How to Calculate the Square Footage of a House

A house’s square footage is the sum of its rooms — but which rooms you’re allowed to count depends on what the number is for. A flooring order counts every surface you’ll cover. A property listing counts only finished, heated living space. Use the wrong rule and the figure is either useless or, on a listing, a problem.

This guide covers measuring a whole house room by room, the difference between total floor area and gross living area, and what counts on each floor. The gross living area calculator applies the listing rules; the multi-room calculator handles a plain total.

Add the rooms — don’t measure the outside

Measure each room’s floor area and add them: hallways, landings, stairs and closets included. What you must not do is measure the exterior footprint and call it the interior area. The difference is the thickness of every wall, inside and out, which typically eats 3% to 6% of the outside measurement. On a 2,000-square-foot footprint that’s 60 to 120 square feet of wall you’d be charging for, or flooring over.

Go room by room with a floor plan and check the pieces off as you measure, so nothing gets counted twice and no closet gets missed.

Gross living area vs total floor area

Gross living area (GLA) is the figure real estate runs on, and it’s narrower than a plain total. GLA counts only space that is finished, heated, and above grade, measured to the outside faces of the exterior walls. A plain total floor area counts everything with a floor, finished or not.

The gap between the two is almost always the basement, the garage and the attic — spaces that have a floor but fail the living-space test.

What counts and what doesn’t

Space	Counts as living area?	Notes
Bedrooms, living areas, kitchens, halls	Yes	Finished, heated, above grade
Finished basement	No — report separately	Below grade, even when finished
Garage	No	Never, finished or not
Finished attic	Sometimes	Needs about 7 ft of ceiling over enough of the floor
Covered porch / sunroom	Only if heated and finished	An unheated sunroom is left out
Stairs	Yes, once	Counted on the floor they descend from

Multiple floors and the stairwell

On a two-story house, measure each floor and add them together. Stairs are counted once — as floor area on the level they start from — and the stairwell opening, the hole the stairs drop through, is subtracted from the floor above, because there’s no floor there to stand on. Miss that subtraction and you count the same footprint on two levels.

Ceiling height matters upstairs. Under a sloped roof, only the part of the floor with a ceiling of roughly 7 feet or more usually counts, so a finished attic can be smaller on paper than it feels underfoot.

Why two “identical” houses measure differently

Two houses built to the same plan can list at different square footages, and it’s usually the measuring standard, not an error. The widely used ANSI Z765 standard sets rules for what to include and how to round; appraisers follow it closely, while a listing agent’s quick measure might fold in a finished basement that ANSI excludes. When the number matters — a sale, a tax assessment, a price-per-square-foot comparison — ask which standard produced it before you trust it.

The mistakes that matter most

1. Using the exterior footprint as the interior area. They differ by the thickness of every wall.
2. Rolling the basement or garage into the headline living-area figure. Report them separately.
3. Counting the stairwell on both floors. The opening comes out of the upper level.
4. Comparing a listing to an appraisal without checking they used the same rules.

Frequently asked questions

How do you calculate the square footage of a house?

Measure each room’s length and width, multiply for the floor area, and add every room together — including hallways, stairs and closets. Don’t measure the outside of the house and use that; the walls make the interior smaller than the exterior footprint by a few percent.

Does a garage count in a house’s square footage?

No. A garage is never counted as living area, whether it’s finished, heated, or attached to the house. The same goes for carports. If you convert a garage into a heated, finished room that’s part of the living space, that converted area can count — but as a garage, it doesn’t.

Does a finished basement add to square footage?

It adds usable space, but not to the gross living area used on listings, because a basement sits below grade. A finished basement is normally reported on its own line — “1,800 sq ft plus a 900 sq ft finished basement” — rather than added into the main figure. Buyers and appraisers expect it kept separate.

Why is my home’s listed square footage different from the tax record?

Different people measure to different standards. Tax assessors, appraisers and listing agents don’t always include the same spaces or round the same way, and older records may predate a renovation. If the gap is large, ask what each figure counted — a finished basement or attic is the usual culprit.

Do you measure the inside or the outside of a house?

For gross living area you measure to the outside faces of the exterior walls, which is how appraisers do it. For a flooring or interior job you measure the actual inside floor of each room. They’re different numbers, so match the method to what the figure is for.

The short version

Getting a house’s square footage right is less about arithmetic than about knowing which spaces the number is allowed to include.

• Measure and add every room; never substitute the exterior footprint.
• Gross living area counts finished, heated, above-grade space only.
• Report basements, garages and unfinished attics separately.
• Count stairs once and subtract the stairwell opening from the floor above.
• When the figure matters, confirm which measuring standard produced it.

For a listing-ready number, the gross living area calculator applies these rules as you go; for a simple whole-house total, the multi-room calculator keeps the running sum.