How Much Paint Do I Need? Room Calculation With a Worked Example

Buy too little paint and you are back at the shop halfway through a wall, hoping the new tin matches. Buy too much and it sits in the store room for years. A five-minute calculation gets you close enough to buy with confidence.

Step 1: measure the walls

Measure the length and width of the room and the wall height. For a rectangular room:

Wall area = 2 × (length + width) × height

A bedroom of 4.5 m × 3.6 m with walls 2.7 m high: 2 × (4.5 + 3.6) × 2.7 = 43.74 m².

Step 2: subtract doors and windows

You don’t paint glass or doors (or if you do, it is a different paint). Typical allowances are about 1.9 m² for a door and 1.5 m² for a window, but measure yours if they are large.

One door and two windows: 1.9 + 1.5 + 1.5 = 4.9 m². Paintable area = 43.74 − 4.9 = 38.84 m².

Step 3: coats and coverage

Paint needed = area × number of coats ÷ coverage per litre

Two coats at 10 m² per litre: 38.84 × 2 ÷ 10 = 7.77 litres. Paint usually comes in 1, 4, 10 and 20 litre packs, so buy 8 litres, for example two 4-litre tins.

What changes the amount

• Surface: rough, textured or porous walls can need 20–30% more.
• Colour change: going from dark to light often needs a third coat or a primer.
• Ceiling: add length × width (16.2 m² here) if you are painting it too, usually in a separate ceiling paint.
• Method: spraying uses more paint than rollers; rollers use a little more than brushes.

Buying tips

• Check the coverage on the actual product you plan to buy; premium paints often cover more per litre.
• Buy all the paint for one room from the same batch if you can.
• Keep a small amount for touch-ups, sealed tightly and labelled with the room and shade.
• Get a quote for labour per square metre and compare it with the area you calculated.

Planning new floors as well? The Flooring Calculator and Tile Calculator use the same room measurements.

Frequently asked questions