Calcylator
Room Area

Square metres of a room:
length × width

Measure a room, convert millimetres or feet, handle L-shapes and alcoves, and turn the result into square feet when a supplier uses them.

Calcylator Editorial Team

Updated · 6 min read

How to calculate the square metres of a room

To calculate the square metres of a room, measure its length and width in metres and multiply them. A room 4.2 m long and 3.5 m wide has an area of 4.2 × 3.5 = 14.7 square metres, written 14.7 m².

Square metres tell you how much floor or ceiling surface there is. It is the number that flooring, paint, carpet and underlay suppliers use, so getting it right before you shop avoids paying for material you cannot use.

Room area formula and example

Area of a rectangular room =area = length × width
length:
longest wall, in metres
width:
shorter wall, in metres
area:
floor area in square metres
The rectangle area calculator multiplies the two values and reports square units; if you enter metres, the answer is in square metres.

  • Length

    4.2 m

  • Width

    3.5 m

Floor area

14.7 m²

4.2 × 3.5 = 14.7 square metres.

  1. Measure the longest wall at floor level, wall to wall.
  2. Measure the width at its widest point.
  3. Convert both to metres, for example 4,200 mm is 4.2 m.
  4. Multiply the two numbers.

L-shaped rooms, alcoves and bay windows

Rooms are rarely perfect rectangles. Split an L-shape into two rectangles, find each area, then add them. A room made of a 5 m by 3 m main part and a 2 m by 1.5 m alcove is 15 + 3 = 18 m² in total.

Adding up a split room
PartLength (m)Width (m)Area (m²)
Main room5315
Alcove21.53
Total18

Walls are not always straight. Measure the length and width at two or three points, and if the readings differ by more than a couple of centimetres, use the larger values for ordering and note the difference for the fitter. Also measure with a steel tape rather than a cloth one, which can stretch.

Once you have the floor area, you can reuse it. A flat ceiling has the same area as the floor, and underlay, carpet and floor paint are all priced per square metre, so one careful measurement serves several purchases.

For an area you want to leave out, such as a fitted wardrobe or a kitchen island, work out its rectangle and subtract it from the total, if the flooring will not run underneath.

Converting to square feet and other units

One square metre is about 10.764 square feet. Your 14.7 m² room is therefore about 158.2 square feet, and the 18 m² L-shaped room is about 193.8 square feet. Multiply by 10.764 to go from square metres to square feet, or divide by it to come back.

  • Millimetres: divide each measurement by 1,000 first, so 4,200 mm becomes 4.2 m.
  • Centimetres: divide by 100 before multiplying.
  • Feet: multiply the two lengths in feet to get square feet, then divide by 10.764 to get square metres.

Common mistakes when measuring a room

  • Mixing units, such as a length in metres and a width in centimetres, which gives a nonsense answer.
  • Measuring along skirting boards that curve around chimney breasts, rather than at a clear floor line.
  • Forgetting that area is not perimeter. A 4.2 by 3.5 room has a perimeter of 15.4 m but an area of 14.7 m².
  • Ordering exactly the room area. Flooring and tiles need an allowance for cutting and waste, often around 10 percent.

Common questions

How do you calculate the square metres of a room?

Multiply the room's length by its width, both in metres. A room measuring 4.2 m by 3.5 m has an area of 14.7 square metres. For L-shaped rooms, split the shape into rectangles and add their areas.

How do I calculate square metres from millimetres?

Divide each measurement in millimetres by 1,000 to get metres, then multiply. A room 4,200 mm by 3,500 mm is 4.2 m by 3.5 m, which gives 14.7 square metres. Converting before multiplying avoids mistakes with the zeros.

How many square feet are in a square metre?

One square metre is about 10.764 square feet. A 14.7 m² room is therefore roughly 158.2 square feet. To convert square feet to square metres, divide by 10.764, which gives the same room in the unit your supplier uses.

How do I measure an L-shaped room?

Divide the L into two rectangles, measure and multiply each, then add the areas. A 5 m by 3 m part plus a 2 m by 1.5 m part gives 15 + 3 = 18 square metres.

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