How to Calculate the Square Footage of a House
A house square footage is the sum of its rooms - but what counts depends on whether you want finished living area, total floor area, or the figure an appraiser uses. Here is how to measure each, room by room or from outside.
By the Calculate My Reno Team / Published
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A house square footage is just the sum of its rooms, floor by floor. The question that trips people up is not the maths - it is what counts. The figure on a listing (gross living area) is not the same as every square foot under the roof.
Skip the math
Add each room as a shape in the square footage calculator - rectangles for the rooms, with the odd triangle or curve - and it totals the whole house in square feet and square metres. Enter feet and inches straight off the tape. Below is how to do it by hand and what to leave in or out.
Method 1: room by room (inside)
- Measure the length and width of every room and multiply for its floor area.
- Include hallways, stairs and closets - they are living space.
- Add every room together.
- For more than one storey, do each floor and add the floors.
This is the most accurate way for an existing home and the easiest to check. Because you measure inside the walls, the total is a little smaller than the exterior method.
Method 2: from the outside
For a simple rectangular house, measure the outside length and width of each storey and multiply. Subtract any part that is not living area, like an attached garage. This is closer to how an appraiser measures, because the standard US method (ANSI Z765, the official US standard for measuring homes) measures single-family homes from the exterior.
Two-storey and split-level homes
Do each floor on its own and add them. A 1,000 sq ft footprint (the area of one floor) over two full storeys is roughly 2,000 sq ft. The stair opening counts once, on the floor it belongs to - do not count the hole in the upper floor and the stairs below as two areas. A split-level home, where floors sit about half a storey apart, works the same way: measure each level and add them, counting any shared landing once.
What counts as living area
| Counts as living area | Usually excluded |
|---|---|
| Bedrooms, living and dining rooms | Attached garage |
| Kitchen, bathrooms, hallways | Unfinished basement |
| Finished, heated basement (reported separately in the US) | Unheated porch or sunroom |
| Stairs and landings | Attic below the ceiling-height rule |
| Finished attic with enough headroom | Open patios and balconies |
The exact rules vary by country and by standard, but the principle holds everywhere: living area is finished, heated space with full ceiling height.
Worked example
A two-storey home, each floor a 30 × 20 ft rectangle, with a 20 × 20 ft garage taking up part of the ground floor:
- Ground floor: 30 × 20 = 600 sq ft, minus the 400 sq ft garage = 200 sq ft living area on that level (the garage is excluded).
- Upper floor: 30 × 20 = 600 sq ft, all living area.
- Living area total: 200 + 600 = 800 sq ft, with the garage reported separately as 400 sq ft.
To convert the total, see square feet to square metres; for the lot the house sits on, how many square feet in an acre.
Frequently asked questions
How do I calculate the square footage of a house?
Measure each room's length and width, multiply for its floor area, and add the rooms together. For a two-storey home, do each floor and add them. Include hallways and stairs but not the garage or an unfinished basement if you want finished living area. The square footage calculator lets you add every room as its own shape and totals them.
How do I measure a two-storey house?
Work out each floor separately and add them. A 1,000 sq ft footprint over two storeys is about 2,000 sq ft of living area. The stair opening counts once, on the floor it sits in - do not double-count it. An attic or loft only counts if it is finished and has the required ceiling height.
Does the garage count toward square footage?
No. An attached garage is not finished living area, so it is excluded from the living-area figure that appraisers and listings use - reported separately if at all. The same goes for an unfinished basement, an unheated porch and an attic that does not meet the ceiling-height rule. They are real floor area, just not living area.
Is square footage measured inside or outside the walls?
It depends on the standard. The widely used US method (ANSI Z765) measures a single-family home from the exterior of the walls, so the figure includes the wall thickness. The UK RICS gross internal area is measured to the inside face of the outer walls. Measuring room by room from the inside gives a slightly smaller number than the exterior method because it leaves out the walls.
Why is the listed square footage different from what I measure?
Listings usually quote gross living area measured from the outside to a standard like ANSI Z765, which includes the wall thickness and stairwells but excludes the garage, an unfinished basement and low-ceiling attic space. Measuring your rooms from the inside leaves out the walls, so it comes out a little lower. Neither is wrong - they are measuring different things.