Percent Error Calculator – Plus the Formula and Examples

Find the error between a theoretical and observed value expressed as a percentage. The theoretical value is the expected value of a measurement while the observed value is the actual value measured.


Percent error between 75 and 100 is -33.333333%

Start with the formula to calculate percent error
a - b
Substitute values in the equation
75 - 100
Simplify and solve

How to Find Percent Error

Percent error is the margin of error in a measurement between the expected and actual values measured. Find percent error using the following formula:

((tv – ov) ÷ tv) × 100

Start by subtracting the observed value (ov) from the theoretical value (tv) to find the difference. Divide the difference by the theoretical value to find the relative error then multiply by 100 to find the percent error.

Our percent change calculator may also be useful for finding the amount of change between 2 values.

Red percent symbol
Example: Find the percent error between a theoretical value of 99 and observed value of 98.

pe = ((tv – ov) ÷ tv) × 100
pe = ((99 – 98) ÷ 99) × 100
pe = (1 ÷ 99) × 100
pe = .010101 × 100
pe = 1.01%

What is Percent Error

Because it is impossible to completely avoid measurement errors there needs to be a way to define how much error there is when measuring. Percent error is a measurement of the relative error between measurements in an experiment. It can tell you the precision, accuracy, or size of the error between measurements in an experiment.

Percent Error Terminology

When conducting an experiment the theoretical value is often referred to as the true value. This is the value expected to be seen when conducting a measurement.

The observed value can also be called the experiment value and is the actually measured when conducting a measurement.

Now you can easily find percent error using the formula or our handy calculator. Use our percentage calculators for more tools for calculating percent values.