Percent Error Calculator

Calculate the percent error between an actual and expected value.

Actual Value

Expected Value

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Enter your values and click Calculate

How It Works

The calculator computes percent error using the standard formula: |actual − expected| ÷ |expected| × 100. First, the absolute difference between the actual (observed or measured) value and the expected (theoretical or true) value is calculated. Dividing by the absolute value of the expected value normalises the error relative to the true reference, and multiplying by 100 converts it to a percentage. Wrapping both the numerator and denominator in absolute values ensures the result is always a positive number, regardless of whether the actual value was higher or lower than expected. This is the widely accepted convention in science and engineering, where the magnitude of the deviation matters more than its direction. If directional information is important, the signed version of the formula can be used by removing the absolute values.

Examples

Lab Measurement

An experiment yields 95 when the expected result was 100.

Result: The percent error is 5%.

Overestimate

A forecast of 110 against an actual outcome of 100.

Result: The percent error is 10%.

Close Measurement

A measured value of 9.98 against an expected 10.

Result: The percent error is 0.2%.

Frequently Asked Questions

What is a good percent error?

It depends on the field. In many scientific experiments, a percent error under 5% is considered acceptable. In precision engineering, the threshold is much tighter.

Why use absolute value in the formula?

The absolute value ensures that over- and under-estimates produce the same positive percentage, making it easier to compare magnitudes of error.

What is the difference between percent error and percent difference?

Percent error compares a measurement to a known true value. Percent difference compares two measured values with no assumed 'correct' one.