How much paint do I need for a room?

Buying paint by guesswork usually ends one of two ways: a half-painted wall on Sunday evening, or three unopened cans in the garage. Both are avoidable with ten minutes and a tape measure. Here is the method, with a worked example you can repeat in the paint calculator.

Step 1: measure the walls

For a rectangular room, the quickest way is the perimeter times the height:

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

Measure the height from floor to ceiling, not to the top of the skirting. If the walls are not all the same height, as under a sloped ceiling, measure each wall separately and add them up.

Step 2: subtract doors and windows

Large openings make a real difference. Measure each one, or use typical sizes as a rough guide:

• a standard interior door: about 21 ft² (2 m²)
• an average window: about 15 ft² (1.4 m²)

Small openings such as a single light switch are not worth subtracting; leaving them in gives you a small, useful margin.

Step 3: check the coverage on the can

Coverage depends on the product, the colour and the surface. The label gives it as square feet per gallon or square metres per litre, for one coat. Many interior wall paints state somewhere around 350–400 ft² per gallon (roughly 9–10 m² per litre), but the label of your paint is the only number that matters.

Rough, porous or patched surfaces soak up more paint than the label assumes. So do strong colour changes, such as painting a light colour over a dark one.

Step 4: decide on the number of coats

Two coats is the usual recommendation for an even finish. Fresh plaster or drywall generally needs primer or a mist coat first, and a dramatic colour change may need a third coat. Multiply the area by the number of coats.

Worked example: a 12 × 14 ft bedroom

The room is 12 × 14 ft with an 8 ft ceiling, one door and two windows.

1. Wall area: (12 + 14) × 2 × 8 = 416 ft²
2. Openings: one door (21 ft²) and two windows (2 × 15 = 30 ft²) = 51 ft²
3. Paintable area: 416 − 51 = 365 ft²
4. Two coats: 365 × 2 = 730 ft²
5. Coverage of 375 ft² per gallon: 730 ÷ 375 = 1.95 gallons

So two gallons for the walls. Because 1.95 is very close to 2, a textured wall or a darker existing colour could tip it over. In that case two gallons plus a spare quart is a sensible buy.

The ceiling is a separate job, often with a different paint: 12 × 14 = 168 ft², or 336 ft² for two coats, which is just under one gallon.

Metric example: a 4 × 3.5 m living room

Ceiling height 2.6 m, one door and one large window (2.5 m²).

1. Wall area: (4 + 3.5) × 2 × 2.6 = 39 m²
2. Openings: 2 + 2.5 = 4.5 m²
3. Paintable area: 34.5 m²
4. Two coats: 69 m²
5. Coverage 10 m² per litre: 69 ÷ 10 = 6.9 litres

Paint is often sold in 2.5 and 5 litre tins, so this is one 5 L tin plus one 2.5 L tin, with a little left for touch-ups.

Tips that save paint

• Stir thoroughly and box the paint: mix all cans of the same colour together in a bucket so small batch differences do not show.
• Keep the leftover. Write the room and date on the lid. Touch-ups with the original paint blend in far better than a new mix.
• Don’t over-thin. Thinning spreads paint further but hides less, so you may need an extra coat after all.

The BuildCalc paint calculator does this arithmetic for several walls at once and rounds the result up to whole gallons.