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How to calculate house plan measurements?

To calculate house plan measurements, start with the scale in the title block, use the written dimensions rather than measuring the drawing, and break the outside wall footprint into rectangles. Multiply length by width for each section, work out any odd shapes separately, and add everything together.

Say a footprint has a 30-foot by 40-foot main rectangle and a 10-foot by 12-foot bump-out. Its gross footprint is 1,320 square feet: 30 × 40 + 10 × 12. A multistory house gets calculated one floor at a time.

Before using the result, decide which measurement standard applies. Architectural plans may measure to exterior framing or exterior wall surfaces, while a room schedule may use interior finished dimensions.

A garage, covered porch, unfinished basement, stair opening, or vaulted area may or may not be included. That depends on whether you are estimating materials, reporting gross building area, or calculating above-grade living area.