How to Calculate the Square Footage of a House

To calculate the square footage of a house, measure each room's length and width in feet, multiply them to get each room's area, and add the rooms together, including hallways, closets and stairs. Split L-shaped or irregular rooms into rectangles first. Count finished, above-grade space as living area, and list the garage, unfinished space and any basement separately.

That gives you an interior, room-by-room figure, which is what you need for flooring, paint and most planning. For a listing or an appraisal, the official method (ANSI Z765) measures the outside of the house instead, so the two numbers differ. This guide covers both, with a worked example, and the free square footage calculator does the arithmetic for you.

The Basic Method: Length × Width, Room by Room

Measure each room wall to wall at floor level, along the longest straight runs, and write the numbers down in feet. Convert inches to decimal feet before multiplying: 6 inches is 0.5 ft, 3 inches is 0.25 ft, so 12 ft 6 in becomes 12.5 ft.

Multiply length by width for each room. A bedroom 12.5 ft by 11 ft is 137.5 sq ft. Do the same for every room, hallway and closet, then add the results. Measure to the nearest inch and round only the final total, so small rounding errors don't pile up.

Work methodically: go around each floor clockwise, room by room, so nothing is skipped or measured twice, and sketch the layout as you go. A rough sketch with the numbers on it is the best way to spot a room you've missed.

Irregular Rooms: L-Shapes, Angles and Bays

Split any room that isn't a rectangle into rectangles, measure each, and add them. An L-shaped living room that is 14 ft by 12 ft with a 6 ft by 5 ft alcove is 168 + 30 = 198 sq ft.

For an angled corner, treat the cut-off corner as a triangle: half its width times its depth. A 12 ft by 12 ft room with a corner cut off 3 ft each way is 144 − (3 × 3 ÷ 2) = 139.5 sq ft. Bay windows that extend to the floor add their own small area; bays that start at a window seat usually don't add floor area.

Rooms with curved walls or many angles are where tape measurements go wrong most often. A scanned floor plan measures the actual outline, which is the easiest way to get these right.

What to Include and What to Leave Out

Living area normally includes every finished, heated room above ground: bedrooms, living and dining rooms, the kitchen, bathrooms, hallways, closets and the stairs (counted on the floor they go down from). Finished areas need a reasonable ceiling height; under ANSI Z765, a finished area needs a ceiling of at least 7 ft over at least half of it, and any part under 5 ft doesn't count.

Report these separately rather than adding them in: the garage, unfinished space such as a utility room or storage attic, covered porches and three-season rooms, and any level that is partly or completely below ground, even if it's finished. Our guide to below-grade vs. above-grade square footage explains the basement rules in detail.

On a two-storey home, don't count the open space of a two-storey foyer or great room on the upper floor; it's open to the floor below, not floor area.

Interior vs. Exterior Measurements

Adding up rooms gives an interior figure: it leaves out the thickness of every wall. ANSI Z765, the American standard used for appraisals that follow Fannie Mae's rules, measures a detached house from the outside of its exterior walls, floor by floor, so the official figure is larger.

The difference is easy to estimate. A simple 40 ft by 30 ft house is 1,200 sq ft measured outside. With 6-inch exterior walls, the space inside them is 39 ft by 29 ft, or 1,131 sq ft, so the outer walls alone account for about 69 sq ft, before any interior walls are subtracted. Neither number is wrong; they measure different things, so say which one you're quoting.

A Worked Example

Take a small single-storey house. The living room is 16 × 14 ft (224 sq ft), the kitchen 12 × 11 ft (132 sq ft), the main bedroom 13 × 12 ft (156 sq ft), the second bedroom 11 × 10 ft (110 sq ft), the bathroom 8 × 5 ft (40 sq ft) and the hallway 12 × 4 ft (48 sq ft). Added together, that's 710 sq ft of interior living area.

The attached garage, 20 × 20 ft (400 sq ft), is listed separately, not added to the living area. If you need materials for one room, carry its own area into the flooring calculator or the paint calculator rather than using the house total.

The same example shows why the room list matters: a buyer, an appraiser or a flooring installer can check each line, which they can't do with a single number.

Measuring Faster With a LiDAR Scan

On an iPhone or iPad Pro with LiDAR, vPlan AR scans each room in one to two minutes and works out its area from the scanned outline, so irregular rooms and angled walls need no extra arithmetic. Room dimensions are typically within 1–2 cm (under an inch) of a tape measure when scanned carefully. Our guide to scanning a whole house covers multi-room and multi-level projects.

On Pro and Business, set each room's area type (living, below-grade finished, garage, unfinished or covered) and export a square footage report that totals each category, with the plan and a room-by-room breakdown. Open a sample square footage report to see what it includes. Because a scan measures the inside of the rooms, the report is an interior, ANSI Z765-style breakdown rather than the exterior figure an appraiser measures.

Which Number to Use Where

For flooring, paint and renovation budgets, use interior room areas; they're what the materials actually cover. For a listing, follow your MLS rules and say how the figure was measured; our ANSI Z765 GLA report guide covers what agents should and shouldn't claim. For an appraisal or a mortgage, the appraiser's exterior measurement is the official number.

For insurance and home inventories, a room-by-room plan is more useful than any single total, because a claim is settled room by room. Real estate agents can see how the whole workflow fits a listing on the real estate agents page.