How far away does a camera have to be to take a picture of a 100 ft. long object? Is there a formula for this?

Does anybody know a formula to figure out the following: Say I wanted to take a picture of a wall that is 100 feet in width and in length. How many feet away from the wall would the camera have to be to capture he entire wall???

I obviously could measure the distance with a long measuring tape once I found my self in a position where the entire wall was in the picture, but my goal is to find the formula for the problem.

Thank you for your help.