I always use the length times width, consider closets, consider cutting around and odd areas.
There is a fudge factor or over buy percent that varies depending on the type of flooring being installed.
I have a higher over buy percent for ceramic than I do for square vinyl tile. Then an even higer fudge factor for wood flooring. One must consider the percent for mistakes, for cutting around hard to do areas, and for the proper centering of the layout which sometimes requires for symetrical cut pieces on each side to make the floor look symetrical.
So, I have gone on and on, but if you come back with the kind of flooring we/I can assist you further.