Plaster Calculator (Cement and Sand)
How many 50 kg bags of cement and how much sand you need to plaster a wall, from its area, the thickness and the mix.
Length × height of each wall, less big openings such as doors and windows.
About 12 mm inside, 15 to 20 mm on rough or outside walls, 6 mm on ceilings.
Cement
9.8 bags
- Sand
- 48 cu ft (1.36 m³)
- Cement by weight
- 489 kg
- Dry mortar
- 1.699 m³
Plastering 1,000 sq ft at 12 mm thick in a 1 : 4 mix needs about 10 bags of cement and 48 cubic feet of sand.
How it's worked out
- Area in square metres
1,000 ÷ 10.76492.9 m² - Wet plaster
92.9 m² × 0.012 m1.115 m³ - Add for uneven walls and waste
× 1.21.338 m³ - Dry materials
× 1.27 (sand and cement settle when wet)1.699 m³ - Cement
1.699 ÷ (1 + 4) × 1,440 kg ÷ 50 kg9.8 bags - Sand
0.34 m³ × 4 × 35.3148 cu ft
- Buy whole bags and a little spare sand. Wet the wall before plastering and keep the plaster damp (curing) for about 7 days so it doesn't crack.
How it is worked out
Dry volume = area × thickness × (1 + extra) × 1.27
The dry volume is split by the mix ratio. In a 1 : 4 mix, one part in five is cement and four are sand. Cement is turned into bags at 1,440 kg a cubic metre and 50 kg a bag, and sand into cubic feet (1 m³ = 35.31 cu ft).
Choosing a mix
1 : 4 is common for outside walls and 1 : 6 for inside walls. A richer 1 : 3 mix is used for repairs and where the wall needs to keep water out. Follow your engineer's specification if you have one.
Getting a good finish
Rake out the joints and wet the wall before you start, apply the plaster in one or two coats, and keep it damp for about 7 days so it cures without cracking.
Questions people ask
How much cement do I need for 100 sq ft of plaster?
About 0.7 bags of 50 kg cement and 5.1 cubic feet of sand for 12 mm plaster in a 1 : 6 mix, allowing 20% for uneven walls and waste.
How much cement for 1,000 sq ft of 12 mm plaster in 1 : 4?
About 9.8 bags (489 kg) of cement and 48 cubic feet of sand, so buy 10 bags.
Why multiply by 1.27?
Wet mortar takes up less space than the dry sand and cement that went into it, as the cement fills the gaps between sand grains. About 1.27 times the wet volume of dry material is needed.
What thickness should plaster be?
About 12 mm on inside walls, 15 to 20 mm on rough or outside walls, and 6 mm on ceilings.
About this calculator
Formula and rules last checked 9 November 2026Written and tested by the CalculatorDex teamReport a problem