In short
Multiply the full length by the full width, as if the room were a rectangle, then subtract the missing corner. A 20 × 15 ft L-shaped room with an 8 × 6 ft corner missing is 300 − 48 = 252 sq ft.
Subtract the corner
Most L-shaped rooms are easiest as one big rectangle minus the corner that is not there. You measure the two long walls, and the two short walls of the missing corner.
Or add two rectangles
The same room as two rectangles: the narrow part, 12 × 6 ft = 72 sq ft, plus the full-width part, 20 × 9 ft = 180 sq ft. Together 252 sq ft, the same answer. Pick the split with the fewest measurements.
Examples
| Full size | Missing corner | Area |
|---|---|---|
| 12 × 10 ft | 4 × 4 ft | 104 sq ft |
| 14 × 12 ft | 6 × 4 ft | 144 sq ft |
| 16 × 14 ft | 6 × 6 ft | 188 sq ft |
| 20 × 15 ft | 8 × 6 ft | 252 sq ft |
| 24 × 18 ft | 10 × 8 ft | 352 sq ft |
| 30 × 20 ft | 12 × 10 ft | 480 sq ft |
How to measure an L-shaped room
- Measure the two longest walls: the full width and the full length.
- Measure the two short walls at the inside corner: that is the missing corner.
- Check: the long wall minus the short wall on the same side is the wall in between. If it is not, measure again.
- Type the four sizes in the calculator, or draw the room in the editor.
Figure 1. One floor of 25 × 17 ft. The rooms add up to 425 sq ft.
About this page · Written by the Squaremetry team. Last checked on October 1, 2026. Found a mistake? Send us a message.