Plaster calculator for walls and ceilings
This plaster calculator tells you how many bags of cement and how much sand you need to plaster a wall or ceiling. Enter the wall length and height (or the area in m² or square feet), choose the plaster work — internal 12 mm at 1:6, external 20 mm at 1:4, ceiling 6 mm at 1:3 — and it works out the mortar volume, cement bags and sand in m³, cft and brass, with a cost if you add prices.
It follows the method used for estimating cement-sand plaster on Indian sites: wet volume = area × thickness, plus an allowance for filling joints and uneven surfaces, multiplied by a dry-volume factor of 1.27 to get the loose materials, which are then split in the mix ratio. Every factor is editable, so you can match your engineer’s or contractor’s own rule.
It is a planning tool for cement plaster and sand-cement render. It does not estimate gypsum skim coats, ready-mix plaster or putty, whose coverage is printed on the bag.
How to use the Plaster Calculator
- Choose metric or imperial units, then whether to work from wall length × height or a known area.
- For walls, enter the total running length of wall, its height, whether one or both faces are plastered, and the area of doors and windows to deduct.
- Pick the plaster work. It sets a typical thickness and mix, or choose your own thickness and cement : sand ratio.
- Keep the 20% extra and 1.27 dry factor or change them, and pick the cement bag size.
- Optionally enter cement and sand prices, then read the bags, sand and cost and copy the summary.
Plaster Calculator features
Common plaster jobs
Internal 12 mm 1:6, rough brickwork 15 mm 1:5, external 20 mm 1:4 and ceiling 6 mm 1:3 presets.
Wall or area input
Running length × height × faces, minus openings, or type the area directly.
Every sand unit
Sand in m³, cubic feet, brass and tonnes; cement in kg and 50 kg, 25 kg or 94 lb bags.
Transparent method
Each step — wet volume, extra, dry volume, cement and sand shares — is shown in the table.
Cost estimate
Cement per bag and sand per m³, cft, brass or tonne in your currency.
How the plaster quantity is calculated
Worked example — 100 m² of internal plaster, 12 mm thick, 1:6: wet volume = 100 × 0.012 = 1.2 m³. Add 20% for joints, unevenness and waste: 1.44 m³. Dry volume = 1.44 × 1.27 = 1.829 m³. Cement = 1.829 ÷ 7 = 0.261 m³ × 1,440 kg/m³ = 376 kg, which is 7.5 bags of 50 kg — order 8. Sand = 1.829 × 6 ÷ 7 = 1.57 m³, about 55 cft.
The 1.27 factor allows for the voids between loose sand and cement grains that the water fills. Some engineers use 1.33 or 1.35 and no separate extra; enter whatever rule you work to. In square feet the same job is 1,076 sq ft at ½ in.
Plaster thickness in mm for walls (IS 1661)
IS 1661:1972 (code of practice for cement and cement-lime plaster) recommends these thicknesses and also lists cement-lime-sand mixes such as 1:1:6 and 1:2:9. Leaner mixes like 1:6 suit internal walls; richer 1:3 and 1:4 mixes are used outside, on ceilings and in wet areas.
| Work | Thickness | Typical mixes (cement : sand) |
|---|---|---|
| Single-coat plaster, internal or external | 10–15 mm | 1:3, 1:4, 1:6 |
| Two-coat plaster (backing + finishing) | about 20 mm total (10–12 + 3–8 mm) | 1:3 to 1:6 |
| Three-coat plaster on very rough surfaces | about 25 mm total | 1:3 to 1:6 |
| In-situ concrete soffits (ceilings) | not more than 15 mm; 6 mm is common practice | 1:3 |
Tips for plastering estimates
- Measure the running length of all walls once and use “Both sides” for internal walls plastered on each face; external walls are usually one side outside and one inside.
- Deduct large openings. Many contractors do not deduct small openings because the reveals (window sides) need plaster too.
- Use clean, well-graded plastering sand; silt and clay cause cracks whatever the mix.
- Estimating the wall itself? Use the brick calculator. For the finish on top, the paint calculator works from the same wall area, and the house construction cost calculator gives the whole budget.
Plaster Calculator FAQ
How many bags of cement are needed for 100 sq ft of plaster?
For 12 mm plaster at 1:6 with 20% extra and a 1.27 dry factor, 100 sq ft (9.29 m²) needs about 0.7 bags of 50 kg cement and 5.1 cft of sand. At 1:4 it is about 1 bag. The plaster calculator works it out for your exact area.
What is the cement sand ratio for plastering?
Common ratios are 1:6 for internal brick walls, 1:4 or 1:5 for external walls and rough surfaces, and 1:3 for ceilings and repair work. IS 1661 lists 1:3, 1:4 and 1:6 cement-sand mixes among its recommended specifications.
What is the standard plaster thickness in mm for a wall?
About 12 mm for internal walls in one coat (IS 1661 gives 10–15 mm for single-coat work), about 20 mm in two coats for external walls, and 6 mm for ceilings.
Why multiply by 1.27?
Dry cement and sand shrink when water is added because the fine particles fill the voids. Multiplying the wet volume by about 1.27 gives the dry volume of materials to buy.
How do I calculate plaster area for a room?
Add the lengths of all four walls, multiply by the height, subtract doors and windows, and add the ceiling area if it is plastered. Example: a 4 × 3 m room 3 m high has 42 m² of walls; less a door (2 m²) and window (1.5 m²) leaves 38.5 m².
Does it work for render in the UK or stucco in the US?
Yes for sand-cement render: choose imperial units, the thickness and a 1:4 to 1:6 mix. It does not cover gypsum plaster, lime-only render or proprietary stucco mixes; use the coverage on their bags.