About this calculator
The Percent Error Calculator finds the relative error between a true (expected) value and an observed (measured) value, expressed as a percentage.
What is percent error?
Percent error measures how far off a measured or experimental result is from the true or accepted value, relative to that true value. It's calculated as the absolute difference between the observed and true values, divided by the true value, then multiplied by 100 to express it as a percentage: Percent Error = |Observed − True| ÷ |True| × 100.
Why does percent error matter?
A raw difference between two numbers doesn't tell you much on its own — being off by 5 units matters a lot if the true value is 10, but barely at all if the true value is 10,000. Percent error puts that difference in context, which is why it's the standard way scientists, engineers, and students report the accuracy of a measurement or experiment.
How to interpret percent error
A lower percent error means the observed value is closer to the true value, and a higher percent error signals a bigger discrepancy. There's no universal "acceptable" percent error — what counts as a small error in a rough estimate might be unacceptable in a precision measurement, so context (the field, the equipment, the required precision) matters when judging the result.