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How to calculate square footage from a floor plan?

Use the written dimensions on the plan whenever they are available. Multiply length by width for each rectangular room or building section; a 12-foot by 15-foot room is 180 square feet. Add the room or section totals for the floor, and for a two-story house work out each level separately before combining them.

L-shaped and other irregular spaces get divided into simple rectangles. Calculate each rectangle and add the results. If part of the footprint has to be excluded, such as an open stairwell or courtyard, find that area and subtract it. A triangular area is one-half of base times height.

Before you settle on a number, confirm what the calculation is for. A flooring takeoff may include closets but deduct fixed cabinets or islands. Gross building area can be measured from exterior dimensions, while a room-area figure may use interior finished dimensions.

A plan with no dependable scale needs calibrating first. Set the PDF against a known labeled dimension, then check the calibration against a second dimension before you measure anything.