Last updated: September 27, 2026
Can gross building area tell you the size of each unit?
No. A building's gross area does not identify its individual units, corridors, stairs, service rooms or measurement boundaries. An exterior-wall calculation can illustrate the space inside a simple building shell, but it cannot replace unit measurements or an area schedule.
Start by identifying the area you have
Before using a square-footage figure in a listing, find its source and definition. An exterior footprint, a total across several floors, an assessor's record and a lease's rentable area may describe different things. Record which floors and spaces are included, the measurement date and any applicable standard. Do not assume that a figure labeled “building SF” is a measured total of apartment interiors.
To calculate an individual unit, use its measured boundaries and a consistent method. Divide an irregular layout into measurable shapes, total their areas, and account for exclusions under that method. To estimate an average from an area schedule, divide the total measured unit area by the number of units. That average still does not establish the size of any particular apartment.
A worked example: exterior walls only
Assume a single-story building is a perfect 100 ft by 100 ft square measured outside its walls. Each exterior wall is exactly 1 ft thick. Ignore all interior partitions and other spaces for this illustration.
Exterior area: 100 × 100 = 10,000 sq ft .
Interior dimensions: (100 − 2 × 1) by (100 − 2 × 1) = 98 ft by 98 ft.
Area inside the exterior walls: 98 × 98 = 9,604 sq ft .
Difference: 10,000 − 9,604 = 396 sq ft , or 3.96% of exterior area.
Multiplying the outside perimeter by wall thickness gives 400 sq ft and a rounded interior estimate of 9,600 sq ft. That shortcut counts the corner areas twice. It is close for this specific shape and wall thickness, but its apparent 4% deduction is not a universal rule.
For a rectangular shell with uniform wall thickness, the geometric interior area is (outside length − 2 × wall thickness) × (outside width − 2 × wall thickness). Use consistent units. This calculation addresses the shell only, not the measurement rules for a lease or the distribution of space among units.
Why a blanket deduction is unreliable
A narrow building, a different wall thickness or an irregular perimeter changes the proportion occupied by walls. Multiple floors, interior partitions, circulation, mechanical rooms and vertical openings introduce further questions. You cannot learn their sizes by subtracting the same percentage from every building.
In the example, the 9,604 sq ft remains an interior-shell figure. It is not automatically usable area, rentable area or the sum of unit areas. Do not divide it by the unit count and advertise the result as each unit's measured size.
Use the method appropriate to the property
BOMA publishes separate standards for office, industrial, retail, multifamily and other property types. Its multifamily standard includes gross and net methods, while office leasing has its own rentable-area framework. The governing method determines boundaries and allocations; an exterior-wall deduction does not substitute for it. See BOMA's standards overview .
For a lease or transaction, reconcile the plans, measured area schedule and contractual definitions. Have a qualified measurement professional resolve differences when the result affects pricing, rent or a representation to a buyer or tenant.
Present the figures clearly
Label a preliminary estimate as an estimate, identify its assumptions, and distinguish it from verified unit and rentable areas. Read our BOMA guide and floor-area comparison for related terminology. Once reviewed, use CREOP's marketing software to present the area schedule and supporting property information consistently.