A prior Tip of the Night discussed the definition of precision and recall. An F1 score calculates the accuracy of a search by showing a weighted average of the precision (the percentage of responsive documents in your search results, as opposed to non-responsive documents) and the recall (the percentage of total responsive documents that show up in the results) scores. The F1 formula is calculated this way:
F1 Score = 2 * (Precision * Recall)
(Precision + Recall)
So if you recall all of the responsive documents, and no non-responsive documents, the F1 score would be 1. Search results containing only non-responsive documents would get an F1 score of zero. The F1 formula weights precision and recall equally.
When the precision percentage is listed in column A, and the recall percentages is give in column B, you can use this Excel formula to calculate the F1 score.