Gather and confirm the data that you have for the F-statistic. You will need the degrees of freedom for each data set. In most experiments, degrees of freedom are equal to the number of data points minus one. You also need the cumulative probability which is the sum of probabilities of the data sets.
Enter your values into the online calculator and calculate the F-value by clicking submit.
Use the formula for the F-statistic if you are interested in calculating the number manually. The concept is explained variance/ unexplained variance. Apply the formula which is as follows:
Explained variance = Sum of: ((number of observations)(Sample mean - Overall mean)^2)/(number of observations - 1)
Unexplained variance = Sum of: (An observation from the second group - mean of the second group)^2 /(number of observations - degrees of freedom)
Plug in the values you have and complete the operation. Divide the result of the explained variance by the unexplained variance.