Building Square Footage Calculator

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.

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.

Building square footage has multiple definitions, and the right one depends on what you're calculating for. The differences can be 15-30% between definitions of the same building.

Gross building area (GBA): everything within the exterior walls of all stories, including walls, columns, stairs, mechanical rooms. Used for total construction cost calculations.

Net usable area (NUA): GBA minus walls, columns, mechanical spaces, and elevator shafts. Typically 80-85% of GBA.

Rentable building area (RBA): commercial leasing standard. GBA minus elevator and stairwell penetrations, but including building common areas. Typically 90-95% of GBA.

Floor plate efficiency: NUA ÷ GBA. Modern office buildings target 80-85% efficiency; older buildings can be 70-75%.

Worked example: a 10,000 sq ft GBA office building has roughly 8,500 sq ft NUA, 9,200 sq ft RBA, and a floor plate efficiency of 85%.

Total building square footage is the sum of all stories, not just the footprint. Each floor is calculated separately to its own outline:

Single-story building: footprint = total sq ft.

Two-story matching: footprint × 2 = total sq ft (assuming both floors match exactly, which is rare).

Multi-story with varying floor plates: calculate each story separately. Many buildings have larger ground floors with smaller upper floors (commercial), or larger upper floors with smaller ground floors (residential with garages).

Mezzanines and partial floors: count actual square footage, not full floor.

Basement: included in GBA if heated/conditioned; excluded if not.

Penthouse, rooftop mechanical: included in GBA if enclosed and conditioned.

Building construction costs vary by use type, materials, and region:

Single-family residential (new): $150-300 per sq ft (NAHB 2024 average).

Custom luxury home: $300-600+ per sq ft.

Multi-family (apartments, condos): $130-250 per sq ft.

Light commercial (office, retail): $200-400 per sq ft.

Medical office buildings: $300-600 per sq ft (more mechanical systems).

Industrial / warehouse: $80-200 per sq ft (less interior finish).

Schools: $250-500 per sq ft (varies by district and design).

Hospitals: $500-1,000+ per sq ft (highest because of mechanical systems and code compliance).

These exclude land cost, site preparation, and permits. Always ask: does the per-square-foot quote include the foundation, mechanical/electrical/plumbing, finishes, or just shell?