How to Calculate Square Footage of L-Shaped and Irregular Rooms
Split L-shaped, angled and curved rooms into simple shapes, measure them accurately, and add or subtract areas to get square footage you can buy flooring from.
A rectangular bedroom takes two measurements. An open-plan kitchen with a bay window, an angled wall and an island takes a little more thought. The method is always the same: break the room into shapes you know how to calculate, then add and subtract.
The four shapes you need
| Shape | Area formula | Where you'll see it |
|---|---|---|
| Rectangle | length × width | Most rooms and alcoves |
| Triangle | base × height ÷ 2 | Angled walls, corner cut-offs |
| Trapezoid | (side a + side b) ÷ 2 × height | Bay windows, rooms with one angled wall |
| Circle or half circle | 3.1416 × r² (÷ 2 for half) | Rounded bays, curved alcoves |
L-shaped rooms: two ways
Add two rectangles. Draw a line that splits the L into two rectangles, measure each and add them.
Subtract the missing corner. Measure the overall length and width as if the room were a full rectangle, then subtract the rectangle that is "missing." The square footage calculator has an L-shape option that does this.
Example: a room is 18 ft by 15 ft overall, with a 6 × 5 ft corner taken by a closet. Overall area: 18 × 15 = 270. Missing corner: 6 × 5 = 30. Room area: 240 sq ft. Splitting it instead into a 12 × 15 ft piece (180) and a 6 × 10 ft piece (60) gives the same 240.
Rooms with an angled wall
If one wall runs at an angle, the room is a trapezoid. Measure the two parallel walls (the long and short sides) and the distance between them, measured straight across at right angles. A room whose parallel walls are 14 and 10 ft, 12 ft apart, has an area of (14 + 10) ÷ 2 × 12 = 144 sq ft.
Bay windows and curves
A typical three-sided bay window is a trapezoid stuck onto the room: measure the width where it meets the room, the width of the front window and how far it projects. A curved bay is close to half a circle; measure the opening width as the diameter. Bays are often small enough that a quick estimate is fine, but include them if you are flooring into the bay.
What to subtract
- Kitchen islands and built-in cabinets if the flooring will not run underneath. Many installers lay flooring under islands anyway; ask before subtracting.
- Fireplace hearths that will stay in place.
- Stair openings on upper floors.
Do not subtract closets you intend to floor, or small obstacles like radiators or vents.
Measuring tips that prevent errors
- Measure at floor level, because walls are not always perfectly vertical.
- Measure each wall twice, at different heights or ends, and use the larger number.
- Check that a "rectangular" room is square by measuring both diagonals; if they differ by more than an inch or two, plan extra flooring for cuts.
- Convert inches to decimals before multiplying: 11 ft 9 in is 11.75 ft.
Worked example: an open-plan kitchen and dining area
| Piece | Dimensions | Area |
|---|---|---|
| Kitchen | 14 × 12 ft rectangle | 168 sq ft |
| Dining area | 12 × 11 ft rectangle | 132 sq ft |
| Bay window | trapezoid: 8 ft and 5 ft sides, 2 ft deep | 13 sq ft |
| Island (subtract) | 6 × 3 ft | −18 sq ft |
| Total | 295 sq ft | |
For flooring, you would then add a waste allowance; our article on the flooring waste factor explains how much.
Going further
If you are covering the floor with something poured or spread, such as self-leveling compound or concrete, multiply the area by the thickness to get a volume. The concrete calculator handles slabs, and the cubic yard calculator works for loose fill.
Further reading from official sources
- Characteristics of New Housing (floor area data) – U.S. Census Bureau
- Unit conversion: area units – National Institute of Standards and Technology (NIST)
