Gross vs Net Square Footage

Every argument about the size of a building comes down to one question: where do you put the tape. Gross square footage is measured to the outside face of the exterior walls, so the walls themselves are inside the number. Net square footage is the floor you can actually stand on once those walls are subtracted. Same building, two honest numbers, and on a typical house they sit about 10 percent apart.

Every other measurement question on this site is about which spaces count. This one is about where the line goes.

The two definitions

The cleanest definitions in print are federal. The Department of Education's Facilities Inventory and Classification Manual, which every college and university in the country uses to report its space, defines gross area as the sum of all areas on all floors of a building included within the outside faces of its exterior walls, measured disregarding cornices, pilasters and buttresses.

Its counterpart, net assignable square feet, is the sum of all areas on all floors of a building assigned to, or available for assignment to, an occupant or specific use. The measurement rule is the part that matters: net is taken from the inside faces of the surfaces that bound the space.

Outside faces versus inside faces. That is the whole distinction, and everything below is a consequence of it.

Two details in the federal definition are worth keeping. Gross counts a stairwell or elevator shaft once on every floor it passes through, because gross is measuring the building, not the usable floor. And net excludes building columns and structural projections, which the manual treats as too small to bother deducting.

The gap is the perimeter times the wall thickness

This is the part nobody publishes, and it is simple enough to do on the back of an envelope.

Take a rectangular footprint, length L and width W, with exterior walls t feet thick. The gross area is L x W. The net area inside those walls is (L - 2t) x (W - 2t). Expand the second one and subtract:

Gap = 2t(L + W) - 4t²

The first term is just the perimeter multiplied by the wall thickness. The second is the four corner squares, and it is tiny. So:

Gap ≈ perimeter x wall thickness

On a 44 by 28 foot footprint with a 6.5 inch wall, the perimeter is 144 feet and the thickness is 0.5417 feet. 144 x 0.5417 = 78.0 square feet. The exact answer is 76.8. The 1.2 square foot difference is those four corners, and it stays 1.2 square feet no matter how big the building gets.

That is worth stating plainly: the exterior walls of a house eat roughly the perimeter, in square feet, times the wall thickness in feet. Not the area. The perimeter.

How thick is the wall, actually

You do not have to guess, because the door industry already answered this. Prehung doors come in two standard jamb depths, and each one is cut to match a wall.

Bob Vila's rundown of standard door sizes puts it directly: the standard interior jamb is 4 9/16 inches, designed for a 2x4 wall built from 1½ by 3½ inch studs with standard drywall on either side, and the alternate is 6 9/16 inches, cut to fit a 2x6 wall with drywall and exterior sheathing.

Run those backwards and the assembly falls out exactly:

Wall	Stud (actual)	Sheathing	Drywall	Total	Jamb sold for it
2x4	3.5 in	0.5 in	0.5 in	4.5 in	4 9/16 in
2x6	5.5 in	0.5 in	0.5 in	6.5 in	6 9/16 in

The jamb is the wall thickness plus a sixteenth of an inch of fitting clearance. That is not a coincidence, it is how the jamb is specified, and it means you can read your own wall thickness off a door frame with a tape measure.

Two notes. A 2x4 is 3.5 inches because of the nominal versus actual convention that runs through framing lumber, the same reason a 2x6 deck board is 5.5 inches wide and a concrete block is 7 5/8 inches tall. And siding sits outside all of this, so measuring to the finished exterior surface adds roughly another half inch for vinyl or a bit less for fiber cement. Standard door sizes covers the rest of the door dimensions.

Small buildings lose a much bigger share

Here is the result that surprises people. The percentage you lose to walls is not a constant. Take a square building of side s: the gap is 4s x t and the gross is s², so the share lost is

4t / s

The size is in the denominator. Double the building and you halve the percentage lost. At a 6.5 inch wall:

Footprint	Gross	Net inside the walls	Lost to walls
20 x 20	400	358	10.5%
30 x 30	900	836	7.1%
40 x 40	1,600	1,515	5.3%
50 x 50	2,500	2,393	4.3%
80 x 80	6,400	6,228	2.7%
100 x 100	10,000	9,785	2.2%

A 400 square foot backyard studio gives up more than a tenth of its floor to its own walls. A 10,000 square foot warehouse on the same construction gives up about a fiftieth. The walls are not thicker on the small building, there is just far more perimeter per square foot of floor.

This is why quoting a small building in gross terms flatters it, and why a tiny-home or garage-conversion figure deserves a harder look than the same number on a full house. The garage conversion guide works through what a converted bay really gives you.

A worked example

A single-storey footprint of 44 by 28 feet, 2x6 exterior walls, an ordinary number of interior partitions.

Gross. 44 x 28 = 1,232 square feet. This is the number an appraiser reports, the number a zoning code counts, and the number on the listing.

