Free • No signup

Plaster Calculator

Calculate cement and sand required for wall plastering, with adjustable thickness and mix ratio. Standard for both internal (1:6) and external (1:4) plasters.

Enter your values

m
m
%
5 %30 %
Plaster Materials
3 bags + 755 kg sand
Wall Area
30 m²
Wet Volume
0.36 m³
Dry Volume
0.551 m³
Cement
3 bags (150 kg)
Sand
755 kg

* Dry volume factor of 1.33 accounts for voids and shrinkage.

* Internal walls typically use 12mm thickness; external 15-20mm.

* Wastage of 15-20% is normal for hand-applied plaster.

Quick answer

Plastering covers brick walls with a thin layer of cement-sand mortar to create a smooth surface ready for painting. The calculator gives you cement bags and sand needed for any wall area, mix ratio, and thickness.

What is Plaster?

Plaster serves two purposes: it smooths out the rough surface of brick walls (so paint sits evenly) and protects the brick from weather/water damage. Internal walls use thinner plaster (12 mm) and a leaner mix (1:6). External walls need thicker plaster (15-20 mm) and stronger mix (1:4) for water resistance.

The math: wall area × thickness gives the wet plaster volume. Multiply by 1.33 (dry volume factor) and add wastage. Then split between cement and sand based on the mix ratio.

Hand-applied plaster has high wastage — 15-20% is normal because mortar drops to the floor as the mason works.

Wall area × thickness × dry-factor

Formula
Wet volume = Area × thickness Dry volume = Wet × 1.33 × (1 + wastage) Cement = Dry × c/(c+s) Sand = Dry × s/(c+s)
Area
Wall area—Length × Height in m²
thickness
Plaster layer—12 mm internal, 15-20 mm external
c, s
Mix ratio—1:6 = c=1, s=6
Worked example
Wall10 × 3 = 30 m²
Thickness12 mm internal
Mix1:6
Wastage15%
Wet volume = 30 × 0.012 = 0.36 m³
Dry volume = 0.36 × 1.33 × 1.15 = 0.55 m³
Cement = 0.55 × 1/7 = 0.079 m³ → 3 bags
Sand = 0.55 × 6/7 = 0.47 m³ → 754 kg
➜ 3 cement bags + 754 kg sand

How to use this calculator

Wall dimensions, plaster thickness, mix ratio, and wastage.

  1. Enter wall length and height

    In metres. Calculate per wall, sum across the room.

  2. Pick plaster thickness

    12 mm for internal walls (smooth), 15 mm for external, 20 mm only for very uneven external.

  3. Pick mix ratio

    1:6 for internal, 1:4 for external (water resistance), 1:3 for ceilings.

  4. Set wastage

    15% standard, up to 20-25% for ceilings (more falls during overhead work).

Plastering applications

Internal wall plastering

12mm × 1:6 — most common. Smooth finish before painting.

External wall plastering

15mm × 1:4 — water-resistant, weather-protective.

Ceiling plastering

12mm × 1:3 — richer mix for adhesion against gravity. Higher wastage (20%+).

Skim coat / finish plaster

2-5 mm of fine plaster over base plaster for ultra-smooth surface. Very low cement usage.

Glossary

Internal plaster
Inside walls. 12mm thickness, 1:6 mix typically.
External plaster
Outside walls. 15-20mm thickness, 1:4 mix for water resistance.
Skim coat
Final fine layer (2-5mm) for ultra-smooth finish.
Dry volume factor
1.33× — accounts for voids and shrinkage when water is added to the dry mix.

Frequently asked questions

What plaster thickness should I use?
12 mm for internal walls (smooth) and 15-20 mm for external walls (need weather protection and uneven surface coverage). Ceilings: 12-15 mm.
What's the right mix ratio?
1:6 (cement:sand) for internal walls — most common. 1:4 for external (more cement = better water resistance). 1:3 for ceilings or critical areas. Stronger mix = costlier but more durable.
Disclaimer: Results are estimates based on the inputs you provide. They are not professional advice. For consequential decisions — financial, tax, medical, or legal — verify with a qualified professional.

Related calculators

Brick

Calculate the number of bricks required for a wall, including mortar, with wastage allowance. Supports standard Indian and modular brick sizes.

Concrete

Calculate the volume of concrete required for slabs, columns, beams, and footings — and the breakdown of cement, sand, and aggregate by mix grade (M10, M15, M20, M25).

Paint

Calculate litres of paint required for walls and ceiling — accounting for room dimensions, doors, windows, number of coats, and paint coverage rate.

Tile

Calculate the number of tiles and boxes required for a floor or wall, with wastage allowance for cuts and breakages.

Steel Weight

Calculate the unit weight of steel rebar, plates, pipes, angles, and channels. Uses precise density (7,850 kg/m³) and standard formula d²/162.28 for rebar.