Building Square Footage Calculator
Calculate total building square footage by summing each floor's footprint. Includes guidance for distinguishing gross building area, rentable square footage, and usable area.
Building Area Calculator
Gross vs. rentable vs. usable
Gross Building Area (GBA) is measured to the exterior wall surface and includes all enclosed space - structural walls, mechanical rooms, stairwells, and elevator shafts. This is what most building square footage calculators report.
Rentable Square Footage (RSF) - used in commercial leasing - includes a tenant's usable space plus a pro-rata share of common areas (lobbies, hallways, restrooms). RSF is typically 85-92% of GBA.
Usable Square Footage (USF) is just the tenant's exclusive space, measured to the inside face of perimeter walls. USF is what you actually occupy.
Multi-story and irregular footprints
For a multi-story building, calculate each floor's area separately. A 60 × 40 ft footprint over 3 stories = 60 × 40 × 3 = 7,200 sq ft GBA. Basement and attic only count if they're finished and code-compliant heated space.
Irregular footprints (L-shapes, courtyard buildings, additions) need to be broken into rectangles. Use the multi-segment calculator above and add a segment for each rectangular section.
Pro tips
Use the BOMA standard for commercial
BOMA (Building Owners and Managers Association) publishes the standard for measuring office space. Lenders and brokers expect BOMA-compliant numbers.
ANSI Z765 for residential
For homes, ANSI Z765 defines what counts as finished, heated square footage. Garages, unheated porches, and unfinished basements don't count.
Measure to the exterior face
For GBA, measure to the outside surface of the exterior wall. Wall thickness counts. For a 60 × 40 ft house with 6-in walls, that's 60.5 × 40.5 ft of GBA per floor.
Atriums count once
Multi-story atriums or open floor cutouts only count on the lowest floor. The void above the floor doesn't count toward upper-floor square footage.