How to Calculate Wall Area for Painting
Getting the wall area right before you buy paint saves money and avoids mid-job trips to the shop. The calculation is straightforward — measure the walls, subtract the openings, account for the number of coats, and add a small buffer. This guide walks through each step with worked examples.
Step 1 — Measure the Walls
For a rectangular room, the quickest method is to calculate the gross wall area using the room perimeter.
Example: Room 5 m × 4 m, ceiling height 2.4 m:
Step 2 — Subtract Doors and Windows
Measure the width and height of each door and window opening and subtract the total from the gross wall area.
Example continued: One door (0.9 m × 2.1 m) and two windows (1.2 m × 1.0 m each):
Step 3 — Add the Ceiling (If Painting It)
Ceiling area is simply the floor area of the room. Calculate it separately — ceilings often use a different paint and may need fewer coats.
Example continued: 5 m × 4 m room:
Step 4 — Decide on Number of Coats
The number of coats affects how much paint you need. Use the table below as a guide.
| Situation | Recommended Coats |
|---|---|
| Repainting — similar colour, sound surface | 1–2 coats |
| Significant colour change (dark to light or vice versa) | 2–3 coats |
| New plaster | 1 mist coat + 2 full coats |
| Bare plasterboard | 1 primer + 2 full coats |
Step 5 — Calculate Paint Quantity
Most standard emulsion paints cover 10–12 m² per litre per coat. Always check the coverage rate on the tin — premium or high-pigment paints often cover less.
Example continued: 38.91 m² net wall area, 2 coats, coverage rate 11 m²/L:
Step 6 — Add Waste
Always order slightly more than the calculated quantity. Cutting in around skirting boards, cornices, and window reveals uses more paint than rolling open wall areas.
| Situation | Waste Factor |
|---|---|
| Simple rectangular room, roller finish | 10% |
| Many corners, alcoves, or detailed trim | 15% |
| Textured or rough surfaces | 15–20% |
Example continued: 7.07 litres + 10% waste:
Worked Examples
Example A — Small bedroom, repaint
- Room: 3.5 m × 3.0 m, ceiling 2.4 m
- Gross wall area: (3.5 + 3.0) × 2 × 2.4 = 31.2 m²
- Openings: 1 door (0.9 × 2.1 = 1.89 m²) + 1 window (1.0 × 1.0 = 1.0 m²) = 2.89 m²
- Net area: 31.2 − 2.89 = 28.31 m²
- 2 coats, coverage 11 m²/L: (28.31 × 2) ÷ 11 = 5.15 L
- +10% waste: 5.15 × 1.10 = 5.66 L → buy 2× 2.5 L tins (5 L) + 1× 1 L tin
Example B — Open-plan living room, colour change
- Room: 7.0 m × 5.0 m, ceiling 2.5 m
- Gross wall area: (7.0 + 5.0) × 2 × 2.5 = 60.0 m²
- Openings: 1 door (0.9 × 2.1 = 1.89 m²) + 2 windows (1.4 × 1.2 × 2 = 3.36 m²) = 5.25 m²
- Net area: 60.0 − 5.25 = 54.75 m²
- 3 coats (dark to light), coverage 10 m²/L: (54.75 × 3) ÷ 10 = 16.43 L
- +15% waste: 16.43 × 1.15 = 18.89 L → buy 4× 5 L tins
Common Paint Coverage Rates
Coverage varies by paint type. Use these as a starting point and always verify against the tin.
| Paint Type | Typical Coverage | Notes |
|---|---|---|
| Standard emulsion (walls) | 10–12 m²/L | Most common interior wall paint |
| Matt emulsion | 10–12 m²/L | Hides surface imperfections well |
| Silk / satin emulsion | 10–11 m²/L | Washable, used in kitchens and bathrooms |
| Eggshell (oil-based) | 8–10 m²/L | Durable, slow drying |
| Masonry paint (exterior) | 4–6 m²/L | Lower coverage due to textured surfaces |
| Primer / undercoat | 8–10 m²/L | Higher pigment, covers less per litre |
Skip the Maths
Use our Paint Calculator to enter your room dimensions, number of coats, and coverage rate — it calculates the exact litres needed in one step.
Related Guides
- How to Calculate Paint Coverage — coverage rates, tin sizes, and surface types explained
FAQ
How do I calculate the wall area of a room for painting?
Measure the perimeter of the room (total length of all walls) and multiply by the ceiling height to get the gross wall area. Then subtract the area of all doors and windows. For a standard rectangular room 4 m × 3 m with 2.4 m ceilings, one door (0.9 × 2.1 m) and one window (1.2 × 1.0 m): perimeter = (4 + 3) × 2 = 14 m, gross area = 14 × 2.4 = 33.6 m², openings = 1.89 + 1.2 = 3.09 m², net area = 33.6 − 3.09 = 30.51 m².
Do I need to include the ceiling when calculating paint coverage?
Only if you're painting the ceiling too. Calculate the ceiling area separately — it's simply the floor area of the room (length × width). Keep ceiling and wall calculations separate because ceilings often use a different paint type (typically matt white) and may need fewer coats if already a light colour.
How much paint do I need per m²?
Most standard emulsion paints cover 10–12 m² per litre for a single coat. Higher-pigment or specialist paints may cover less — always check the manufacturer's stated coverage rate on the tin. For two coats on new plaster or a significant colour change, divide your wall area by the coverage rate and multiply by 2.
How many coats of paint do I need?
For repainting a similar colour on a sound surface: 1–2 coats. For a significant colour change (e.g. dark to light or vice versa): 2–3 coats. For new plaster or bare walls: a diluted mist coat first, then 2 full coats. New plaster is highly absorbent — skipping the mist coat leads to uneven finish and peeling.
Should I add waste when ordering paint?
Yes. Add 10% for straightforward rooms. Add 15% for rooms with many corners, alcoves, or detailed trim work where more cutting-in is required. It's better to have a small amount left over — paint from the same batch ensures a consistent colour match if you need to touch up later.
How do I measure an irregular or L-shaped room?
Break the room into rectangular sections. Measure each wall individually (width × ceiling height), add them all together for the gross wall area, then subtract doors and windows as normal. Don't try to calculate the perimeter of an L-shape in one step — measuring wall by wall is more accurate and less error-prone.