There will most likely be two values given: The approximate value and the exact value. Subtract the approximate value from exact value.
Example: I estimated 260 people, but 325 came.
260 - 325 = -65, ignore the "-" sign, so my error is 65
Show the error as a percent of the exact value. So divide by the exact value and make it a percentage:
65/325 = 0.2 = 20%