How to Calculate Percent Error: A Step-by-Step Guide with Examples

Percent error tells you how far a measured or estimated value is from the true or accepted value, expressed as a percentage. It is the standard way to judge accuracy in science labs, engineering tests, and even everyday estimates — a recipe that calls for 250 ml of water when you poured 260 ml is a 4% error. Here is the formula, how to use it, and the traps that trip people up.

The Percent Error Formula

Percent error = |measured value − true value| ÷ true value × 100

The vertical bars mean absolute value — you take the difference as a positive number, because what matters is how far off you are, not whether you were over or under.

Step-by-Step: Three Simple Steps

• Step 1 — Subtract: measured value minus true value.
• Step 2 — Absolute value: drop the negative sign if there is one.
• Step 3 — Divide and convert: divide by the true value, then multiply by 100 to get a percentage.

Example: a lab experiment measures the density of water as 1.03 g/ml, but the accepted value is 1.00 g/ml.

Step	Calculation	Result
1. Subtract	1.03 − 1.00	0.03
2. Absolute value	|0.03|	0.03
3. Divide and convert	0.03 ÷ 1.00 × 100	3%

So the measurement has a 3% error — a very good result for most lab work.

More Worked Examples

Scenario	Measured	True	Percent Error
Odometer check	101 km	100 km	1%
Recipe water	260 ml	250 ml	4%
Shipping weight estimate	5.4 kg	5.0 kg	8%
Temperature probe	98.6°F	98.6°F	0%
Budget vs actual spend	$1,320	$1,200	10%

Percent Error vs. Percent Difference

These two get confused constantly, but they answer different questions:

• Percent error compares a measurement against a known true value. Use it when there is an accepted reference (a constant, a specification, a target).
• Percent difference compares two measurements with no true value — e.g., two scales reading 101 kg and 99 kg. It divides by the average of the two: |101 − 99| ÷ 100 × 100 = 2%.

Pick the one that matches your situation. Using percent difference when you have a true value (or vice versa) inflates or deflates the result and can send the wrong signal in a report.

Common Mistakes

• Dividing by the measured value instead of the true value — the denominator must be the accepted/true number.
• Reporting a negative error — the absolute value makes the result positive; the sign only tells direction, not accuracy.
• Forgetting the × 100 — a 0.03 result is 3%, not 0.03%.
• Confusing it with percent difference when no true value exists.

Percent error is just a percentage in disguise — the arithmetic is subtraction, division, and a × 100. If you want to check your math or convert the result into a decimal, percentage, or fraction, the percentage calculator does the conversion instantly, and the statistics calculator can handle the mean and spread of your repeated measurements.