Wall area calculator: walls, floor and ceiling of a room in m²

The wall area of a room is its perimeter times the ceiling height, less doors and windows: a 4 × 3.5 m room with a 2.50 m ceiling has 15 × 2.50 = 37.5 m² of wall, plus 14 m² of floor and the same again of ceiling.

Perimeter × height: the wall area

The walls of a room form a band that runs all the way round. Its length is the perimeter, twice the length plus twice the width, and its height is the ceiling height. For a 4 × 3.5 m bedroom, the perimeter is 2 × (4 + 3.5) = 15 m; with a 2.50 m ceiling, the walls come to 15 × 2.50 = 37.5 m².

The floor and the ceiling have the same area, length times width: 4 × 3.5 = 14 m². Walls and ceiling together come to 37.5 + 14 = 51.5 m², the area to paint when you redecorate the whole room.

Areas of rectangular rooms with a 2.50 m ceiling

Gross areas, before taking off doors and windows. Example: a 4 × 3.5 m bedroom has a 15 m perimeter; with a 2.50 m ceiling, its walls come to 15 × 2.50 = 37.5 m², or 51.5 m² with the ceiling.

Taking off doors and windows

Each opening comes off as its width times its height. Measure them rather than assuming a size: internal doors, French doors and windows all vary. In the example bedroom, a 0.90 × 2.10 m door (1.89 m²) and a 1.20 × 1.25 m window (1.50 m²) take off 3.39 m², leaving 34.11 m² of wall.

For skirting, only the width of the doorways comes off the perimeter: 15 − 0.90 = 14.10 m. A window does not interrupt the skirting, but a French door does, so enter it as a door.

An L-shaped room

An L-shaped room is described by its outer rectangle and the corner that is missing. The floor is the large rectangle less the missing corner: 6 × 5 m with a 2 × 2 m corner missing gives 30 − 4 = 26 m². The perimeter does not change: the two walls around the missing corner are exactly as long as the sides they replace, so it stays at 2 × (6 + 5) = 22 m. With a 2.50 m ceiling, that is 55 m² of wall before openings.

From square metres to materials

Once the area is worked out, the buttons under the result open the calculator you need with the figure already filled in: the net wall area for paint or wallpaper, the floor area and skirting length for tiles or flooring, and the ceiling area for a plasterboard ceiling. The coverage of the paint or the number of tiles per m² is then set in each calculator, from the product's data sheet.

Special cases

• A wall under a sloping ceiling: work it out separately as a trapezium, (low height + high height) ÷ 2 × length, and add it on. A 4 m wall rising from 1 m to 2.50 m is (1 + 2.50) ÷ 2 × 4 = 7 m².
• A ceiling that changes height: use the average height of each wall.
• An irregular room: split it into rectangles, work out each one and add the floors together; measure the perimeter with a tape along the walls.
• Small openings such as sockets or air bricks: their area is negligible, so there is no need to take them off.

Mistakes that throw the area out

• Multiplying length by width by height: 4 × 3.5 × 2.5 = 35 is a volume in m³, not the wall area (37.5 m²).
• Leaving out two of the four walls: length plus width is only half the perimeter.
• Taking windows off the skirting length.
• Measuring the height under a beam or a bulkhead that only covers part of the room.

With these areas, the paint calculator gives the litres to buy and the tile calculator the number of tiles. For skirting, the mitre angle calculator gives the cut for each corner, and the cubic metres to tonnes converter helps with the sand for a screed.