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How to add square feet?

Work out the area of each separate section first, then add the results together. If one room is 120 square feet, another is 180 square feet, and a hallway is 40 square feet, the combined area is 340 square feet.

Each rectangular section is length times width. A 10-foot by 12-foot room is 120 square feet; a 15-foot by 12-foot room is 180 square feet. An irregular room gets split into rectangles or triangles, each piece calculated and then added. Triangles are one-half of base × height.

Only add areas that were measured under the same definition. Interior usable floor area and exterior gross building area should not go into one unlabeled total. A house's above-grade living area, finished basement, garage, covered porch, and deck all have square footage, yet they are normally reported as separate categories.

A worksheet listing every space, its dimensions, its area, and anything included or excluded makes the total easy to check.