October 24, 2026 · Building & Materials · 5 min read
How Much Paint Do You Need? A Room-by-Room Guide
Nothing stalls a painting project like running out two-thirds of the way through — or buying three unopened tins that the store won't take back. Paint quantity is one of the easiest estimates in home improvement once you know the method decorators use.
Step 1: Measure the walls
For each wall, multiply width by height and add the results. A typical 4 m × 5 m room with 2.5 m ceilings has about (4 + 5) × 2 × 2.5 = 45 m² of wall, plus the ceiling at 20 m². Height mistakes are the most common error — newer builds often have 2.7 m ceilings, older houses 2.3 m.
Step 2: Subtract the openings
- A standard door: about 2 m²
- A standard window: about 1.5 m²
In our example room, one door and two windows remove 5 m², leaving 40 m² of wall.
Step 3: Multiply by coats
Two coats is the standard for most repaints; a drastic color change or bare plaster can take three or a separate primer. Two coats on 40 m² of wall means 80 m² of coverage to deliver.
Step 4: Divide by real coverage
Tins claim 10–14 m² per litre, but real-world coverage depends on surface texture, roller nap, application thickness and color contrast. A safe planning figure is 10 m² per litre. For our room:
That's two 5-litre tins with a comfortable margin, or roughly 3 gallons in US terms.
Cost and sanity checks
Multiply litres by price per litre for the paint cost, and remember rollers, brushes, tape and drop cloths typically add 15–20% to the budget. Glossy and dark paints often need an extra coat; light colors over dark ones almost always do. And if the estimate lands on an awkward number like 7.4 litres, round up — leftover paint is useful for touch-ups, a shortfall is not.
Planning more than paint? Tile Math: How Many Tiles and Boxes Do You Really Need? uses the same measure-subtract-waste method for flooring.