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Paint and Wallpaper Calculator

Enter a room's length, width and wall height, plus doors, windows, coats and the coverage on the tin, to see how much paint to buy and the best mix of tins — or switch to wallpaper for the number of rolls. A 4 × 3 m room with 2.5 m walls and two coats needs about 5.8 litres.

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How to use the Paint and Wallpaper Calculator

  1. 1Choose Paint or Wallpaper, and metric or US units.
  2. 2Enter the room's length, width and wall height.
  3. 3For paint, add doors, windows, coats and the coverage printed on the tin; for wallpaper, the roll size and pattern repeat.
  4. 4Read how much to buy and which tins or how many rolls.

How it works

Wall area is the room's perimeter multiplied by its height: 2 × (length + width) × height. Doors and windows are subtracted using typical sizes you can change, and the ceiling can be added. The paint needed is that area multiplied by the number of coats, divided by the paint's coverage, plus an allowance for waste.

Coverage is printed on the tin, usually as square metres per litre or square feet per gallon. Interior wall paints typically cover about 10–14 m² per litre, or 350–400 ft² per gallon, on a smooth surface that has been painted before. Bare plaster, rough textures and big colour changes use more.

The tin suggestion searches every combination of standard sizes — 10, 5, 2.5 and 1 litre, or 5-gallon buckets, gallons and quarts — for the fewest containers that cover what you need.

Wallpaper is counted in drops, the full-height strips. Each drop is the wall height plus the pattern repeat plus a trimming allowance; a roll gives as many whole drops as fit its length; and the room needs as many drops as the roll width divides into the perimeter. Doors and windows are not subtracted, because offcuts rarely match the pattern.

The formula

paint = (2 × (L + W) × H − doors − windows) × coats ÷ coverage × (1 + waste)
L, W, H
room length, width and wall height
coverage
the area one litre or gallon covers in one coat
waste
the allowance for spills, rollers and touch-ups

Wallpaper rolls = ⌈perimeter ÷ roll width⌉ ÷ ⌊roll length ÷ (height + repeat + trim)⌋, rounded up.

Worked example

A 4 × 3 m bedroom with 2.5 m walls, one door, one window and two coats

  1. Walls: 2 × (4 + 3) × 2.5 = 35 m².
  2. Minus a 1.9 m² door and a 1.5 m² window: 31.6 m².
  3. 31.6 m² × 2 coats ÷ 12 m² per litre = 5.27 L, plus 10% = 5.79 L.

Result: About 5.8 litres — one 5 L tin and one 1 L tin.

Tips and common mistakes

  • Use the coverage printed on the tin you plan to buy; it varies between paints.
  • Allow for more paint on new plaster, rough surfaces and strong colour changes, or use a primer first.
  • Buy all the paint or wallpaper for a room at once; batches can differ slightly in colour.
  • Keep a little left over for touch-ups, labelled with the room and the colour name.

Frequently asked questions

How much does a litre of paint cover?
Typically 10–14 m² per coat for interior wall paint on a smooth surface. The exact figure is printed on the tin.
Do I need two coats?
Usually. A second coat evens out colour and sheen, and is almost always needed when changing colour significantly or painting new plaster.
Does it include the ceiling?
Tick Include the ceiling to add length × width. Ceilings are usually painted with a different, flat paint, so you may prefer to work them out separately.
How many extra rolls of wallpaper should I buy?
The calculation already ignores doors and windows, which leaves some spare. Buy one more roll from the same batch number if you want room for mistakes or later repairs, because rolls from different batches can differ slightly in colour.
How do I measure a room for paint?
Measure the length and width of the floor and the height of the walls. The wall area is the perimeter, 2 × (length + width), multiplied by the height. A 4 × 3 m room with 2.5 m walls has 35 m² of wall.
Should I buy extra paint?
The calculation already adds a waste allowance. Any small amount left over is useful for touch-ups later.
What is a pattern repeat on wallpaper?
The vertical distance before the pattern starts again. Each strip has to be cut so the pattern lines up with its neighbour, so a larger repeat means more waste and more rolls.
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