Plaster & Screed Calculator
Cement and sand for walls, ceilings and floor screeds, with IS 1661 thickness guidance.
Surface
Doors, windows and other openings to leave out
Thickness and mix
Mortar settings
Prices optional
Materials
| Item | Quantity | Unit | Cost |
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About the Plaster & Screed Calculator
Work out the cement and sand for plastering walls inside or out, plastering a ceiling or laying a sand–cement floor screed. Measure the surface as length × height (or type the area), leave out doors and windows, pick the thickness and the mix, and get the wet mortar, the loose dry materials, cement bags and sand in m³, cft, tonnes and brass — plus the cost if you add prices.
As you choose a thickness the calculator tells you what it means under IS 1661:1972: a single coat is 10–15 mm, two-coat plaster is normally not more than 20 mm and plaster on in-situ concrete ceilings not more than 15 mm. For screeds it shows whether the thickness is typical of a bonded, unbonded or floating screed.
How to use it
- Choose inside wall, outside wall, ceiling or floor screed — the usual thickness and mix are filled in.
- Enter the length and height (or width) of the surface and how many identical ones there are, or switch to Area and type it. Add openings such as doors and windows to leave them out.
- Set the thickness — tap a common size or type your own, such as
12 mmor1/2 in— and the mix, and check the wastage. - Read the cement bags and sand, add prices for a cost, and copy or download the result.
Examples
30 m² → wet mortar 0.36 m³ → dry materials 0.527 m³ (× 1.33 + 10%) → cement 2.17 bags (108 kg), sand 0.451 m³ = 15.9 cft
32.4 m² → wet mortar 0.648 m³ → cement 5.46 bags (273 kg), sand 0.758 m³ = 26.8 cft
12 m² → cement 0.76 bag (37.9 kg), sand 0.079 m³ = 2.79 cft
20 m² → wet mortar 0.8 m³ → cement 6.74 bags (337 kg), sand 0.936 m³ = 33.1 cft
How it is calculated
Wet mortar = net area × thickness. Loose cement and sand take up more room than the mortar they make, so the dry materials are the wet volume × a dry-volume factor (1.27–1.33 is the usual range; 1.33 is the default) plus a wastage allowance for mortar that falls, uneven backgrounds and raked joints. The dry volume is split in the mix ratio: for 1:6, cement is 1/7 and sand 6/7. Cement is converted at 1,440 kg/m³ (IS 875 Part 1) and divided by the bag size.
Example: 30 m² of 12 mm plaster is 0.36 m³ wet, × 1.33 × 1.10 = 0.527 m³ dry. Cement = 0.527 ÷ 7 × 1,440 = 108 kg (2.17 bags of 50 kg); sand = 0.527 × 6 ÷ 7 = 0.451 m³.
Plaster thickness and mixes (IS 1661)
- Single coat: 10–15 mm, cement : sand 1:3, 1:4 or 1:6 (or 1:1:6 / 1:2:9 with lime) — IS 1661 Table 2.
- Two coats: a 10–12 mm backing coat and a 3–8 mm finishing coat; the total is generally about, and should not normally exceed, 20 mm — cl. 7.3.2.
- Ceilings: two-coat plaster on in-situ concrete soffits should not exceed 15 mm — cl. 7.3.2.
- Three coats: about, and not normally more than, 25 mm, for very rough backgrounds.
- Rough finishes: 1 part cement to 3–4 parts sand is recommended — cl. 7.4.2.
- Curing: keep each coat damp until the next is applied, and the finished plaster wet for at least seven days — cl. 16.
The presets are common specified thicknesses: 6 mm thin coats on smooth concrete, 12 mm and 15 mm single coats, 20 mm in two coats.
Floor screed thickness
Sand–cement levelling screeds are covered by BS 8204-1. The commonly quoted thicknesses are 25–40 mm for a screed bonded to a prepared concrete base, a minimum of about 50 mm for an unbonded screed on a separating membrane, and more than 65 mm for a floating screed over insulation (75 mm for more heavily loaded floors). Most sand–cement screeds are mixed between 1:3 and 1:4.5.
Sources
- IS 1661:1972, Code of practice for application of cement and cement-lime plaster finishes, cl. 7.3, 7.4, 16 and Table 2 — archive.org
- IS 875 (Part 1):1987, Table 1 — cement 1,440 kg/m³, dry sand 1,540–1,600 kg/m³ — archive.org
- Screed thickness ranges as summarised from BS 8204 by Designing Buildings Wiki — Screed; BS 8204-1:2002+A1:2009 is the standard to follow
- Dry-volume factor 1.27–1.33: Indian estimating convention, editable
Limitations
- Uneven or hollow backgrounds, deep raked joints and dubbing out use more mortar — raise the wastage or measure the extra depth.
- This is for cement–sand mixes. Cement–lime plaster, gypsum plaster and ready-mixed, flowing or self-levelling screeds have their own coverage figures.
- The dry-volume factor is a convention; actual sand use depends on its moisture and grading.
- Screed thicknesses are typical guidance — follow your specification, especially over insulation or underfloor heating.
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Frequently asked questions
How much cement is needed for 1 m² of 12 mm plaster?
With a 1:6 mix, a dry-volume factor of 1.33 and 10% wastage, about 3.6 kg of cement and 0.015 m³ of sand per m² — so 100 m² takes about 7.2 bags of 50 kg.
What is the standard thickness of plaster?
IS 1661:1972 gives 10–15 mm for single-coat plaster. Two-coat plaster is about, and normally not more than, 20 mm (a 10–12 mm backing and a 3–8 mm finishing coat), plaster on in-situ concrete ceilings should not exceed 15 mm, and three-coat work is normally not more than 25 mm.
Which mix should I use for plastering?
Use the mix in your specification. IS 1661 Table 2 lists cement : sand mixes of 1:3, 1:4 and 1:6 for single coats and backing coats (or 1:1:6 and 1:2:9 with lime), and cl. 7.4.2 recommends 1 part cement to 3–4 parts sand for any rough finish.
How thick should a floor screed be?
For sand–cement screeds the figures commonly quoted from BS 8204 are 25–40 mm for a screed bonded to the slab, at least about 50 mm for an unbonded screed on a membrane and more than 65 mm for a floating screed on insulation — 75 mm under heavier loads. Most are mixed between 1:3 and 1:4.5.
Why is the dry volume more than the wet volume?
Loose cement and sand contain air. Once mixed with water the mortar settles into a smaller volume, so you need roughly 27–33% more dry material than the finished plaster. Change the factor under Mortar settings if your own measurements differ.
How long should plaster be cured?
IS 1661 asks for each coat to be kept damp continuously until the next coat goes on (for up to 7 days), and for the finished plaster to be kept wet for at least seven days, protected from extreme heat and weather.