House Square Footage Calculator

The ANSI Z765 standard - used by appraisers and most real estate listings - counts only finished, heated, fully enclosed space measured to the exterior wall surface. This means: above-grade finished rooms count; finished basements count separately (and are usually not added to "main" square footage).

Garages, screened porches, unfinished attics, decks, and patios do NOT count as living square footage. They're reported separately in listings ("attached 2-car garage", "covered porch").

Walk the exterior with a tape measure. For each side, measure the longest exterior dimension at the foundation (or, if measuring to siding, account for cantilevers and bump-outs). Sketch the footprint and label every dimension.

For a two-story house with the same footprint upstairs and down, multiply the footprint by 2. For homes with smaller upper floors (Cape Cod, story-and-a-half), measure each floor separately. For tri-levels, measure each level.

House square footage has three distinct meanings, and listings often blur them. Knowing the difference protects you when comparing properties.

Above-grade (finished) area: the heated, finished space above ground level. Excludes basements even if finished. This is what ANSI Z765 considers 'gross living area' (GLA) for residential.

Below-grade (finished basement): heated, finished basement space. Listed separately, even when habitable.

Total finished area: above-grade + below-grade. The 'big number' that listings often lead with.

Why this matters: a 2,500 sq ft 'total' home with 1,000 sq ft of finished basement is really 1,500 sq ft above-grade. Appraisers, tax assessors, and most real estate comparables use above-grade square footage. Comparing a 2,500 sq ft GLA home to a 2,500 sq ft total home (with 1,000 below-grade) is misleading — they're not equivalent products.

The most accurate way to calculate house square footage is room by room from exterior wall to exterior wall. ANSI Z765 specifies the methodology:

Measure each room's length and width to the inside surface of finished walls. Multiply for room square footage.

Stair openings count toward the floor below, not the floor above (the opening is space in the lower floor, not the upper).

Closets count if they're heated and at least 5 ft tall.

Bathrooms, hallways, and circulation space count if heated.

Attached garages, screened porches, attics with less than 7 ft ceiling height: don't count.

Sloped ceilings (in attics): only count area with 5+ ft ceiling height. Area between 5-7 ft counts at half value in some standards.

Sum all qualifying rooms for total above-grade GLA.

Real estate appraisers use specific methodologies that affect your home's listed and assessed square footage:

Exterior measurement: appraisers typically measure the exterior dimensions of the house using a tape measure or laser. The interior square footage is calculated by subtracting standard wall thicknesses (usually 6 inches per exterior wall).

Doorways and openings: included as part of the room they're in. Don't subtract for door swings.

Vaulted ceilings: counted as a single floor — the cathedral ceiling area only counts once, not twice (it's the same air, just taller).

Multi-story foyers: the floor count is whichever floor the floor surface is on. A two-story foyer is part of the first floor; the upper-floor surface that overlooks it doesn't count (because there's no floor there).

Realtor disputes happen frequently: if you think your house is bigger than the listed square footage, hire a measure specialist ($150-300) for an official report you can attach to listings.