The measurement behind every real-estate listing
Square footage is length × width, and almost every dispute about it traces to what got included. ANSI guidelines count finished, habitable space with adequate ceiling height; garages and unfinished basements are excluded. Split odd-shaped rooms into rectangles, sum them, and the number survives scrutiny.
Sources and standards
Where these figures come from
Every formula on this page is implemented from the published reference material below. Where a value depends on a regional code, the calculator lets you override it rather than assuming one jurisdiction.
- NIST Office of Weights and Measures — the US customary and SI conversion factors used throughout this tool.
- International Code Council — the International Residential Code and International Building Code.
Figures, formulas and reference tables last verified: 19 September 2026.