Take out the exterior walls. The inside dimensions are 44 - 2(0.5417) = 42.92 by 28 - 2(0.5417) = 26.92. That gives 42.92 x 26.92 = 1,155 square feet. The walls took 77 square feet, or 6.2 percent, and the perimeter shortcut predicted 78.

Take out the interior partitions. Interior walls are thinner, usually 4.5 inches, but there are a lot of them. A typical house with the usual run of bedrooms, bathrooms and closets loses somewhere near 5 percent of the remaining floor to them, so about 58 square feet. That leaves roughly 1,097 square feet of actual floor.

The total gap is 135 square feet, or 10.9 percent. Measure that house room by room from the inside and you will land near 1,097. Measure it from the outside and you get 1,232. Neither tape is wrong.

One more comparison. Build the same footprint with 2x4 walls instead of 2x6 and the interior grows to 1,179 square feet, 23 square feet more floor for the same gross number. That is the trade the thicker wall buys: better insulation, slightly less room.

Which number are you being quoted

The same building has a different official size depending on who is asking.

Context	Number used	Measured
US home appraisal	Gross living area	Outside the exterior walls
Zoning and permitting	Gross floor area	Outside the exterior walls
Retail leasing	Gross leasable area	Outside walls, centerline between tenants
Office leasing	Rentable square feet	Usable space plus a share of common area
Office planning	Usable square feet	Inside the space you occupy
University and institutional records	Net assignable square feet	Inside faces of the bounding walls
Most metric-country listings	Net internal area	Inside the walls

The last row catches a lot of people. A European or Australian listing quoting 100 square metres is usually quoting interior floor, so converting it to 1,076 square feet and comparing it to a US gross living area figure understates it by around 10 percent. The square meters to square feet converter handles the units, but the measurement convention is the part that does not convert.

For a US home, gross living area is the number that matters and ANSI Z765 is the rulebook behind it. On the commercial side, commercial square footage covers rentable, usable and the load factor that sits between them, and gross floor area covers the zoning version. If your question is who measured your house and why their number disagrees with the tax record, how square footage is measured answers that one.

Common mistakes

Comparing a gross figure to a net one. This is the whole problem in one line. A house quoted at 1,232 and an apartment quoted at 1,100 may have the same floor.

Adding up rooms and expecting the listing number. Room-by-room interior measurement is a net figure. It will come in roughly 10 percent under the gross number on a house, and the gap is not an error.

Assuming net is always smaller by a fixed percentage. It scales with perimeter, not area. A compact rectangle loses less than a sprawling L-shaped plan with the same floor area, because the L has more wall.

Deducting interior walls from a gross figure. Gross does not deduct anything inside. Interior partitions, columns and stairwells are all inside the gross number.

Reading "gross" as "the bigger catch-all." In appraisal, gross describes where the tape goes, not a bucket that sweeps in the basement and the garage. That is a separate question, covered in what counts as square footage.

Frequently asked questions

What is the difference between gross and net square footage?

Gross square footage is measured to the outside face of the exterior walls, so the walls are counted inside the number. Net square footage is measured from the inside faces of those walls, so it is the floor you can actually use. On a typical house the two numbers sit about 10 percent apart.

What is net square footage?

The floor area inside the walls, measured from the inside faces of the surfaces that bound the space. Institutions call it net assignable square feet, office leasing calls it usable square feet, and most metric-country listings quote it as net internal area. It excludes the exterior walls and, depending on the definition, the interior partitions, corridors and mechanical space too.

How much smaller is net than gross?

The exterior walls alone take roughly the perimeter multiplied by the wall thickness. On a 44 by 28 foot house with 6.5 inch walls that is about 77 square feet, or 6.2 percent. Interior partitions usually take another 5 percent or so, putting the total near 10 percent. Small buildings lose a much larger share, because the loss scales with perimeter rather than area.

Does gross square footage include the walls?

Yes. Gross is measured to the outside face of the exterior walls, so the full thickness of every exterior wall is inside the figure, and every interior wall, column and stairwell is inside it too. Nothing within the outline is deducted.

Is gross living area a gross or net measurement?

Gross. Gross living area is measured to the outside of the exterior walls, which is why an appraiser's figure for a house comes in above what you get adding the rooms up from the inside. The word gross there refers to the measurement method, not to a total that includes the basement and garage.

Why is my apartment smaller than the square footage says?

Apartment and condo figures are often quoted inside the unit while house figures are quoted to the exterior walls, so the two are not directly comparable even when the numbers match. Check which convention the listing used before comparing it to anything.

How do I calculate net square footage?

Subtract twice the wall thickness from each outside dimension, then multiply. For a 44 by 28 foot footprint with 6.5 inch walls: 44 - 1.083 = 42.92 and 28 - 1.083 = 26.92, so 42.92 x 26.92 = 1,155 square feet. Deduct interior partitions after that if your definition of net excludes them. The calculator at the top of this page does both steps.

Related tools and guides

Gross vs Net Square Footage