How much cement and sand do I need to plaster 100 square metres?

For 100 m² at 12 mm thickness with a 1:5 mix, a 1.30 dry factor and 10% wastage you need about 9 bags of cement (50 kg) and roughly 0.5 cube of sand (about 50 cubic feet). Thicker plaster or a richer mix raises both. Use the calculator above for your exact thickness and mix.

What is the standard plaster thickness for internal and external walls?

Internal walls are usually plastered at 12 mm in a single coat. External walls and rough block or brick surfaces take 15–20 mm, often in two coats. Ceiling soffits and skim coats run 6–10 mm. The calculator defaults to 12 mm and lets you switch presets or enter a custom thickness.

What mix ratio is used for wall plastering in Sri Lanka?

1:5 (one part cement to five parts sand) is the common all-round internal plaster mix. External and exposed surfaces use a richer 1:4 or 1:3 for strength and weather resistance; dry internal areas sometimes use a leaner 1:6. Richer mixes need more cement per cube of sand.

How many cement bags are in one cube of plaster mortar?

For a 1:5 dry mortar, one cubic metre needs 4.8 bags of cement, so one cube (2.832 m³) of dry 1:5 mortar carries about 13.6 bags. A 1:4 mix needs 5.76 bags per m³. The calculator shows the bags-per-m³ figure so you can cross-check against published consumption norms.

How do I convert plaster sand from cubic feet to cubes?

One cube is 100 cubic feet (2.832 m³), the unit Sri Lankan hardware stores sell sand in. Divide cubic feet by 100 to get cubes: 50 ft³ ÷ 100 = 0.5 cube. The calculator reports sand in all three — cubes, cubic feet and cubic metres — so you can order in whatever the supplier quotes.

Why does the calculator multiply by a dry-volume factor?

The plaster you measure on the wall is the wet volume. When you mix dry cement and sand, the loose material takes more space than the finished mortar because voids fill and the mix compacts on troweling. The dry factor (1.20–1.40, default 1.30) converts the wet coat back to the dry cement and sand you must buy.

Should I deduct doors and windows from the plaster area?

Yes, for a tight material order. Enter the net surface after subtracting large openings such as doors and picture windows. Small openings are often left in as a wastage buffer. If you plaster window reveals and jambs, add that surface back. The 10% wastage allowance covers minor uneven walls.

Does this include waterproofing, lime or gypsum plaster?

No. This version estimates plain cement-sand plaster only. Waterproofing additives, lime rendering, gypsum plaster and textured or machine-applied finishes change the material mix and coverage, so they are outside the current scope. For those, follow the product's own coverage rate on the bag.

Sri Lanka · Construction

Sri Lanka Plastering Calculator (Cement & Sand)

Find how many 50 kg cement bags and cubes of sand you need to plaster a wall or ceiling. Enter the area, pick a thickness and mix ratio, and get a hardware-ready list — with an optional rupee cost. Free, no signup.

By Induwara Ashinsana— Executive Director, Ryzera TechnologiesUpdated Jul 16, 2026

Plastering material estimatecement & sand

Dry-volume method · CIDA basis

Plaster area

Net wall or ceiling surface to plaster. Deduct large openings (doors, windows) for a tighter estimate.

Plaster thickness

Standard internal wall

Mix ratio (cement : sand)

Internal walls — the common all-round mix

Wastage allowance (%)

Extra to cover droppings, uneven walls and mixing loss. 10% is typical for plastering.

Cement

9bags (50 kg)

412 kg total

Sand

0.51cube

50.5 ft³ · 1.43 m³

Plaster area

100m²

1,076 ft² · mix 1 : 5

Volume method (transparency)

Wet volume

1.2 m³

42.4 ft³

Dry mortar

1.56 m³

wet × dry factor

After wastage

1.72 m³

4.8 bags/m³ dry

Sources: CIDA Building Schedule of Rates (SL plastering norms and site units), SLS 107 (50 kg bag, cement density 1440 kg/m³). 1 cube = 100 ft³ = 2.832 m³. Estimates for ordering only — confirm quantities with your mason for the actual wall condition.

How it works

The calculator uses the standard dry-volume (dry-mortar) method for cement-sand plaster, the same estimating basis in the Construction Industry Development Authority (CIDA) Building Schedule of Rates. Every step is deterministic, so the same inputs always give the same material list.

Normalise the area to m². If you enter square feet, it is multiplied by 0.092903 to get square metres.
Wet volume. Multiply area by thickness in metres: V_wet = A × t. A 12 mm coat is 0.012 m, so 100 m² × 0.012 = 1.20 m³ of finished plaster.
Dry volume. Dry loose cement and sand occupy more space than the compacted wet coat, so multiply by a dry-volume factor (1.20–1.40, default 1.30): V_dry = V_wet × f.
Add wastage. Multiply by (1 + wastage%). A 10% allowance covers droppings, uneven walls and mixing loss.
Split by the mix ratio. For cement:sand = 1 : s, the parts sum to (1 + s). Cement volume = V_dry_w × 1 ÷ (1 + s) and sand volume = V_dry_w × s ÷ (1 + s).
Convert cement to bags. Multiply cement volume by the SLS 107 bulk density 1440 kg/m³ to get kilograms, divide by the 50 kg bag, then round up to whole bags — what you actually buy.
Report sand in trade units. Sand volume is shown in m³, cubic feet (× 35.3147) and cubes (÷ 2.832, since one cube = 100 ft³), the unit Sri Lankan suppliers quote.

As a cross-check the tool also computes cement bags per cubic metre of dry mortar directly from the ratio — 1:5 gives 4.8 bags/m³ and 1:4 gives 5.76 bags/m³, matching published consumption norms. Both routes agree, so the bag count on the results panel is auditable against a known figure.

Worked examples

Internal wall — 100 m², 12 mm, mix 1:5

Wet volume: 100 m² × 0.012 m = 1.20 m³
Dry volume: 1.20 × 1.30 = 1.560 m³
Plus 10% wastage: 1.560 × 1.10 = 1.716 m³
Sum of parts: 1 + 5 = 6
Cement: 1.716 × 1/6 = 0.286 m³ × 1440 = 411.8 kg ÷ 50 = 8.24 → 9 bags
Sand: 1.716 × 5/6 = 1.430 m³ = 50.5 ft³ = 0.505 cube

External wall — 60 m², 20 mm, mix 1:4

Wet volume: 60 m² × 0.020 m = 1.20 m³
Dry + 10% wastage: 1.20 × 1.30 × 1.10 = 1.716 m³
Sum of parts: 1 + 4 = 5
Cement: 1.716 × 1/5 = 0.3432 m³ × 1440 = 494.2 kg ÷ 50 = 9.88 → 10 bags
Sand: 1.716 × 4/5 = 1.373 m³ = 48.5 ft³ = 0.485 cube
Richer 1:4 mix correctly needs more cement than the 1:5 internal coat

Area in square feet — 1,076 ft², 12 mm, mix 1:5

Convert area: 1,076.4 ft² × 0.092903 = 100.0 m²
From here it matches example 1 exactly
Wet 1.20 m³ → dry + wastage 1.716 m³
Cement: 9 bags · Sand: 0.505 cube
Entering feet or metres lands on the same answer

Frequently asked questions

Sources & references

The mix ratios, thicknesses, dry-volume factor and unit conversions on this page were last cross-checked against these sources on 2026-07-16. Cement and sand prices are user-entered, not fetched, because they vary by district and week — a stale hard-coded price would mislead.

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