Cement plaster calculator (plaster and render)

How to calculate cement and sand for plastering

Multiply the area by the plaster thickness to get the wet volume, multiply by about 1.33 for the dry volume, then split it by the mix ratio. The cement share × 1,440 kg/m³ gives kilograms of cement.

Area        = length × height × faces − openings
Wet volume  = area × thickness
Dry volume  = wet volume × 1.33 × (1 + wastage)
Cement, kg  = dry volume × cement ÷ (cement + sand) × 1,440
Sand        = dry volume × sand ÷ (cement + sand)

The 1.33 factor allows for the voids in loose cement and sand; it's a site convention rather than a value from a standard, and quoted figures run from about 1.27 to 1.35, so the calculator lets you change it. Cement's bulk density of 1,440 kg/m³ is from IS 875 Part 1.

Worked example: a 4 × 3 m internal wall, 12 mm thick

One face of a 4 × 3 m wall is 12 m². At 12 mm that is 0.144 m³ of plaster; in a 1:6 mix it needs 39.4 kg of cement (0.79 of a 50 kg bag) and 0.164 m³ of sand.

1. Area: 4 × 3 = 12 m².
2. Wet volume: 12 × 0.012 = 0.144 m³.
3. Dry volume: 0.144 × 1.33 = 0.1915 m³.
4. Cement: 0.1915 × 1 ÷ 7 × 1,440 = 39.4 kg.
5. Sand: 0.1915 × 6 ÷ 7 = 0.164 m³ (5.8 cft).

Plaster thickness and mix: what to use

Common site practice is 12 mm on internal walls in a 1:6 mix, 20 mm in two coats outside in 1:4, and 6 mm on ceilings. IS 1661 recommends 10–15 mm for a single coat and caps two-coat work at 20 mm.

IS 1661 lists 1:3, 1:4 and 1:6 cement-sand mixes for plaster, and recommends 1 part cement to 3 or 4 of sand for rough finishes. It says two-coat work should not normally exceed 20 mm, or 15 mm on concrete soffits. The 6 mm ceiling and 12 and 20 mm wall figures are item thicknesses from Indian (CPWD) schedules and site practice, not values from the code; 6 mm is thinner than IS 1661's single-coat range. Uneven brickwork needs more: enter the average thickness you expect.

Gypsum plaster vs cement plaster (render)

In the UK, "plaster" usually means gypsum plaster on internal walls, and sand-cement on external walls is called render. Gypsum is sold by the bag with a coverage per bag, so the calculator works it out by weight instead of mix ratio.

Example: a 2 mm skim over 20 m² uses 50 kg, which is 2 bags of 25 kg. Coverage per bag is British Gypsum's published figure for each Thistle product, as given in the Thistle Hardwall data sheet and repeated by merchants for Multi-Finish and Bonding. For other brands, use the coverage on the bag. Undercoats (Hardwall, Bonding) and the finish coat are separate calculations.

How to use the calculator

Choose cement and sand or gypsum, set the thickness, then add each wall or ceiling, or enter a total area.

1. Region presets set the units, the usual cement bag size and, for the UK, gypsum plaster.
2. Pick a quick thickness or type your own.
3. Add walls with length and height, or ceilings with length and width. Choose Both faces for a partition plastered on each side.
4. Deduct doors and windows as an area per wall; it is deducted from each face.
5. Coming from the brick calculator? Its wall area arrives as Other area to plaster.

Assumptions and limits

The calculator converts area and thickness into materials. It doesn't account for the surface's suction, bonding agents or beads.

• Thickness is an average; hollow or uneven walls take more.
• Sand is assumed dry. Damp sand bulks and needs more volume.
• Reveals around openings, corners and beads aren't counted.
• Costs appear only for prices you enter.

Next, work out the paint with the paint calculator.

Sources

Constants and limits come from these standards, manufacturers' data and trade bodies. Where practice varies, the calculator lets you change the value.

• IS 1661:1972 Cement and cement-lime plaster finishes on walls and ceilingsBureau of Indian Standards · 1972, reaffirmed
• IS 875 (Part 1):1987 Unit weights of building materials (Table 1)Bureau of Indian Standards · 1987
• Thistle Hardwall product data sheet: coverageBritish Gypsum · current

Found a value that doesn’t match your local practice? Tell us and we’ll review it.