Cement Calculator
Cement, sand and water quantities for plastering, flooring screed and masonry mortar — in bags, kilograms and cubic metres.
Inputs
Formula
- Wet volume = area × thickness
- Dry volume = wet volume × 1.33
- Cement = dry volume × 1 / (1 + n); bags = m³ × 1440 / 50
- Sand = dry volume × n / (1 + n)
Mortar shrinks as the sand voids fill with cement paste and the mix is compacted, so the dry ingredient volume must exceed the finished volume. A factor of 1.33 (sometimes 1.30–1.35) is standard for plaster and mortar, against 1.54 for concrete which also contains coarse aggregate.
Step-by-step calculation
Wet volume
area × thickness
1.2 m³
Dry volume
wet × 1.33 × (1 + waste)
1.7556 m³
Cement volume
dry × 1/7
0.2508 m³
Bags
volume × 1440 / 50
7.22
Sand
dry × 6/7
1.5048 m³
Worked example
- 100 m² of 12 mm internal plaster in 1 : 6.
- Wet volume = 100 × 0.012 = 1.2 m³; dry = 1.2 × 1.33 = 1.596 m³ (+10% wastage = 1.756 m³).
- Cement = 1.756 / 7 = 0.251 m³ = 7.2 bags; sand = 1.505 m³.
Assumptions
- Dry volume factor 1.33; cement bulk density 1440 kg/m³; sand 1600 kg/m³.
- Water for plaster is roughly 20% of the total dry mix mass.
Tips
- External plaster should be 15–20 mm and internal 12 mm; ceilings 6 mm.
- Never gauge more mortar than can be used in 30 minutes after adding water.
Standards & references
- IS 1661 (plastering)
- IS 2250 (masonry mortars)
- IS 269 (OPC)
Frequently asked questions
How many cement bags for 100 m² of 12 mm plaster?
About 7 bags in a 1 : 6 mix, or roughly 11 bags in a stronger 1 : 4 mix, including 10% wastage.
Why is the dry volume factor 1.33 and not 1.54?
1.54 applies to concrete, which contains coarse aggregate with much larger voids. Cement–sand mortar bulks less, so 1.30–1.35 is used.
How much water does plaster need?
Around 20% of the dry mix weight — enough to give a workable, non-slumping consistency. Excess water causes shrinkage cracks.
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