Paint quantity calculator

How to use the calculator

1. Choose "By rooms" and enter length, width and height of each room plus doors and windows (or "Whole flat" to work from the area).
2. Enter the spreading rate from the paint data sheet or can, the number of coats and the waste.
3. Enter the pack sizes you will buy (for example 9 L, 2.7 L and 0.9 L).
4. Read the litres and the packs, and open "How we calculated" to see the formula with your numbers.

The formula

Litres = (wall area + ceiling area) x coats / spreading rate per coat (m²/L) x (1 + waste)

Wall area = room perimeter x height - openings. Ceiling area = length x width. The common mistake is to use the floor area: paint goes on the walls and ceiling.

Worked example: a 4 x 4 m room

Height 2.9 m, one door 0.9 x 2.1 m and one window 1.2 x 1.4 m, two coats on walls and ceiling, spreading rate 10 m²/L, waste 10%:

• Walls: 2 x (4 + 4) x 2.9 = 46.40 m², minus 1.89 (door) and 1.68 (window) = 42.83 m²
• Ceiling: 4 x 4 = 16 m², so the total is 58.83 m²
• Paint: 58.83 x 2 / 10 x 1.10 = 12.94 L
• Packs: two 9 L buckets (18 L), or one 9 L plus two 2.7 L (14.4 L)

For a 100 m² flat with a net ratio of 85% and a wall factor of 2.5: 212.5 + 85 = 297.5 m², about 65.45 L, so eight 9 L buckets.

Where does the spreading rate come from?

The spreading rate (coverage) is how many square metres one litre covers in one coat. It is on the product's technical data sheet or on the can. Published examples differ: one Jotun interior product page in Egypt lists 9.06 m²/L, a Jazeera product page lists 14 m²/L, and the sheets say the figure is theoretical and the real one is lower. So the default is a conservative 10; copy your own product's number. If the product is sold by weight, switch the unit to kg and enter the kg rate from its sheet.

Common mistakes

• Measuring the floor instead of the walls and ceiling.
• Forgetting coats: the sheet rate is usually per coat.
• Using the highest theoretical coverage figure.
• Mixing paint with filler and sealer: each has its own rate.
• Buying in several batches, which can show slight colour differences.

From quantity to cost

Add the price of the large pack and your labour rate per m² to get material and total cost. To price a whole flat item by item, use the finishing cost calculator (Arabic) and the tile calculator.