Start with the area: length × width for a floor, or the perimeter × height for walls, minus doors and windows. Then divide by the coverage of your product and add an allowance for waste. For a 5 m × 4 m room with 2.4 m walls, two coats of emulsion need about 6.4 litres, or 7.5 litres with 10% spare.
Step 1: measure the area
For a rectangular floor, area = length × width. For walls, add up the width of every wall to get the perimeter, then multiply by the height. Measure openings such as doors and windows so you can take them off. For an odd shape, break it into rectangles and triangles and add the results, or use the Area and square footage tool, which gives square metres, square feet and more.
If you work in feet, remember 1 square metre is about 10.76 square feet.
How much paint do you need?
As a rule of thumb, one litre of emulsion covers around 12 square metres per coat, but coverage varies, so check the tin label.
Worked example with the Paint calculator tool: a room 5 m long, 4 m wide and 2.4 m high, with one door and two windows, walls only, two coats.
- Wall area: 2 × (5 + 4) × 2.4 = 43.2 m².
- Take off the door (1.85 m²) and two windows (1.4 m² each): about 38.6 m².
- Two coats: 77.2 m² ÷ 12 m² per litre = 6.4 litres.
- With 10% spare: 7.5 litres.
If you also paint the ceiling (20 m²), the total rises to 9.8 litres, or 11 litres with spare. Rough, dark or very porous surfaces may need an extra coat or a primer.
How much wallpaper do you need?
Wallpaper is bought in rolls, but you hang it in vertical strips, called drops. A standard UK roll is commonly 10.05 m long and 0.53 m wide. The pattern repeat matters because you must match the pattern at each join, which wastes paper.
The Wallpaper calculator tool works like this:
- Each drop is the wall height plus 10 cm for trimming, rounded up to the pattern repeat.
- Number of strips = total wall length ÷ roll width, rounded up.
- Strips per roll = roll length ÷ drop length, rounded down.
- Rolls = strips ÷ strips per roll, rounded up.
Example: 18 m of wall, 2.4 m high, one door and two windows. With a 32 cm repeat each drop is 2.56 m and you get 3 strips per roll, so you need 11 rolls. With a plain paper (no repeat) each drop is 2.5 m and you get 4 strips per roll, so 8 rolls are enough. The repeat adds three rolls in this case, which shows why it matters. Buy one spare roll and make sure they come from the same batch.
How many tiles do you need?
Divide the area by the area of one tile, including the grout gap, then add waste. Allow about 10% for straight layouts and 15% for diagonal or patterned layouts, because of cuts and breakages.
Example with the Tile calculator tool: a 4 m × 3 m floor (12 m²) with 60 cm × 30 cm tiles, 3 mm grout gaps and 10% waste.
- Tiles without waste: 66.
- With 10% waste: 73 tiles.
- Boxes of 6: 13 boxes, which is 78 tiles.
Tiles are sold in boxes, so you always round up to whole boxes. Keep a few spares for future repairs, because a later batch may differ slightly in colour.
Common mistakes to avoid
- Forgetting a second coat of paint or the ceiling.
- Not taking off enough (or taking off too much) for large windows and alcoves.
- Ignoring the pattern repeat on wallpaper.
- Buying exactly the number of tiles with no waste allowance.
- Mixing batches of paint, wallpaper or tiles, which can differ in shade.
Working out the waste allowance is a simple percentage calculation, which the guide on How to calculate percentages explains.
Note: These are estimates for planning. Always check coverage on the product label and ask your supplier if you are unsure, especially for expensive materials.
Try the calculator
Use Paint calculator, Wallpaper calculator or Tile calculator to get quantities for a room, and Area and square footage to measure irregular spaces.
Open the free Paint calculator
Frequently asked questions
How much waste should I allow for tiles?
About 10% for a straight layout and 15% or more for diagonal or patterned layouts.
How many square metres does a litre of paint cover?
Around 12 square metres per coat for typical emulsion, but check the tin because coverage varies by brand and surface.
How wide and long is a standard UK wallpaper roll?
Commonly 0.53 m wide and 10.05 m long, though some papers differ, so check the label.
Do I need to subtract windows and doors?
Yes, for paint and wallpaper it saves money. For tiles, subtract only areas that will not be tiled.