Gross vs Net Square Footage
The difference in gross vs net square footage is where the tape goes: gross is measured to the outside faces of the exterior walls, so the walls count, and net is measured from the inside faces. In campus and commercial buildings net also leaves out corridors, stairs, restrooms and mechanical rooms. On a 44 by 28 foot house the exterior walls alone take about 6 percent, and about 11 percent with the calculator's 5 percent allowance for interior walls; on a typical classroom building the gap is about a third.
- At a glance
- Calculate net
- Wall thickness
- Small buildings
- Campus and commercial
- Building code
- Which number
- FAQ
Gross vs net at a glance
Both definitions come from the federal Facilities Inventory and Classification Manual that US colleges use to report their space.
All floors within the outside faces of the exterior walls. Stairs and shafts count on every floor they pass through (NCES facilities manual, gross area).
Space assigned to an occupant or use, measured from the inside faces of its walls. Circulation, mechanical, building service and structural areas are out; columns aren't deducted (NCES, net assignable area).
How to calculate net square footage
Subtract twice the wall thickness from each outside dimension and multiply. The wall loss comes out close to the perimeter times the wall thickness, whatever the building's area.
Net inside the walls = (L - 2t) x (W - 2t) ≈ gross - perimeter x t
- Convert the wall thickness to feet. 6.5 in ÷ 12 = 0.5417 ft.
- Take 2t off each outside dimension. 44 ft becomes 42.92 ft and 28 ft becomes 26.92 ft.
- Multiply. 42.92 x 26.92 = 1,155 sq ft inside the exterior walls.
- Take out interior partitions if your definition of net excludes them. The calculator applies a percentage allowance for this.
Check the shortcut: the 44 by 28 house has a 144 ft perimeter, and 144 x 0.5417 = 78 sq ft, within 1 sq ft of the exact answer. With the calculator's 5% partition allowance the net drops to 1,097 sq ft, 10.9% under gross. The rectangle calculator gives any footprint's perimeter.
For a footprint made of several rectangles, the main calculator adds the pieces into one gross figure before you take the walls off.
How thick are exterior walls?
A 2x6 wall is about 6.5 inches and a 2x4 wall about 4.5 inches before siding. A nominal 2x6 stud is actually 5 1/2 in deep and a 2x4 is 3 1/2 in, per the American Softwood Lumber Standard (NIST PS 20); the totals below add 1/2 in drywall and 1/2 in sheathing (7/16 in OSB sheathing makes them 1/16 in thinner).
| Wall | Stud, actual | Total with drywall and sheathing | Inside a 44 x 28 ft footprint |
|---|---|---|---|
| 2x4 | 3 1/2 in | 4.5 in | 1,179 sq ft |
| 2x6 | 5 1/2 in | 6.5 in | 1,155 sq ft |
Measure your own wall at an exterior door or window opening, from the inside wall surface to the outside face of the siding. ANSI Z765 sketches use exterior dimensions (Fannie Mae's ANSI FAQ) taken outside the cladding (Home Innovation Research Labs, the standard's developer), so pick the calculator's "plus siding" option. Door and opening sizes are in standard door sizes.
Why small buildings lose more
The share lost to walls is about 4t ÷ s for a square building with side s, so halving the size doubles the percentage. On the same walls, a 400 sq ft studio loses about five times the share of a 10,000 sq ft building.
Shape matters too: an L-shaped plan has more perimeter than a rectangle of the same area, so it loses more. Small conversions feel it most; garage conversion square footage works out what a converted bay really adds.
Net square feet in campus and commercial buildings
On campus buildings net is typically 55% to 72% of gross across the University of New Mexico guideline's general building types, because corridors, stairs, mechanical rooms, restrooms and structure all come out. That building efficiency guideline (revised 2007) puts classroom buildings at 62% to 71% net, 67% in the middle, with circulation the largest share of the rest.
Office leasing works the other way round: BOMA's office standard exists mainly to calculate rentable area, the figure office rent is charged on. Commercial square footage covers rentable, usable and the load factor.
Gross and net floor area in the building code
The 2024 International Building Code measures gross floor area inside the exterior walls, not outside, so code gross is already smaller than an appraiser's gross. Net floor area is the occupied part only. Both definitions are in IBC 2024 Chapter 2:
Inside perimeter of the exterior walls, minus vent shafts and courts. Corridors, stairs, closets, interior walls and columns aren't deducted.
The actual occupied area, without corridors, stairways, ramps, toilet rooms, mechanical rooms and closets.
The code uses them to set occupant load, and IBC 2024 Table 1004.5 names which one each use takes, in square feet per occupant:
Stores and sales floors.
Educational classroom area.
Unconcentrated assembly.
Which number are you being quoted?
Homes are quoted gross, campus records net, and offices rentable, so check the method before comparing two figures. The garage has a rule of its own, covered in whether a garage is included in square footage.
| Context | Figure used | Measured |
|---|---|---|
| US home appraisal | Gross living area | Outside the exterior walls, under ANSI Z765 |
| Zoning and FAR | Gross floor area | Usually outside the exterior walls (varies by city) |
| Office and retail leasing | Rentable area or gross leasable area | See commercial square footage |
| College space records | Net assignable square feet | Inside faces of the walls |
| Building code | Gross or net floor area | Inside the exterior walls |
| German apartment listings | Wohnfläche | Clear inside dimensions between walls |
German listings usually quote Wohnfläche, measured between the finished wall surfaces under the Wohnflächenverordnung, § 3, so a 100 m² flat is a net figure. Converting it with the square meters to square feet converter fixes the units, not the method.
"Gross" in gross living area is about where the tape goes, not about sweeping in the basement or garage; what counts as square footage covers which spaces count. If the appraiser, listing and tax record disagree, how square footage is measured explains why, and the house square footage calculator adds up a home floor by floor.
Frequently asked questions
What is the difference between gross and net square footage?
Gross includes the exterior walls because it's measured to their outside faces. Net is measured from the inside faces and, in commercial buildings, also drops corridors, stairs and mechanical rooms.
Does gross square footage include the walls?
Yes, in appraisal, zoning and campus records, where gross runs to the outside faces of the exterior walls and nothing inside is deducted. The building code is the exception: IBC gross floor area starts at the inside face of the exterior walls.
How much smaller is net than gross on a house?
Around 6% for 6.5 inch exterior walls on a 44 by 28 foot house, and near 11% with the calculator's 5% allowance for interior partitions. Smaller houses lose a larger share.
Is gross living area a gross or net measurement?
Gross. Appraisers measure it to the outside of the exterior walls, which is why adding up rooms from the inside comes in lower.
Why does my apartment measure smaller than a house with the same square footage?
The two may be measured differently. ANSI Z765, the standard for measuring houses, doesn't apply to apartment buildings, and for a condo in an apartment building Fannie Mae has appraisers use interior perimeter measurements (Fannie Mae ANSI FAQ). Ask whether a figure is inside the unit or to the outside walls.