Percent error in simple words
Percent error tells you how far a measurement is from the true value, as a percentage of the true value. A small percent error means your measurement was close.
There are three steps. Subtract to find the gap, ignoring any minus sign. Divide the gap by the true value. Multiply by 100 to get a percent.
Example: a pencil is really 20 cm long, but you measure 19 cm. The gap is 1 cm. 1 ÷ 20 = 0.05, and 0.05 × 100 = 5%. Your percent error is 5%.
The signed error keeps the minus sign: 19 − 20 = −1 shows that your measurement was too short.
Formula and method
Percent error = |measurement − reference| / |reference| × 100. A zero reference has no defined relative error. Both quantities must use the same unit.
Worked example
A measurement of 98 against a reference of 100 has absolute error 2, signed error −2 and percent error 2%.
Step by step
The calculator shows both steps with your numbers. For a measurement of 9.5 against an accepted value of 10, the absolute error is |9.5 − 10| = 0.5 and the percent error is 0.5 ÷ |10| × 100 = 5%. The signed error, −0.5, shows the measurement was below the reference.