Square Footage of L-Shaped and Irregular Rooms

Every irregular room is a set of simple shapes. The skill is in choosing the split so each piece is easy to measure and nothing is counted twice. Below are the shapes you'll meet, each with worked numbers you can check against the square footage calculator.

How do you find the square footage of an L-shaped room?

There are two ways, and doing both is a good check. Add: split the L into two rectangles that only touch along a line, and add their areas. Subtract: take the rectangle that encloses the whole room and subtract the missing corner.

The missing dimensions come from the ones you have: the short leg of the L is 18 − 6 = 12 ft, and the deep part is 14 − 5 = 9 ft. If you measured walls in feet and inches, convert inches as twelfths (12'-6" is 12.5 ft, 12'-4" is 12.33 ft) before multiplying.

T-shaped and U-shaped rooms

Same idea with one more piece. A T-shaped great room with a 24' × 10' top bar and a 12' × 14' stem:

A U-shape is a box minus the notch in the middle. Whatever the shape, draw the split on a sketch before you measure, so you know which lengths you need.

How do you handle an angled wall?

An angled wall across a corner cuts a triangle out of the rectangle. Its area is half of base × height, measured along the two square walls, not along the angled wall itself.

If you only know the angled wall's length, and it runs at 45°, each leg of the triangle is that length ÷ 1.414: a 5'-8" angled wall has legs of about 4 ft. When a whole side of the room is angled, the room is a trapezoid: average the two parallel walls and multiply by the distance between them. A room 12 ft wide whose side walls are 15 ft and 18 ft long is (15 + 18) ÷ 2 × 12 = 198 sq ft.

Bay windows

A bay that extends the floor is usually a trapezoid. Measure its width at the wall, its width at the front and its depth, then average the two widths and multiply by the depth.

For a house's official square footage, ANSI Z765 counts a bay only if it has floor at that level; a window that projects from the wall above the floor is left out. See ANSI Z765.

Curved walls

Measure the room as if the curved wall were straight (along the chord, the straight line between its ends), then add the area of the curved part. For a half circle, that's π × r² ÷ 2: a bump with a 5 ft radius adds 39.3 sq ft. For a shallower arc, measure the chord and how far the curve bulges out from it (the rise):

The exact formula uses the radius: r = chord² ÷ (8 × rise) + rise ÷ 2, which is 7.25 ft here. The angle the arc spans is θ = 2 × asin(chord ÷ 2r), and the area is r² ÷ 2 × (θ − sin θ) with θ in radians. In practice the 2/3 estimate or eight segments is close enough: here the gap is under half a square foot.

Closets, bump-outs and alcoves

Treat each as its own rectangle and decide whether it belongs in the total. That depends on what the number is for.

• Flooring: include every closet and bump-out that gets the same floor, then add waste. See how much flooring you need.
• Room dimensions for a listing: usually the main rectangle, but practices vary. See room dimensions for the MLS.
• A house's total: you don't add rooms at all. Measure the outside walls, and closets come along automatically. See square footage from a floor plan.

Common mistakes

Doing it in FloorFigures

FloorFigures skips the splitting. It's free and runs in your browser. Open the plan, click inside the room (M), and the room finder traces it to the wall faces, however many corners it has. To draw one yourself, trace it corner by corner (N), typing exact wall lengths as you go, with a 45° lock for angled walls. The cutout tool subtracts areas such as a chimney or an opening. Curved walls are traced with short straight segments, which, as the table above shows, gets within about 2% of the curved part with eight of them.