How Do I Work Out the Square Metres of a Room?
Multiply length by width in metres for a rectangle. For L-shapes and alcoves, split the room into rectangles and add them. What to measure and how much extra to order.
In short
For a rectangular room, multiply the length by the width in metres: a room 4.2 m by 3.5 m is 14.7 square metres. For an irregular room, divide it into rectangles, calculate each and add them together. Measure in metres to two decimal places, take each measurement at the widest point, and add 5 to 10 percent when ordering flooring or tiles for cuts and waste.
Floor area is the number every quote, every flooring order and every paint tin depends on, and it is straightforward to get right if you measure in the right units and split the room sensibly.
The basic rectangle
Length multiplied by width, both in metres.
A room 4.2 m long and 3.5 m wide: 4.2 × 3.5 = 14.7 m².
Measure in metres from the start. Measuring in centimetres and converting at the end works but invites a factor-of-a-hundred slip, and measuring in feet and inches makes the multiplication genuinely awkward.
Irregular rooms
Most rooms are not clean rectangles. There is an alcove, a chimney breast, a bay window or an L-shape. The method is the same every time: divide the floor into rectangles, calculate each, add them up.
An L-shaped room that is 5 m by 4 m with a 2 m by 1.5 m section cut out of one corner:
- Full rectangle: 5 × 4 = 20 m²
- Missing corner: 2 × 1.5 = 3 m²
- Room: 20 minus 3 = 17 m²
Or measure it as two rectangles directly and add them. Both routes give the same answer.
A bay window adds a small area. Measure its depth and width and treat it as an extra rectangle, or a half-hexagon if you want to be precise, which is rarely worth it.
Where to measure
Wall to wall at floor level, at the widest point, and in at least two places along each dimension. Rooms are rarely square, and a difference of 5 cm between the two ends is common. Use the larger figure for ordering.
Skirting boards reduce the visible floor slightly. Measure from wall to wall, not skirting to skirting, since flooring goes under or up to the skirting depending on the type.
Converting units
- Square feet to square metres: divide by 10.764
- Square metres to square feet: multiply by 10.764
- Square yards to square metres: multiply by 0.836
A 12 by 10 foot room is 120 sq ft, which is 11.15 m².
Ordering material
Never order the exact area. Cuts, pattern matching, breakages and the offcut that turns out to be just too short all eat material.
- Laminate, vinyl planks, tiles: add 5 to 10 percent. Add 15 percent for diagonal layouts or complex rooms.
- Carpet: comes in fixed roll widths, usually 4 m or 5 m. The fitter needs the room dimensions, not just the area, because a 3.5 m wide room from a 4 m roll wastes half a metre along the whole length.
- Paint: area of the walls, not the floor. Perimeter times height, minus doors and windows.
A worked example
A living room measured at 5.1 m by 3.9 m, with a chimney breast 1.2 m wide projecting 0.4 m:
- Main area: 5.1 × 3.9 = 19.89 m²
- Chimney breast: 1.2 × 0.4 = 0.48 m²
- Floor: 19.41 m²
- Order for laminate with 10 percent: about 21.4 m², so round up to the pack size above that.
Write the measurements down and keep them. The next time anything needs replacing, the room will still be the same size.
Common questions
How do I convert square feet to square metres?
Divide square feet by 10.764, or multiply square metres by 10.764 to go the other way. A 150 square foot room is about 13.9 square metres.
Do I subtract fireplaces and built-in cupboards?
For flooring, only if they are large and the floor genuinely will not go under them. Small features are not worth the complication and the cuts around them waste material anyway. For carpet, measure the full rectangle and let the fitter cut.
What about a circular or odd-shaped area?
A circle is π times the radius squared. For genuinely irregular shapes, draw them on squared paper to scale and count squares, or break them into triangles: half the base times the height for each.
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