How to Calculate Square Footage of a Room or House

Updated October 2, 2026

To find square footage, multiply length by width in feet. A 12 × 15 ft room is 180 sq ft. For anything that isn't a simple rectangle, split it into rectangles, triangles and circles, work out each one, and add them up. The square footage calculator lets you add shapes one at a time.

What you need

A 25 ft tape measure (or a laser measure for big rooms)
Paper to sketch the floor plan and write down each measurement

Rectangular rooms

square feet = length (ft) × width (ft)

If you measured in feet and inches, convert the inches to a decimal first: 12 ft 6 in = 12.5 ft. (The calculators on this site accept 12' 6" directly.)

L-shaped and irregular rooms

Split the room into rectangles that don't overlap:

Sketch the room and draw a line that cuts the L into two rectangles.
Measure each rectangle. For example 12 × 20 ft and 8 × 10 ft.
Add them: 240 + 80 = 320 sq ft.

Other shapes

Shape	Formula	Example
Rectangle	length × width	10 × 14 = 140 sq ft
Triangle	base × height ÷ 2	6 × 4 ÷ 2 = 12 sq ft
Circle	3.1416 × radius²	10 ft across: 3.1416 × 5² = 78.5 sq ft
Half circle	circle area ÷ 2	10 ft across: 39.3 sq ft

A bay window, for example, is often a rectangle plus two small triangles.

Square footage of a whole house

There are two common ways to count it, and they give different answers:

Outside measurement: appraisers and listings in the US commonly measure the exterior walls of the finished, heated living space, floor by floor. Garages, unfinished basements and open porches are usually listed separately.
Room by room: adding up each room's interior measurements gives a smaller number, because it leaves out the walls themselves. This is the number you want for flooring and paint.

So if your listing says 1,800 sq ft but your rooms add up to 1,600, nothing is wrong: the difference is mostly wall thickness, stairs and closets.

Converting square feet

To get	Do this
Square yards (carpet)	divide sq ft by 9
Square meters	multiply sq ft by 0.0929
Acres (land)	divide sq ft by 43,560
Roofing squares	divide sq ft by 100

Turning square feet into materials

Once you have the area, each project adds its own rules:

Flooring: add 10% for cuts, then divide by the coverage per box.
Paint: you need wall area, not floor area: perimeter × wall height.
Tile: add 10–15% and divide by the tile size.
Mulch and gravel: multiply by depth to get volume.