Name: Period: Date:
Percent error handout
We humans often assume that each measurement we make in the laboratory is very accurate.
Along with reporting significant digits, percent error calculations are used to determine how
accurate their experimental values really are.
The value that the student comes up with is called the measured value. A value that can be
found in reference tables is usually called the actual value, or the standard value. The percent
error can be determined when the actual value is compared to the measured value according to
the equation below:
% error = | measured value - actual value | x 100 %
actual value
Note: The absolute value of the difference between the measured and actual value is used. This
means that % error is always represented by a positive number.
Example 1: You recently purchased a gold ring. Wanting to check if the gold was really “pure
gold,” you measure the density your ring. Your measurement is 17.5 g/mL.
To see how accurate your measurement was you look up the density of gold and find that the
actual value is 19.3g/mL. What is the percent error of your measurement?
% error = | 17.5 – 19.3 | x 100 % = 9.3%
19.3
When the experiment yields multiple measured values (either it is repeated or results are
compiled from a group of students doing the same experiment), the measured value used to
calculate percent error is the average of all of the measured values reported.
Example 2: Five students measure the density of a brand new lead ball. The manufacturer of
the ball states that the ball has a density of 11.34 g/mL. The students report the density as:
10.5 g/mL, 10.9 g/mL, 11.2 g/mL, 11.4g/mL and 11.5g/mL.
Step 1: Find the average of the students measured results.
(10.5 + 10.9 + 11.2 + 11.4 + 11.5) / 5 = 11.1 g/mL
Step 2: Preform the percent error calculation using the average as the measured value and the value from
the manufacturer as the actual value.
% error = | 11.1 – 11.34 | x 100 % =
11.34
2.1 %
