Quick answer: Measure the exterior of each finished, above-grade floor, break it into rectangles, multiply length × width for each, and add them. Leave out garages, unfinished spaces and porches. Basements are reported separately. The total is the home’s finished square footage, often called gross living area (GLA).
A home’s square footage drives its list price, appraisal, property taxes and even insurance quotes, yet two people measuring the same house often get different numbers. Most of the difference comes down to what you count, not how you multiply.
What counts toward a home’s square footage
In the US, the most widely referenced method is the ANSI Z765 standard for single-family homes, and Fannie Mae requires it for most appraisals. Exact rules vary by market and purpose, so treat this as a practical summary rather than legal advice.
Generally included:
- Finished, heated living space on above-grade floors
- Stairs, counted on the floor they descend from
- Closets, hallways and finished bonus rooms that connect to the house
Generally excluded from finished above-grade area:
- Garages, even attached or heated ones
- Unfinished attics, storage and utility areas
- Porches, decks, patios and balconies
- Openings to the floor below, such as the upper part of a two-story foyer
- Spaces with ceilings that are too low. Under ANSI rules, finished areas generally need at least a 7 ft ceiling, and under sloped ceilings the parts below 5 ft aren’t counted.
Reported separately:
- Basements and any level partly below grade, even if they’re beautifully finished. They add value but are listed apart from above-grade living area.
How to measure a house, step by step
1. Sketch the footprint
Walk around the outside and draw the shape of each level. Mark every corner where the wall changes direction.
2. Measure the exterior walls
Measure each wall from corner to corner along the outside. A laser measure or a 100 ft tape and a helper makes this quick. Write each length on your sketch, and check that opposite sides add up.
3. Break the floor into rectangles
Divide the footprint into rectangles (and the odd triangle for angled walls). Calculate each one with length × width. Our square footage calculator and the irregular room guide help with tricky shapes.
4. Subtract what doesn’t count
Deduct the attached garage, and any porch that falls inside your outline.
5. Repeat for each floor and add them up
Measure upper floors from the outside too. Subtract open-to-below areas such as a two-story foyer or great room.
Worked example: a two-story home
First floor
- Exterior rectangle: 60 ft × 30 ft = 1,800 sq ft
- Minus attached garage: 22 ft × 24 ft = 528 sq ft
- First-floor living area: 1,272 sq ft
Second floor
- Exterior rectangle: 38 ft × 30 ft = 1,140 sq ft
- Minus open-to-below foyer: 12 ft × 10 ft = 120 sq ft
- Second-floor living area: 1,020 sq ft
Total above-grade finished area: 1,272 + 1,020 = 2,292 sq ft
If the home also has a 900 sq ft finished basement, it would typically be described as “2,292 sq ft above grade plus 900 sq ft finished basement.”
Exterior vs interior measurements
- Exterior measurements are the norm for a home’s total square footage. They include the thickness of the walls.
- Interior measurements (wall to wall inside each room) are what you want for flooring, paint and furniture. They run smaller because walls take up space. See how much flooring do I need.
Why listings, appraisals and tax records disagree
- Different standards: a tax assessor may include a finished basement that an appraiser reports separately.
- Old records: additions and finished bonus rooms aren’t always updated in public records.
- Rounding and method: interior vs exterior measurements alone can differ by 5% or more.
If you’re buying or selling, ask how the number was measured, and when it really matters, have the home measured to a stated standard.
Price per square foot
Once you have a reliable number, price per square foot makes homes easier to compare:
Price per sq ft = Price ÷ Finished square footage
A $450,000 home with 2,292 sq ft works out to 450,000 ÷ 2,292 = $196.34 per sq ft. Only compare like with like: above-grade to above-grade, in similar neighborhoods.
Measure on the go with Areo
With the Areo app you can sketch each level’s outline as you walk the property, type in the wall lengths, and get the area instantly in square feet and square meters. Save each floor as a shape so it’s ready for the listing, the appraisal discussion or a renovation quote.
Skip the formulas next time
Draw any shape in Areo and get square feet, square meters, acres and the total cost instantly. Try it free for 3 days.