Plastering Calculator
Plaster volume = area × coat thickness; the dry mix is 1.27 × that, split by the cement : sand ratio, at 1,440 kg/m³ per 50 kg bag. A 200 ft² wall at ½ in in a 1:4 mix is 8.33 ft³ wet: about 1.7 bags of cement (buy 2) and 8.5 ft³ of sand. Method: Cement Calculator.
How it's calculated: Wet volume = area × thickness; dry volume = wet × 1.27; cement = dry × cement part ÷ total parts, in 50 kg bags at 1,440 kg/m³; sand = dry × sand part ÷ total parts.
Result appears here
Fill in the fields and press Calculate.
Formula, calculation, assumptions and sources
Wet plaster volume = Area × Thickness
Dry volume = Wet volume × 1.27
Cement volume = Dry volume × cement-part ÷ (cement-part + sand-part); bags = cement volume × 1,440 kg/m³ ÷ 50
Sand volume = Dry volume × sand-part ÷ (cement-part + sand-part)
Run the calculation to see each step with your own numbers.
A 200 ft² interior wall, 12 mm (0.5 in) coat, 1:4 mix, 10% waste.
Wet volume = 200 ft² × 0.5 in ≈ 8.33 ft³ (0.236 m³)
Dry volume = 0.236 × 1.27 = 0.30 m³
Cement = 0.30 × 1/5 = 0.060 m³ → 86.3 kg → 1.73 bags → order 2 bags (10% waste)
Sand = 0.30 × 4/5 = 0.240 m³ (≈8.5 ft³)
- A single coat at the stated thickness over the full area entered — deduct openings first.
- 1.27 dry-volume factor for a cement-sand mortar with no coarse aggregate; a two-coat job (backing + finish) is two separate runs of this calculator, one per coat thickness.
- Cement bulk density 1,440 kg/m³ (loose) and a 50 kg bag, matching the Cement Calculator's constants.
- Water, lath/mesh (for a repair or over dissimilar substrates) and bonding agent are not included.
- Why is plaster's dry-volume factor lower than concrete's?
- Concrete's 1.54 factor accounts for the voids between cement, sand AND coarse aggregate. Plaster mortar has no coarse aggregate, so there are fewer voids to fill when water is added — 1.27 is the standard figure for a sand-cement-only mix.
- Does this cover a two-coat render?
- Run it twice — once for the backing (scratch) coat thickness and mix, once for the finish coat — and add the two results. External renders are often two coats; interior single-coat skim plaster is usually just one.
- What ratio should I use?
- 1:4 or 1:5 cement:sand is typical for general internal plastering; a richer 1:3 mix suits a fine ceiling finish coat; a leaner 1:6 suits a rough backing or external coat. Follow your project spec where one exists.
- Another plaster calculator gave me a different cement quantity — why?
- Dry-volume factors for plaster aren't fully standardized across sources: some tools reuse concrete's 1.54 factor, others use around 1.33–1.35 (a blended bulking + wastage allowance), and this calculator uses 1.27, the figure specific to a coarse-aggregate-free cement-sand mix. All are defensible conventions — the difference is which bulking assumption is baked in, not an error in either tool.
- How much cement do I need to plaster 100 square feet?
- About 0.9 of a 50 kg bag for a ½ in coat at 1:4 — one bag to buy — plus about 4.2 ft³ of sand. A 10% allowance still fits in one bag.
- How thick should a coat of plaster be?
- The calculator's guidance is about 12 mm (½ in) for a single coat and 15–20 mm for a two-coat external render. Run each coat separately with its own thickness and mix.
Dry-volume factor for plaster mortar
1.27 dry-volume allowance for a coarse-aggregate-free cement-sand mortar, consistent with the field volume-batching convention (IS 1200 Part 12 method of measurement for plastering; CPWD specification practice) used elsewhere in BuildNumerics's Cement Calculator.
Cement bulk density
Loose Ordinary Portland Cement ≈ 1,440 kg/m³ (≈ 90 lb/ft³).
About the Plastering Calculator
The Plastering Calculator works out the plaster (render) volume for a wall or ceiling, then splits it into cement bags and sand using a site-mix ratio, the same way a mason batches by volume on site. It uses a 1.27 dry-volume factor — lower than concrete's 1.54 because plaster mortar has no coarse aggregate — to account for the voids that close up once water is added. For laying brick or block rather than finishing it, use the Mortar Calculator; for a full cement/sand/aggregate concrete mix, the Cement Calculator.
What you enter
- Wall or ceiling area
- Plaster thickness — 12 mm (½ in) is typical for a single coat; 15–20 mm for a two-coat external render.
- Mix ratio (cement : sand)
- Waste allowance — Default 10%. Adjust to your own allowance — the exact figure is always shown too.
What you get back
- Plaster volume (wet) — shown as the exact figure and, separately, a recommended figure with your waste allowance added.
- Cement (50 kg bags) — shown as the exact figure and, separately, a recommended figure with your waste allowance added.
- Sand — shown as the exact figure and, separately, a recommended figure with your waste allowance added.
The formula, a worked example, the assumptions behind it and the sources are shown in full below the calculator.
Related calculators
- Mortar Calculator — Mortar quantity for a brick or block wall.
- Brick Calculator — Number of bricks for a wall of a given area and brick size.
- Drywall Calculator — Drywall sheets, screws, mud and tape for a room.
- Excavation Calculator — Bank (in-situ) excavation volume and the larger, swollen loose volume once soil is dug out, plus an estimated weight.
Related articles
- Plaster vs Drywall: Where Each Is Commonly Used — Plaster and drywall both produce a finished interior wall surface, but they're applied very differently and each remains common in different contexts.
Part of the Masonry calculators.