Plaster Calculation: Cement & Sand for 12 mm, 15 mm and 20 mm Plaster (With Example)

Plastering looks simple, but it is one of the places where cement is wasted most on a building site. If you know the correct thickness, mix ratio and the dry volume method, you can work out the exact cement bags and sand for any wall in two minutes. This guide gives you the formula, a ready table for 100 m², and a full worked example for a room.

🧮 Free tool: skip the hand calculation and use our Plaster Calculator.

Standard plaster thickness and mix ratio

Where	Usual thickness	Usual mix (cement : sand)
Internal walls (smooth side of brick wall)	12 mm	1:6 (1:5 for a harder finish)
Internal walls (rough side / uneven brickwork)	15 mm	1:6
External walls	18–20 mm (often in two coats)	1:4 (1:5 in dry areas)
Ceiling (RCC slab)	6 mm	1:3
Water tanks, toilets, wet areas	12–15 mm	1:3 or 1:4 with waterproofing compound

Rule: do not apply more than about 15 mm in one coat. If the wall needs more, apply a first coat, scratch it, let it set, and then apply the second coat.

Formula: cement and sand for plaster

1. Wet volume = Area (m²) × Thickness (m)
2. Add 20% for filling brick joints, uneven surface and wastage: Wet volume × 1.20
3. Dry volume = the above × 1.27 (mortar shrinks when water is added, so you need about 27% more dry material)
4. Cement (m³) = Dry volume × cement part ÷ (sum of parts). For 1:6, the sum is 7.
5. Cement (bags) = Cement (m³) × 1,440 ÷ 50 (density of cement 1,440 kg/m³, one bag = 50 kg)
6. Sand (m³) = Dry volume × sand part ÷ (sum of parts). Multiply by 35.315 to get cft.

Example: 100 m² of 12 mm plaster in 1:6

• Wet volume = 100 × 0.012 = 1.20 m³
• Add 20% = 1.20 × 1.20 = 1.44 m³
• Dry volume = 1.44 × 1.27 = 1.829 m³
• Cement = 1.829 × 1/7 = 0.261 m³ = 0.261 × 1,440 = 376 kg = 7.5 bags
• Sand = 1.829 × 6/7 = 1.567 m³ = 55.4 cft

So one bag of cement plasters about 13.3 m² (about 143 sq ft) of wall at 12 mm in 1:6. This is a very useful site check. If your mason is using much more than this, the plaster is too thick or there is too much wastage.

Ready table: material per 100 m² of plaster

Thickness	Mix	Cement (bags)	Sand (m³)	Sand (cft)
6 mm (ceiling)	1:3	6.6	0.69	24.2
12 mm	1:6	7.5	1.57	55.4
12 mm	1:4	10.5	1.46	51.7
15 mm	1:6	9.4	1.96	69.2
15 mm	1:4	13.2	1.83	64.6
20 mm (external)	1:4	17.6	2.44	86.1

Worked example: plaster for one room

Room size 4.0 m × 3.5 m, wall height 3.0 m. One door 1.0 × 2.1 m and one window 1.2 × 1.2 m. Inside walls 12 mm in 1:6, ceiling 6 mm in 1:3.

Step 1: Area of walls

• Perimeter = 2 × (4.0 + 3.5) = 15.0 m
• Gross wall area = 15.0 × 3.0 = 45.0 m²
• Openings = door 2.10 + window 1.44 = 3.54 m²
• Net wall area = 45.0 − 3.54 = 41.46 m²

Step 2: Walls (12 mm, 1:6)

• Cement = 41.46 × 0.0752 = 3.1 bags
• Sand = 41.46 × 0.554 = 23.0 cft

Step 3: Ceiling (6 mm, 1:3)

• Area = 4.0 × 3.5 = 14.0 m²
• Cement = 14.0 × 0.066 = 0.9 bag
• Sand = 14.0 × 0.242 = 3.4 cft

Total for the room: about 4 bags of cement and 26.4 cft of sand. Order 5 bags so the work does not stop for half a bag.

Deduction rules for openings (measurement)

In the worked example we deducted the full openings to find the real material. For billing a contractor, India follows the measurement rules of IS 1200. In simple words:

• Small openings up to 0.5 m²: no deduction, and the jambs (sides) are not added.
• Openings from 0.5 m² to 3 m²: deduct the opening from one face only. Jambs are not added.
• Openings above 3 m²: deduct from both faces, and add the area of jambs, sills and soffits.

The idea is that plastering around a small opening takes as much labour as plastering the hole. Check your contract, as some contracts change these rules.

Site steps for good plaster

1. Rake the joints of brickwork about 10–12 mm deep while the mortar is green, so the plaster grips well.
2. Hack RCC surfaces (columns, beams, slab soffit) or apply a bonding coat. Plaster does not stick well to smooth concrete.
3. Fix chicken mesh (about 150 mm wide) at every joint between brick and RCC. This is the most common place for hairline cracks.
4. Wet the wall before plastering so the bricks do not suck the water out of the mortar.
5. Fix dots (bull marks) about 1.5–2 m apart to keep the thickness and line correct.
6. Use mortar within about 30 minutes of adding water. Do not re-mix old mortar with more water.
7. Cure for at least 7 days. Start the next day and keep the plaster wet. See our guide on curing; the same idea applies to plaster.

Why plaster cracks (and how to avoid it)

• Too rich a mix (too much cement) shrinks more and gives fine map cracks.
• Fine or silty sand needs more water and shrinks more. Use clean, well-graded plaster sand.
• No curing or plastering in hot wind.
• Thick single coat of 20 mm or more.
• No mesh at brick–RCC joints. Brick and concrete move differently, so the joint cracks in a straight line.

Frequently asked questions

How much area does one bag of cement cover in plaster?

About 13.3 m² (143 sq ft) at 12 mm thickness in 1:6. In 1:4 it is about 9.5 m² (102 sq ft).

Why multiply by 1.27?

When water is added, the fine cement fills the gaps between sand grains, so the wet mortar has less volume than the dry materials. About 1.27 times the wet volume of dry material is needed.

Which ratio is best for internal plaster?

1:6 is the normal choice for internal walls. 1:4 is used for external walls and wet areas, and 1:3 for ceilings.

How many days of curing are needed for plaster?

At least 7 days of wet curing. Wait until the plaster is fully dry (usually 3–4 weeks) before putty and paint.

Also useful: how to calculate bricks, cement and sand for brickwork and the free building estimate Excel sheet.