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How to calculate the square footage of a house

Square footage sounds like one number, but a listing, an appraisal, a tax record and a flooring quote can each give a different one for the same house, because they count different things. Here is the method appraisers use, how to do it yourself from a tape measure, and which number fits which job.

Updated 2026-10-19. This follows the ANSI Z765 standard in its 2021 edition. Your appraiser, assessor or local multiple listing service may apply its own rules; theirs decide their number.

The standard: ANSI Z765

ANSI Z765 is the American National Standard for measuring single family houses. Fannie Mae has required it for appraisals on the loans it buys since April 2022, so it is the method most appraisers use. Its main rules:

• Measure the outside. Finished area is measured to the exterior finished surface of the outside walls, not room by room inside.
• Finished and heated only. Finished area is space you can live in year round, with walls, floors and ceilings finished like the rest of the house. A garage, an unheated porch or an unfinished room is not finished area.
• Ceiling height. Finished area needs a ceiling of at least 7 feet. Under a sloped ceiling, at least half of the room must reach 7 feet, and any part lower than 5 feet is not counted.
• Above grade and below grade apart. A floor that is partly or fully below ground is reported separately from the above grade area, even when it is finished.

Do it yourself, step by step

1. Sketch each floor. Draw the outline of the house, floor by floor, with the garage and porches marked.
2. Measure the outside walls. Walk around the house and measure each straight run of wall. Note every corner where the wall steps in or out.
3. Split the outline into rectangles. An L shaped house becomes two rectangles; a house with a bay becomes a large rectangle and a small one.
4. Multiply and add. Length times width for each rectangle, then add them. Subtract any attached garage or unfinished space that sits inside the outline.
5. Do each floor separately. On a second floor, count the floor area only once: an open two story foyer or living room counts on the lower floor, not again above.
6. Check against the rooms. Add up the room sizes from inside. The total should come out somewhat smaller than the outside figure, because walls take up area.

A worked example

A one story house is 40 ft by 28 ft outside, with a 12 ft by 10 ft bump out at the back and a 22 ft by 22 ft attached garage beside it.

Which number for which job

• Appraisal, sale or listing: the outside finished area above, with below grade space listed apart.
• Flooring, paint and drywall: the inside area of each room, which is smaller. Our drywall worked example shows a room from tape to sheets.
• Heating and cooling: the conditioned area, but equipment should be sized by a load calculation, not by square footage alone.
• Property tax: the figure your county assessor has on record, which follows the assessor's own rules. If it looks wrong, the assessor's office explains how to ask for a review.

Common mistakes

• Adding up inside room sizes and calling it the house's square footage.
• Counting a finished basement in the above grade total.
• Counting the open space over a two story room as floor area.
• Including a converted garage or porch that is not heated or finished like the rest of the house.

Where we fit

We draw the floor plan of your home from your measurements and photos, with the room sizes on it, so the areas are written down once and the same for everyone who uses them. The first plan is free with FIRSTPLAN and each extra floor is $10. We are not appraisers: an appraisal or a listing uses the appraiser's or agent's own measurement.

A plan drawn from your measurements is not an appraisal, a survey or a tax determination. Appraisers, assessors and listing services apply their own rules to square footage.