Percent Error Calculator

Calculate percent error between experimental and theoretical values.

By Konstantin Iakovlev · Updated September 2026 · Source: Khan Academy

Percent Error

1.02%

Absolute Error

0.1000

Calculation

Experimental9.7000
Theoretical9.8000
|Exp - Theo|0.1000
Percent Error1.02%

Use the Percent Error Calculator above to calculate your results. Enter your values and see instant results — all calculations run in your browser.

Disclaimer: This calculator is for informational purposes only. Results are estimates based on the information you provide and the assumptions described on this page.

How It Works

Percent error measures how far an experimental result strays from a known theoretical value, expressed as a share of that reference. The figure earns its keep across lab science, engineering, and even financial forecasting, wherever a measurement needs to be checked against what was expected. Picture a solar panel prototype that should convert 28.5% of incoming sunlight into energy; if your test instead reads 27.8%, this tool reports the gap between the two at once: a 2.46% error.

The calculation takes the absolute difference between the experimental and theoretical values, divides by the theoretical value, and multiplies by 100 to land on a percentage. Written out, that is |Experimental Value - Theoretical Value| / |Theoretical Value| * 100%. The absolute value is what keeps the error positive whether your measurement came in above or below the target.

Mismatched units are the quiet source of bad results, so confirm everything is on the same scale before you enter it. Dropping the absolute value is the other frequent slip, since it lets negative signs creep in and distort how large the deviation really looks. As a reading guide, a small percent error points to high accuracy, while a large one flags a discrepancy worth digging into.

Example: Checking an Algae Biofuel Yield

  1. 1 A new algae strain is predicted to produce 150 gallons of biofuel per acre. Your test plots yield 142 gallons per acre.
  2. 2 Experimental Value = 142 gallons, Theoretical Value = 150 gallons. Percent Error = |142 - 150| / |150| * 100% = |-8| / 150 * 100% = 8 / 150 * 100% = 0.05333 * 100% = 5.33%.
  3. 3 The calculator shows a percent error of 5.33% and an absolute error of 8.0000.
  4. 4 This 5.33% error indicates that your experimental yield came in below the prediction, which points to growing conditions, cultivation technique or measurement worth checking. The absolute value hides the direction, though: the percent error would also be 5.33% if the plots had yielded 158 gallons.

Source: Khan Academy · Last updated: September 2026

Frequently Asked Questions

How do I calculate percent error?
Percent error equals the absolute value of (experimental value minus theoretical value) divided by the theoretical value, times 100. For example, if you measured 9.8 and the accepted value is 10.0, the percent error is |9.8-10.0|/10.0 x 100 = 2%.
Is percent error always positive?
By convention, percent error uses the absolute value and is always positive. However, some contexts report signed percent error to indicate whether the measurement was above (positive) or below (negative) the expected value.
What is an acceptable percent error?
It depends on the field. In chemistry labs, 5-10% is often acceptable. In engineering and manufacturing, less than 1% may be required. In social sciences, higher errors are common. The acceptable range depends on measurement precision and the application.

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