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How to calculate floor area?

Multiply the length of a rectangular space by its width. A room that measures 12 feet by 15 feet has 180 square feet of floor area, since 12 × 15 = 180. For a whole floor, measure each room on its own, work out each area, and add the results together.

An irregular shape needs to be split first. Divide it into rectangles, triangles, or other pieces you can handle, calculate each one, and combine them. A triangular section is one-half of the base multiplied by the height. If your purpose calls for it, subtract areas that do not count, such as an open stairwell, a shaft, or a courtyard.

Be clear about what "floor area" means for the job at hand. A flooring takeoff might include closets but leave out cabinets, islands, or openings, depending on the installation scope. Gross building area, rentable area, and residential gross living area each follow their own rules.

Record whether your measurements are interior finished dimensions, exterior dimensions, or plan dimensions, and stick with one method throughout.