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Percent Error Calculator

Calculate percent error online — free, step by step. The deviation between a measured value and the true value in percent, with full working.

Quick answer
How do you calculate percent error?
Percent error is the absolute deviation divided by the absolute true value, times 100: |measured − true| ÷ |true| · 100. Example: measured 9.8, true 9.81 → |9.8 − 9.81| ÷ 9.81 · 100 ≈ 0.10%.
The tool

Enter values — get full working

Comma or dot as decimal separator, negative values allowed.
Step-by-step
Press Calculate to see every step.
HowTo

Percent Error Calculator — step-by-step

How do you calculate percent error?
  1. 1
    Step 1 of 4

    Note the measured and true value

    Set the measured value and the accepted (true) value. Example: measured = 9.8, true = 9.81.

  2. 2
    Step 2 of 4

    Take the absolute deviation

    Absolute difference: |9.8 − 9.81| = 0.01.

  3. 3
    Step 3 of 4

    Divide by the true value

    0.01 ÷ 9.81 ≈ 0.00102.

  4. 4
    Step 4 of 4

    Multiply by 100

    0.00102 · 100 ≈ 0.10%.

Examples

Percent Error Calculator — examples

Worked examples with full working
9.8 vs 9.81
|9.8 − 9.81| ÷ 9.81 · 100
≈ 0.10%
102 vs 100
|102 − 100| ÷ 100 · 100
2%
48 vs 50
|48 − 50| ÷ 50 · 100
4%
3.14 vs 3.14159
|3.14 − 3.14159| ÷ 3.14159 · 100
≈ 0.05%
19 vs 20
|19 − 20| ÷ 20 · 100
5%
995 vs 1,000
|995 − 1000| ÷ 1000 · 100
0.5%
Theory

What is percent error?

Percent error measures how much a measured or estimated value deviates from the actual (true, theoretical) value — relative to the true value. The formula is |measured value − true value| ÷ |true value| · 100. The absolute value in the numerator ensures the error is always reported as a positive quantity, regardless of whether the measurement was too high or too low. Percent error is a standard measure in physics, chemistry and engineering for assessing the accuracy of measurements, experiments and approximations. A small percent error means high accuracy.

Pitfalls

Common mistakes

Wrong reference value

Divide by the true value, not the measured value.

Forgetting the absolute value

Percent error is always positive — the magnitude of the deviation counts.

Confusing it with relative deviation

Percent error uses the true value as reference; relative deviations may use others.
FAQ

Frequently asked questions

Glossary

Glossary — key terms explained simply

Whole (base)
The reference value that equals 100%.
Part (value)
The amount that belongs to a percentage.
Rate
The percentage (per hundred).
Difference
The result of a subtraction (new − old).
Relative
Expressed against a reference, dimensionless.
Absolute
In the unit of the quantity, without reference.
Factor
Number you multiply by (e.g. 1.25 for +25%).
Percentage point
Absolute difference between two percentages.