Plaster and Paint Calculator
Estimate plaster mortar (cement and sand) and paint for a room from its size, openings, plaster thickness and number of coats.
How the calculator works
- Wall area:
2 × (length + width) × height − openings - Ceiling area (optional):
length × width - Wet plaster volume:
area × thickness, plus wastage - Dry mortar: wet volume × 1.27, split by cement : sand ratio
- Paint:
area × coats ÷ coverage per litre, plus wastage
Cement converts to bags using 1,440 kg/m³ and 50 kg per bag, rounded up.
Worked example: 4 m × 3 m room, 3 m high, 4 m² openings
- Walls: 2 × (4 + 3) × 3 = 42 m², minus 4 m² = 38 m²
- Ceiling: 4 × 3 = 12 m², so the total area is 50 m²
- Wet plaster at 12 mm: 50 × 0.012 = 0.60 m³
- Dry mortar: 0.60 × 1.27 = 0.762 m³
- For a 1:4 mix: cement 0.152 m³ (about 5 bags) and sand 0.61 m³
- Paint, 2 coats at 10 m²/L: 50 × 2 ÷ 10 = 10 litres (before wastage)
Typical plaster thickness and mix
| Surface | Thickness | Common mix |
|---|---|---|
| Internal wall | 12 mm | 1:4 to 1:6 |
| External wall | 15-20 mm | 1:4 to 1:5 |
| Ceiling | 6-10 mm | 1:3 to 1:4 |
| Rough or uneven masonry | 20 mm and above | 1:4, in two coats |
Frequently asked questions
Should I deduct windows and doors?
Yes, deduct the full opening area for plaster and paint. Reveals and returns around openings add a little area back, so don't over-deduct on small estimates.
How thick should wall plaster be?
12 mm is standard for internal walls on even brickwork. Use thicker plaster on external or uneven walls, applied in two coats rather than one heavy layer.
Does the paint figure include primer?
No. Primer coverage is usually similar to paint, so run the calculator again with one coat to estimate primer.
Why use a dry-volume factor of 1.27?
Mortar shrinks when water is added and the mix compacts, so the dry materials take up more volume than the finished plaster. Estimators use factors from about 1.25 to 1.33.
How accurate is this estimate?
It is a planning figure. Wall surface, absorption, application method and local mix practice all affect real consumption, so add contingency for rough surfaces.
