What is the difference between percent error and percent difference?

Percent error compares a measurement to a known true value. Percent difference compares two measurements with no designated true value, using their average as the denominator.

Is a negative percent error bad?

Not necessarily. It simply means the measured result was lower than the accepted value. Whether that matters depends on context and required precision.

Should I use the absolute value of actual in the denominator?

Yes. This calculator uses |Actual| so the sign of the percent error reflects the direction of the error rather than the sign of the reference value.

Percent Error Calculator

Calculate signed percent error between a measured and a true value. Negative = underestimate; positive = overestimate. Returns zero when actual is zero.

By AbraCalc Editorial Team · Reviewed by AbraCalc Methodology Review · Last reviewed: 2026-06-25

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How to use this tool

Enter measured value and actual (true) value in the fields above.
Results update instantly as you type — or click Calculate.
Read your percent error and the full breakdown beneath it.

Formula

% Error = ((Measured - Actual) / |Actual|) * 100

How it works

Subtract actual from measured, divide by the absolute value of actual, multiply by 100. Negative = underestimate.

Worked example

Measured 9.8, actual 10.0

Error = (9.8-10)/|10| * 100
= -0.2/10 * 100 = -2%

Common mistakes to avoid

Dividing by the measured value instead of the actual (true) value; the formula requires |Actual| in the denominator.
Ignoring the sign of the result; a negative percent error means the measured value was too low, which is meaningful information.
Entering zero as the actual value, which makes the denominator zero and the result undefined.

Key terms

Frequently asked questions

What is the difference between percent error and percent difference?: Percent error compares a measurement to a known true value. Percent difference compares two measurements with no designated true value, using their average as the denominator.
Is a negative percent error bad?: Not necessarily. It simply means the measured result was lower than the accepted value. Whether that matters depends on context and required precision.
Should I use the absolute value of actual in the denominator?: Yes. This calculator uses |Actual| so the sign of the percent error reflects the direction of the error rather than the sign of the reference value.