How to Calculate How Much Paint You Need for Any Room (Without Wasting Money)

Buying too much paint wastes money. Buying too little means a mid-project trip back to the store — and hoping the new tin matches the old one. Getting the quantity right before you start is a five-minute calculation that saves you from both problems.

This guide walks through exactly how painters and contractors estimate paint quantities, what the formula looks like, and the common errors that lead people to buy the wrong amount.

What the Calculation Is Measuring

Paint coverage is calculated in square metres (or square feet) per litre. You measure the total paintable surface area of the room, divide by how much area a litre of your chosen paint covers, and that gives you the litres needed. Then you multiply by the number of coats.

The key word is paintable — windows, doors, and other openings don't get painted, so they're subtracted from your wall area before you calculate.

The Formula

Total Paintable Area = (Perimeter of Room × Wall Height) − (Area of Doors + Area of Windows)

Litres Required = Total Paintable Area ÷ Coverage per Litre × Number of Coats

Standard paint coverage varies by product:

• Economy / budget paint: 8–10 m² per litre
• Mid-range paint: 10–12 m² per litre
• Premium paint: 12–15 m² per litre

When in doubt, use 10 m² per litre as a safe estimate. Check the specific coverage rate on the tin you plan to buy — it's always printed on the label.

Step-by-Step Example

You're painting a bedroom with these dimensions:

• Room: 4 m × 3.5 m, walls 2.7 m high
• 1 door: 2 m × 0.9 m
• 2 windows: each 1.2 m × 1 m
• Planning 2 coats
• Paint coverage: 10 m² per litre

Step 1 — Perimeter of the room: (4 + 3.5 + 4 + 3.5) = 15 m

Step 2 — Total wall area before deductions: 15 m × 2.7 m = 40.5 m²

Step 3 — Area of openings: Door: 2 × 0.9 = 1.8 m² Windows: 2 × (1.2 × 1) = 2.4 m² Total openings: 4.2 m²

Step 4 — Net paintable wall area: 40.5 − 4.2 = 36.3 m²

Step 5 — Litres for one coat: 36.3 ÷ 10 = 3.63 litres

Step 6 — Litres for two coats: 3.63 × 2 = 7.26 litres

Round up to 8 litres to account for slight wastage and brush loading.

What the Result Means

Your calculation gives you the minimum paint needed under ideal conditions. The "round up" step isn't padding — it accounts for real-world factors: uneven roller loading, edge cutting, slight surface texture absorption, and touch-ups after the paint dries.

For walls with a rough or textured finish (like brick effect, stippled plaster, or exposed stone), add an additional 10–20% on top of your calculated amount. Texture dramatically increases the surface area relative to what a flat-wall formula assumes.

For ceiling paint, calculate the ceiling area separately (length × width) and treat it as a distinct surface — most ceiling paints have different coverage rates than wall paints.

Common Mistakes People Make

Skipping the deduction for doors and windows. On a standard bedroom wall, doors and windows can account for 10–15% of the total measured area. Ignoring them means buying noticeably more paint than you need.

Using a single coverage figure for all paints. Coverage varies significantly between paint types and brands. A high-hide premium emulsion might cover 14 m² per litre while a budget option covers 8 m². Always use the specific coverage rate printed on the tin you're actually buying — not a generic estimate.

Calculating for one coat when you need two. Most interior repaints over a changed colour require two coats for full, even coverage. Calculating for one coat and then discovering you need another part-way through — when you can't exactly match a new tin to an already-opened one — is a frustrating and avoidable problem. Decide on the number of coats before calculating.

When You Should Recalculate

Recalculate if you change the paint brand or product (different coverage rate), if you decide to add or skip a coat after starting, or if you're extending the project to an additional room. Each room should be calculated separately since dimensions, opening sizes, and surface conditions vary.

Related Calculators

• Use the Room Paint Calculator to enter your room dimensions and get the exact litres needed automatically
• Use the Wallpaper Calculator if you're considering wallpaper instead of paint for an accent wall
• Use the Tile Calculator to estimate tile quantities for flooring or a bathroom wall in the same project