Buying the right amount of paint saves you a wasted trip back to the store and avoids the frustration of running out mid-wall. Paint estimating is straightforward once you know the formula.
Step 1: Calculate Your Wall Area
For a rectangular room:
Wall area = 2 × (Length + Width) × Height
Example: Room 4m × 3.5m, ceiling height 2.4m:
Wall area = 2 × (4 + 3.5) × 2.4 = 2 × 7.5 × 2.4 = 36 m²
Step 2: Subtract Doors and Windows
Standard sizes to subtract:
| Feature | Approximate area |
|---|---|
| Interior door | 1.8 m² (0.8m × 2.1m) |
| Exterior door | 2.0 m² (0.9m × 2.1m) |
| Small window | 0.7 m² (0.6m × 1.2m) |
| Medium window | 1.2 m² (0.9m × 1.3m) |
| Large window | 1.8 m² (1.2m × 1.5m) |
Example: Room has 1 door and 2 medium windows:
Subtract: 1.8 + (2 × 1.2) = 1.8 + 2.4 = 4.2 m²
Net wall area: 36 − 4.2 = 31.8 m²
Step 3: Know Your Paint Coverage
Coverage rates vary by paint type:
| Paint type | Coverage per litre |
|---|---|
| Emulsion (standard) | 10–12 m² |
| Emulsion (thick/high-opacity) | 8–10 m² |
| Gloss/satinwood | 10–14 m² |
| Primer/undercoat | 8–10 m² |
| Masonry paint | 4–6 m² |
| One-coat paint | 7–9 m² |
Always check the specific product's datasheet — manufacturers list coverage on the label.
Step 4: Calculate Litres Needed
Litres needed = Wall area / Coverage per litre
Example: 31.8 m², standard emulsion (11 m²/litre):
Litres = 31.8 / 11 = 2.89 litres
Step 5: Account for Number of Coats
Most rooms need 2 coats — especially if you're covering a dark colour or using a pale colour.
Total litres = Litres per coat × Number of coats
Example: 2 coats:
Total = 2.89 × 2 = 5.78 litres
Round up to the nearest standard tin size — typically 5L and 2.5L.
Buy: 1 × 5L tin (5L) + 1 × 2.5L tin = 7.5L. You'll have some left over for touch-ups.
Complete Room Estimate (Walls + Ceiling)
Add the ceiling area:
Ceiling area = Length × Width = 4 × 3.5 = 14 m²
Total area (walls + ceiling):
31.8 + 14 = 45.8 m²
At 11 m²/litre, 2 coats:
Litres = 45.8 / 11 × 2 = 8.33 litres
Buy approximately 10 litres to be safe.
Note: Ceilings usually use a different product (ceiling paint, typically matt white). Calculate and buy separately.
How Many Tins to Buy
| Amount needed | Buy |
|---|---|
| Up to 2.5L | 1 × 2.5L |
| 2.6–5L | 1 × 5L |
| 5.1–7.5L | 1 × 5L + 1 × 2.5L |
| 7.6–10L | 2 × 5L |
| 10.1–12.5L | 2 × 5L + 1 × 2.5L |
Always round up. Running out of paint mid-wall with a batch that doesn't match perfectly is a nightmare.
Coverage Table: Room Size Reference
| Room size | Wall area (approx) | Paint needed (2 coats) |
|---|---|---|
| Small bedroom (3×3m, 2.4m ceiling) | ~24 m² | ~4.5 L |
| Standard bedroom (4×3.5m) | ~32 m² | ~6 L |
| Large bedroom (5×4m) | ~40 m² | ~7.5 L |
| Kitchen/diner (6×4m) | ~50 m² | ~9 L |
| Open plan living (8×6m) | ~70 m² | ~13 L |
| Full house interior | ~200–300 m² | ~40–60 L |
Special Situations
Dark to light colour change: Add an extra coat, or use a tinted primer to reduce the number of top coats needed.
New plaster: Use a mist coat first (highly diluted emulsion, 70% paint / 30% water) to seal the surface. This dries quickly and gives subsequent coats better adhesion.
Textured walls (Artex, rough plaster): Increase your coverage estimate by 15–20% — texture absorbs more paint.
Woodwork (skirting, doors, window frames): Roughly 1 litre of gloss covers 14 m²; for a standard room, 0.5L is usually enough for woodwork.
Quick Coverage Calculator
Litres needed = (Wall area − doors/windows) ÷ coverage rate × coats
Always buy 10–15% more than calculated. Excess paint is perfect for future touch-ups.