How to Calculate Square Footage for Odd-Shaped Rooms (and Square Metres)
Every flooring, paint or AC calculation starts with area. Here’s how to measure rooms that aren’t simple rectangles, with worked examples in feet and metres.
The one rule: split it into simple shapes
Any room, however awkward, can be broken into rectangles, triangles and parts of circles. Measure each piece, work out its area, and add them up. ENERGY STAR gives exactly this advice for sizing a room air conditioner: subdivide complex rooms into squares, rectangles and triangles, then add the areas.
Rectangle = length × width Triangle = base × height ÷ 2 Circle = π × radius² (half-circle: ÷ 2) Trapezoid = (side a + side b) ÷ 2 × height
Feet and inches: convert first
Turn inches into decimal feet before you multiply: divide inches by 12. ENERGY STAR’s example is a room 14 ft 9 in long by 10 ft wide. 9 in is 0.75 ft, so the area is 14.75 × 10 = 147.5 ft². Multiplying 14.9 × 10 by mistake gives 149, and the error grows on bigger rooms. Useful conversions: 3 in = 0.25 ft, 6 in = 0.5 ft, 9 in = 0.75 ft.
In metric, measure in metres to two decimal places (e.g. 3.65 m). To convert between systems: 1 m² = 10.764 ft² and 1 ft² = 0.0929 m², from the exact definition 1 ft = 0.3048 m.
Worked examples
An L-shaped room
Picture a living room that is 20 ft along the long wall. The first 12 ft of it is 14 ft deep, and the remaining 8 ft is only 10 ft deep. Split it with an imaginary line:
- Part A: 12 × 14 = 168 ft²
- Part B: 8 × 10 = 80 ft²
- Total: 248 ft²
Check it a second way. The bounding rectangle, 20 × 14 = 280 ft², minus the missing corner, 8 × 4 = 32 ft², is 248 ft². When both methods agree you can trust the measurements.
A room with an angled wall
A bedroom is 12 ft wide. One side is 14 ft long and the other 10 ft, because one corner is cut off at an angle. That is a trapezoid: (14 + 10) ÷ 2 × 12 = 144 ft². Or split it into a 12 × 10 rectangle (120 ft²) plus a triangle 12 ft by 4 ft (12 × 4 ÷ 2 = 24 ft²), which gives the same 144 ft².
A bay window
A semicircular bay with a 6 ft opening has a radius of 3 ft: π × 3² ÷ 2 = 14.1 ft². Add that to the main room. A three-sided (box or angled) bay is a trapezoid instead.
Metric: an open-plan kitchen-diner
Kitchen 3.2 × 2.8 m = 8.96 m², dining area 4.1 × 3.5 m = 14.35 m², total 23.31 m² (about 251 ft²).
Which area do you need?
- Flooring and tile: the floor area, plus closets and alcoves. Then add a waste allowance; the tile and flooring calculator does both.
- Paint: wall area, which is perimeter × height minus doors and windows. The floor area doesn’t help here. Use the paint calculator.
- Room air conditioners: the floor area of the space to be cooled. The ENERGY STAR chart in our BTU calculator is based on it.
- Mulch, gravel and concrete: area × depth gives the volume to order.
Measuring tips that save money
- Measure wall to wall at floor level, not at the skirting or baseboard top, for flooring.
- Measure twice, at both ends. Rooms are rarely perfectly square. If the two widths differ, use the larger for buying materials.
- Sketch the room with each measurement written on the drawing. It makes the split into shapes obvious and catches missing pieces.
- Round up at the end, not the start. Keep decimals through the arithmetic and round only the final quantity you buy.
- A laser measure is quicker and more accurate than a tape for long runs, but a tape is fine if you hold it level and straight.
Frequently asked questions
How do I calculate square footage of an L-shaped room?
Split it into two rectangles, multiply length × width for each, and add them. For example, 12 × 14 + 8 × 10 = 248 ft².
How do I convert feet and inches for square footage?
Divide the inches by 12 to get decimal feet before multiplying. 14 ft 9 in is 14.75 ft.
How do I convert square feet to square metres?
Multiply square feet by 0.0929, or divide square metres into square feet by multiplying by 10.764.
How do I measure a room with an angled wall?
Treat it as a trapezoid, (long side + short side) ÷ 2 × width, or split it into a rectangle and a triangle and add them.