "good" percent error?
It depends entirely on the context, there is no single universal cutoff. A physics experiment measuring the speed of light might target well under 1% error, while a rough estimate in an introductory chemistry lab might be considered reasonable at 5 to 10%. Check your assignment, lab manual, or field's own conventions for what counts as acceptable in your specific case.
Why can't the theoretical value be zero?
Percent error divides by the theoretical value, and division by zero is undefined. If your accepted or expected value is genuinely zero, percent error isn't a meaningful way to express the result, since any non-zero experimental value would technically be an infinite percent off from zero.
Why is the result always positive, even if my experimental value was lower than expected?
Percent error uses the absolute value of the difference, so it reports how far off a measurement was without regard to direction, by convention. This calculator separately notes whether the experimental value came in higher or lower than the theoretical one, so that direction is not lost, just kept apart from the percentage itself.
How is this different from the Percentage Calculator?
The Percentage Calculator's percentage change panel compares two values as a signed change from a starting point to an ending point (useful for prices, populations, and similar changes over time). Percent error instead compares a measured value against a known, accepted value to judge accuracy, always unsigned, the standard framing used in science and engineering.
Can the experimental value be zero or negative?
Yes. Some real measurements are legitimately zero or negative depending on what is being measured (temperature differences, net charge, and similar quantities), so this calculator accepts any number for the experimental value. Only the theoretical value is restricted, since it cannot be zero.