Plaster Calculator
Home & ConstructionCement, sand and water for plastering, with openings deducted and the right thickness for the surface.
Runs entirely in your browser — nothing is uploaded
Plastering quantities go wrong in two places: the thickness, which follows the surface rather than preference, and the dry volume factor, which is the reason a job ordered on wet volume arrives a third short. This takes several walls at once, deducts the doors and windows, and gives cement, sand and water for the whole job.
What people do next
Features
- Several walls and openings in one estimate, in feet or metres.
- Presets for internal, external, ceiling and backing coats — each with its usual thickness and mix.
- Doors and windows deducted from the gross area.
- Cement in bags, sand in m³, cft and kilograms, plus the water.
- Wastage allowance and cost at your own rates.
- Calculated in your browser — nothing is uploaded.
How to use the Plaster Calculator
- 1Choose what is being plastered — the thickness and mix follow.
- 2Add each wall as a length, a height and a count.
- 3Add the doors and windows to deduct.
- 4Read the cement bags, sand and water needed.
Frequently asked questions
How thick should plaster be?
It follows the surface, not preference. An internal wall takes 12 mm in a single coat. An external wall takes 15–20 mm, usually in two coats, because it has weather to keep out. A ceiling takes 6 mm — anything thicker tends to lose its grip and fall. A rough backing coat over uneven blockwork is 15 mm.
Why is the dry volume more than the wet volume?
Because you buy cement and sand dry, and dry material is full of air. Once mixed with water and pressed onto a wall the mix consolidates and loses about a third of its bulk, so 1 m³ of finished plaster needs about 1.33 m³ of dry material. Ordering on the wet volume is the single most common way a plastering job runs short.
How much cement is needed to plaster 100 square metres?
At 12 mm in a 1:6 mix, about 7 bags of cement and 1.37 m³ — roughly 48 cft — of sand. The wet volume is 100 × 0.012 = 1.2 m³, which becomes 1.596 m³ dry, split one part cement to six parts sand.
Which mix should I use — 1:4 or 1:6?
1:6 for internal walls, where it is cheaper and perfectly adequate. 1:4 for external faces and ceilings, which need the extra strength and water resistance. 1:3 only for repairs and waterproofing — a rich mix is stronger but shrinks more as it cures, so using it everywhere buys cracks rather than durability.
Are openings deducted the way a contractor measures them?
Not exactly. This deducts every opening in full. IS 1200 ignores an opening up to 0.5 m² and deducts only one face between 0.5 and 3 m², which is why a measured bill and a drawing-measured quantity rarely agree. For ordering materials the difference is small, and erring high on a few bags of cement is the cheaper mistake.