How to Calculate the Square Footage of a House
Why your number and the listing's number differ
Two entirely different measurements are both called "square footage":
| Gross living area (listings, appraisals) | Room-by-room total | |
|---|---|---|
| Measured | Outside face of exterior walls | Inside face of each room |
| Includes | Interior walls, stairs, hallways, closets | Only floor you can walk on |
| Excludes | Garage, unfinished space, usually below-grade space | Whatever you did not measure |
| Typical result | Larger | Smaller, often by 10% or more |
Exterior walls are commonly 6 to 8 in thick with sheathing and cladding, and interior partitions around 4.5 in. Across a whole house that adds up quickly, which is the entire explanation for the gap — neither number is wrong, they are answers to different questions.
How to measure gross living area
- Walk the exterior and measure each wall to the outside face, at ground level, in feet and inches. Sketch the footprint as you go.
- Split the footprint into rectangles and calculate each with the square footage calculator, using the running total.
- Repeat for each above-grade storey. An upper floor is often smaller than the ground floor where there are vaulted areas or overhangs.
- Subtract open-to-below areas. A two-storey entry or a vaulted great room is counted once, not twice — the upper floor has no floor there.
- Exclude the garage and any unfinished or unheated space.
What counts and what does not
| Space | Usually counted? |
|---|---|
| Finished, heated rooms above grade | Yes |
| Hallways, stairs, closets | Yes |
| Finished basement | Usually listed separately as below-grade area, not in GLA |
| Finished attic with adequate ceiling height | Sometimes, subject to headroom rules |
| Attached garage | No |
| Enclosed but unheated porch or sunroom | No, in most conventions |
| Open deck, patio, balcony | No |
| Area open to the floor below | No — counted once, on the lower floor |
Where the room-by-room number is the right one
Gross living area is for valuing and comparing houses. For actual work, you want interior measurements:
- Flooring, tile, carpet: room by room, interior, including closets you will floor.
- Paint: wall surfaces, not floor area at all.
- HVAC sizing: conditioned area plus insulation, window and orientation factors — a professional load calculation, not a square footage rule of thumb.
- Cleaning or refinishing quotes: interior floor area.
A practical workflow
- Sketch the exterior footprint, storey by storey.
- Enter each rectangle in the square footage calculator and press add to running total.
- Note the total for each storey separately, then add them — keeping the split is useful for listings and comparisons.
- Keep the sketch. Anyone questioning the figure will want to see how it was built up, and a sketch with dimensions settles it in seconds.
Calculate the area of a room or any regular shape, add rooms into a running total, and convert the result into square yards, square meters and a material cost.
Frequently asked questions
Is square footage measured inside or outside the walls?
Published house square footage is normally measured to the outside face of the exterior walls, which includes the wall thickness and interior partitions. Measuring room interiors gives a smaller figure — often 10 percent or more smaller.
Does a finished basement count in square footage?
In most conventions it is reported separately as below-grade finished area rather than being added to gross living area, even when it is fully finished and heated. Practice varies by jurisdiction and appraiser, so check the standard that applies to you.
Does the garage count towards square footage?
No. Attached garages, open decks and patios, and unheated enclosed porches are excluded from living area in the common conventions.
How is a two-storey entry counted?
Once, on the floor that actually has a floor. The open area above it is not counted on the upper storey, because there is no living area there.
- How to Measure the Square Footage of a Room
- Square Feet, Square Yards and Square Meters: Converting Without Errors
- How Much Flooring Do I Need?
- Linear Feet vs Square Feet: Which One Are You Buying?