Home & Construction⏱ 4 min read
How to Calculate Square Metres of a Wall (Any Shape)
Painting, tiling, or insulating a wall all need accurate square metre calculations. Here is how to measure standard walls, account for doors and windows, and handle sloped or triangular sections.
Getting the area of a wall right prevents costly over- or under-ordering of materials. These calculations are straightforward once you know the right approach for different wall shapes.
Standard Rectangular Wall
Area = Width x Height
Example: wall 4.5m wide, 2.4m high
Area = 4.5 x 2.4 = 10.8 m2
Deducting a door (0.9m x 2.1m):
Door area = 0.9 x 2.1 = 1.89 m2
Net area = 10.8 - 1.89 = 8.91 m2
Deducting a window (1.2m x 1.0m):
Window area = 1.2 x 1.0 = 1.20 m2
Net area = 8.91 - 1.20 = 7.71 m2
Add 10% for waste (tiles, wallpaper) or 5% (paint):
Paint: 7.71 x 1.05 = 8.10 m2
Tiles: 7.71 x 1.10 = 8.48 m2
Wall With a Sloped Ceiling (Loft / Attic)
Treat as a rectangle plus a triangle:
Rectangle portion: width x straight wall height
Triangle portion: (base x height of triangle) / 2
Example: 5m wide wall, 1.8m straight section, then slopes to 3.0m at apex
Triangle height = 3.0 - 1.8 = 1.2m
Rectangle: 5 x 1.8 = 9.0 m2
Triangle: (5 x 1.2) / 2 = 3.0 m2
Total: 12.0 m2
Staircase Wall (Parallelogram)
A staircase wall is a parallelogram — a rectangle tilted sideways.
Area = base width x perpendicular height
(not the slant height, the vertical height)
Or use: Area = Width x Average height
Average height = (tallest point + shortest point) / 2
Staircase wall: 4.5m wide, 1.0m at bottom, 2.8m at top
Average height = (1.0 + 2.8) / 2 = 1.9m
Area = 4.5 x 1.9 = 8.55 m2
Entire Room Wall Area (For Painting)
Calculate all 4 walls as rectangles, subtract openings:
Room: 4m x 3m, 2.4m ceiling
Wall 1 (4m): 4 x 2.4 = 9.6 m2 (has door: -1.89) = 7.71 m2
Wall 2 (3m): 3 x 2.4 = 7.2 m2
Wall 3 (4m): 4 x 2.4 = 9.6 m2 (has window: -1.20) = 8.40 m2
Wall 4 (3m): 3 x 2.4 = 7.2 m2
Total wall area: 7.71 + 7.2 + 8.4 + 7.2 = 30.51 m2
Add ceiling if painting (4 x 3 = 12.0 m2):
Total to paint: 42.51 m2
Standard paint coverage: 10-12 m2/litre (first coat)
8-10 m2/litre (second coat)
Litres for two coats: 42.51 / 10 + 42.51 / 9 = 4.25 + 4.72 = 8.97 litres
→ buy 10 litres (two 5-litre tins)