The experimental value is an empirical number not given by some generic equation, but by the outcome of an experiment. Every experiment has an experimental procedure explaining the steps needed to get an ideal outcome. Before the experiment begins, calculate the theoretical value using hypothetical quantities outlined in the experimental procedure as if they were not subject to human error. When you're done with the experiment, enter the theoretical value and experimental value into an equation to get the percent error. The purpose of this percent error calculation is to account for the inherent human error in every experiment.
Calculate your theoretical value by following the steps outlined in your experimental procedure. Enter the ideal quantities for each step. No physical work should be done at this point.
Find your experimental value by following the same steps as in step 1, only you will be physically measuring and performing the tasks as listed in the experimental procedure.
Subtract the theoretical value from step 1 from the experimental value in step 2.
experimental value - theoretical value
Now divide that difference by the theoretical value to get a decimal representation of percent error.
(experimental value - theoretical value) / theoretical value
Multiply the quotient from step 4 by 100 to obtain the percent error.
percent error = ([experimental value - theoretical value] / theoretical value) * 100