How to Calculate How Much Paint You Need

Start with the wall area

Every paint calculation starts with the same measurement: the total surface area you are going to paint. For a rectangular room, measure the length and width of the floor, and the height of the walls.

The wall area is the room's perimeter multiplied by its height. A room 4 m long and 3 m wide has a perimeter of 2 × (4 + 3) = 14 m. With 2.4 m ceilings, the total wall area is 14 × 2.4 = 33.6 m².

Subtract doors and windows

You don't paint the doors or windows, so subtract them. Unless they're unusual sizes, use these standard approximations:

• Standard door: 0.9 m × 2.1 m ≈ 1.9 m²
• Standard window: 1.2 m × 1.2 m ≈ 1.4 m²
• Large sliding door or feature window: measure it — width × height

Apply the number of coats

Two coats is the norm for repainting in a similar colour. Three coats are common when painting light over dark, over fresh plasterboard, or with a bold colour change. The paint maths is simply the per-coat amount multiplied by the coat count.

Divide by coverage

Every paint tin states its expected coverage in m² per litre per coat — most Australian interior acrylics fall between 10 and 16 m²/L. The figure varies with surface texture, porosity and colour.

For the example room at the conservative end (10 m²/L) with 2 coats: 30.3 ÷ 10 × 2 ≈ 6 litres of paint. Buy the nearest larger tin combination rather than trying to hit the number exactly — you will use extra on cutting in and touch-ups.

Common tin sizes in Australia

Interior paint is commonly sold in 1 L, 2 L, 4 L, 10 L and 15 L tins. A typical bedroom repaint lands around 4–6 litres; a full living room often needs a 10 L tin. Ceiling paint is bought separately if you are doing the ceiling.