How to measure the floor area of a room
Floor area is length × width. Anything that is not a rectangle is a few rectangles pushed together, so split it up, work each one out and add them.
Few rooms are one clean rectangle, though. Add a chimney breast, an alcove or a bay and you are juggling part-areas on a scrap of paper; drawing the shape as you measure does the adding up for you. Draw the room and read the area off →
Rectangular rooms
A room 4.2 m by 3.5 m is 4.2 × 3.5 = 14.7 m² (158 sq ft). Working in feet, turn any inches into a decimal first by dividing by 12 — 11 ft 6 in is 11.5 ft — because feet multiplied by inches gives a number that means nothing.
Measure at floor level along the skirting, not at waist height, and measure both walls of each pair. If they differ by a centimetre or two, use the longer for a flooring order; if by more than about 5 cm, the room is not a rectangle and the next section applies.
L-shaped and irregular rooms
Draw a line across the inside corner to cut the room into two rectangles, measure each one, and add the areas.
Rectangle A is 4.0 × 2.5 = 10.0 m², rectangle B is 2.0 × 2.5 = 5.0 m², so the room is 15.0 m². The same answer comes from measuring the full 5.0 × 4.0 rectangle, 20.0 m², and subtracting the missing 2.5 × 2.0 corner.
Alcoves, chimney breasts and bay windows
A chimney breast is a rectangle taken away, an alcove is one added back, a square bay is one added on. An angled bay is the only shape that will not split neatly, so treat it as a trapezium:
More than two or three steps in the wall? Rectangles on paper get error-prone fast. Walk the perimeter instead, measuring each wall in turn; the shape, and so the area, follows from that list. Tap the corners in floorarea and a mistyped wall shows up straight away: the room stops closing.
Square feet to square metres
Multiply square feet by 0.0929 for square metres, or square metres by 10.764 for square feet. For square yards it is 0.8361 and 1.196. Convert the finished area rather than each length, so you only round once. As a sense check, 9 m² is a little under 100 sq ft.
How much flooring to order
The floor area is not the amount to buy: every cut at a wall, doorway or pipe wastes material, and a box never divides evenly into a room.
| Flooring | Add to the measured area |
|---|---|
| Laminate, engineered wood or LVT laid straight | 5–10% |
| Planks laid diagonally | 15% |
| Herringbone or chevron | 15–20% |
| Large-format tiles | 10% |
| Carpet or sheet vinyl | roll width, see below |
A 15.0 m² room in straight-laid laminate needs 15.0 × 1.10 = 16.5 m², rounded up to whole boxes. Keep the spare box; it is the only way to match a damaged board later.
Carpet and sheet vinyl come off a roll of fixed width, usually 4 m or 5 m, so you pay for the room's longest dimension times the roll width. A 4.2 by 3.5 m room in 4 m carpet needs a 4.2 m length: 16.8 m², not 14.7 m².
Three mistakes worth avoiding
- Mixing units. A wall written as 350 next to one written as 4.2 is the usual reason an area comes out ten times too big.
- Measuring to the door frame. The flooring runs under the door to the threshold, so measure to where the floor of this room actually ends.
- Counting built-ins. A fitted wardrobe with no floor under it is not floor area. A bath is. Check what the trade you are ordering for needs.
Common questions
How do you calculate the floor area of a room?
Multiply the length of the room by its width. A room 4.2 m by 3.5 m has a floor area of 14.7 m². If the room is not a rectangle, split it into rectangles, work each one out and add them up.
How do you work out the area of an L-shaped room?
Draw a line across the inside corner to cut the L into two rectangles, measure each one and add the two areas. The same answer comes from measuring the full rectangle the room would be and subtracting the missing corner.
How many square feet are in a square metre?
One square metre is 10.764 square feet, and one square foot is 0.0929 square metres. Convert the finished area rather than each length, so you only round once.
Does a bay window count as floor area?
Yes, if there is floor in it. Treat a square bay as a rectangle added to the room. For an angled bay, average the width across the back and the width where it meets the room, then multiply by its depth.