Wall Plaster Calculator
Calculate cement bags, sand and water needed to plaster a wall at any thickness and mix ratio.
Inputs
Formula
- Wet volume = area × thickness
- Dry volume = wet volume × 1.33
- Cement = dry × 1/(1+n), Sand = dry × n/(1+n)
Plaster volume is simply the area times the thickness — 12 mm internally, 15–20 mm externally. Mortar shrinks, so multiply by 1.33 to get the dry material. Split that by the mix ratio: 1:6 means one part cement to six parts sand. One bag of cement is 50 kg or 0.0347 m³.
Step-by-step calculation
- 1
Wet volume
area × thickness
0.557 m³
- 2
Dry volume
wet × 1.33
0.741 m³
- 3
Cement
dry ÷ 7 × 1440 ÷ 50
3.05 bags
- 4
Sand
dry × 6/7
0.635 m³
Worked example
- 1500 sq ft (46.45 m²) at 12 mm → 0.557 m³ wet.
- 2Dry volume = 0.557 × 1.33 = 0.741 m³.
- 31:6 mix → cement 0.106 m³ = 3.05 bags, sand 0.635 m³.
Tips
- Cure plaster with water for at least 7 days or it will crack.
- Use 1:4 outside and 1:6 inside — a richer mix outside resists rain better.
Frequently asked questions
How many cement bags for 100 sq ft of plaster?
About 0.6 bags at 12 mm in a 1:6 mix, so roughly 6 bags for 1,000 sq ft.
What thickness is standard?
12 mm for internal walls, 15 mm for external, and 20 mm on rough blockwork.
Why multiply by 1.33?
Dry sand and cement have air voids; the mortar loses about a third of its loose volume once mixed and compacted.
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