SqFt Math

How to measure the square footage of a house

By Rohit GaurPublished October 11, 2026Last updated October 11, 2026Report a mistake

Drafted with an AI assistant. A separate automated check matched each number to the sources below.

A house’s square footage is the finished area of each floor, measured at floor level to the outside face of its exterior walls. On an appraisal, the part above ground is the gross living area (GLA), which Fannie Mae now calls above-grade finished area. Under ANSI Z765-2021, the standard Fannie Mae and Freddie Mac require for appraisals that inspect a house inside and out, only finished, year-round space counts, and ceilings must generally be at least 7 feet high. A level that is even partly below ground (below grade) gets its own figure and is never added to the rest. Garages, unfinished space, open porches and decks, and openings to the floor below don’t count. To add up your own house this way, use the house square footage calculator.

What counts in a house’s square footage?

Space Does it count?
Finished rooms on a level entirely above ground, closets and halls included Yes: above-grade finished area
Stairs Yes, on the floor they go down from
An opening to the floor below (a two-story entry hall, an open stairwell) No
A finished basement, walkout included Only as below-grade finished area, a separate figure
Unfinished space No; it can be listed separately as unfinished area
A garage Never, however it is finished
A finished attic or bonus room, reached through finished space Yes, if its ceilings pass the height rules
A finished room you can only reach through unfinished space No; it is reported separately as nonstandard finished area
An attached finished room with no way in from inside the house No; unless it is an accessory dwelling unit, it is reported separately as noncontinuous finished area
A room with a flat ceiling under 7 ft (6 ft 4 in under beams, ducts) No; nonstandard finished area
A room with a sloped ceiling The part under 5 ft never counts, and at least half of the room’s floor must be 7 ft or higher
An open porch, deck or balcony No; an enclosed porch or sunroom finished like the house and usable all year can

Finished area, in the standard’s words, is an enclosed area “suitable for year-round use” with walls, floors and ceilings “similar to the rest of the house”. The standard’s definition, in its public draft, sets no separate heating test; what matters is whether the space is suitable for year-round use. A level doesn’t count as above grade because it is nicely finished or has big windows.

A house's upper floor in plan, 40 by 30 feet, with the stairs drawn in and a dashed outline around the opening down to the entry hall, which is not counted

The example house’s upper floor, to scale: the floor that counts, the stairs, and, dashed, the opening to the entry hall below, which does not.

How do you measure a house’s square footage, step by step?

  1. Sketch each floor, with the garage, porches and any openings to the floor below marked.
  2. Measure the outside walls at floor level, to the exterior finished surface: the outside of the siding or brick, not the inside of the room. Take each length to the nearest inch or tenth of a foot.
  3. Split each floor into rectangles, multiply length by width, and add them up.
  4. Take off what doesn’t count: the garage, openings to the floor below, and any room that fails the access or ceiling rules below.
  5. Add the floors above ground for the above-grade figure, keep any level that is partly below ground as a separate below-grade figure, and round only the totals, to the nearest whole square foot.

The example house’s main floor: 40 × 30 ft to the outside of the walls is 1,200 sq ft. The attached 22 × 22 ft garage (hatched) is 484 sq ft that never counts.

Measuring inside the rooms gives a smaller number. With 6-inch exterior walls, the same floor is 39 × 29 ft inside, 1,131 sq ft, before a single interior wall is taken off. That is why rooms measured with a tape inside, or added up on the square footage calculator, come to less than the house’s figure. Closets and hallways need no separate measuring: measured outside the walls, they are already in.

Does a basement count in square footage?

Not as part of the main figure. A level is below grade if any part of its walls is not entirely at or above the ground, however it is finished and whatever its windows are like. A walkout basement, open to the yard on one side and dug into the slope on the other, is below grade. Its finished rooms are reported as below-grade finished area, a second number that is never added to the first.

A walkout basement: the ground reaches 7 ft up the back wall and the basement floor at the front. Because part of the level is below ground, all of it is below-grade area.

This is one reason house sizes don’t agree. The Census Bureau’s figures for new homes count finished basements and attics in the floor area, so a Census-style size and an appraisal of the same house can differ by the whole basement. See average house size for the Census numbers.

Does a garage count in square footage?

Never, however it is finished. A finished room over the garage, often called a bonus room, is a different case, and it depends on how you get to it:

Does a finished attic or bonus room count in square footage?

It counts when you reach it through finished space and its ceilings pass the height rules. If you can only get there through unfinished space, or its ceilings are too low, it is reported separately as nonstandard finished area. An appraiser still counts its bedrooms and bathrooms; its area just isn’t in the main above-grade figure.

A finished room needs a ceiling at least 7 ft high. Under beams, ducts and other obstructions, 6 ft 4 in is enough, and under stairs there is no minimum. A room with a flat ceiling lower than that is nonstandard finished area. The 6 ft 4 in allowance is for obstructions only, not for sloped ceilings, which have their own rule.

Where the ceiling slopes, as in many attics, two things apply to each room. No part with a ceiling under 5 ft counts, and at least half of the room’s finished floor must have a ceiling of 7 ft or more; if it doesn’t, the whole room is nonstandard finished area.

The example house’s room over the garage, 22 ft wide: 2 ft knee walls and a roof rising a foot for every foot in. The hatched strips are under 5 ft, the dark middle 7 ft or more.

In the example, the room over the garage is reached by a finished hall from the upper floor, and is 22 × 22 ft, 484 sq ft. The strips under 5 ft are 132 sq ft and come off, leaving 352 sq ft. The middle, at 7 ft or more, is 264 sq ft, more than half the room however you measure it (half of 352 is 176; half of 484 is 242), so 352 sq ft counts. Had only 150 sq ft been 7 ft or higher, the whole room would have been nonstandard.

Do stairs count in square footage?

Yes, once: stairs are counted on the floor they go down from, and an opening to the floor below isn’t counted at all. A two-story entry hall is floor on the ground level and a hole in the floor above it.

The example house’s upper floor: the opening over the entry hall (white, 12 × 10 ft) is not floor, but the stairs in it (12 × 3.5 ft) count here, the floor they go down from.

The upper floor is 40 × 30 ft, 1,200 sq ft, outside the walls. The opening is 120 sq ft and comes off; the stairs inside it, 42 sq ft, go back on. The upper floor counts 1,122 sq ft.

A whole house, worked out

Space Measured Counts as
Main floor 1,200 sq ft 1,200 sq ft above-grade finished
Garage 484 sq ft Nothing
Upper floor 1,200 sq ft 1,122 sq ft above-grade finished
Room over the garage, reached by a finished hall 484 sq ft 352 sq ft above-grade finished
Basement rooms 900 sq ft 900 sq ft below-grade finished
Basement mechanical room 300 sq ft 300 sq ft below-grade unfinished

The statement for this house is 2,674 sq ft of above-grade finished area and 900 sq ft of below-grade finished area, with 300 sq ft of unfinished area below grade. Adding the basement in to call it a 3,574 sq ft house is exactly what the standard rules out. The house square footage calculator gives the same figures from the same inputs.

Keeping a finished basement as a separate figure, or a low attic out of the main one, doesn’t by itself lower what a house is worth: Fannie Mae says that, done correctly, following the standard does not change the value, that rooms left out of the above-grade count “may add substantially to the value”, and that the appraiser adjusts for them. This page explains how the method works; it is not an appraisal of your house.

Who uses ANSI Z765, and since when?

ANSI Z765 is the American National Standard for measuring single-family houses. Home Innovation Research Labs served as secretariat for the committee that wrote it and sells the 2021 edition as a 16-page PDF for $25.

Fannie Mae applies the standard to detached and attached houses, including townhouses, rowhouses and manufactured homes, and to every accessory dwelling unit (ADU). Apartments are different: for Fannie Mae, units in an apartment-style condominium or co-op building are measured inside their walls, not by ANSI Z765, while a townhouse-style condo unit is measured by the standard.

The rules on this page come from Fannie Mae’s and Freddie Mac’s published guidance and, where those don’t spell a rule out, from the standard’s free 2020 public draft. Home Innovation says the 2021 revision mostly clarified the standard, mainly in its commentary, so the draft’s wording may differ in places from the final text.

Why do the listing, the appraisal and the tax record disagree?

Because they don’t all measure the same way. Fannie Mae notes that the figures in MLS listings and assessor records “may not have been derived using the ANSI standard”: they can come from inside measurements, plans, or a total with the basement added in. The Census counts finished basements, and an appraisal with an inspection for a loan Fannie Mae or Freddie Mac will buy uses ANSI Z765. A finished basement alone can make two honest figures for one house far apart. If the number matters for a price or a loan, ask how it was measured.

How is a house measured in Canada, the UK and Australia?

Measure your own house

  1. Measure each floor around the outside walls, at floor level, and split the outline into rectangles. The floor plan area calculator handles L-shapes and other odd outlines, and trace a floor plan works from a plan image. Measuring room by room inside on the square footage calculator works too, but expect a smaller total.
  2. Open the house square footage calculator, enter each floor or carry your rooms over, and put each on its level, above or below grade.
  3. Mark the garage, unfinished space, low ceilings and rooms with no inside way in, and read off the two figures, above grade and below grade, with the sentence to write down.

Where these numbers come from

Open the calculator