How Much Cement and Sand Is Required for Plastering? A Simple Plaster Calculator

For 12 mm plaster in a 1:4 cement-sand mix, you need about 4.4 kg of cement and 0.012 cubic metres of sand per square metre, or about 0.41 kg of cement per sq ft. One 50 kg bag covers roughly 120 sq ft. These figures use a stated assumption for dry volume, explained below, and exclude wastage.

Plastering is one of the quantities owners most often get wrong, because the mason says "so many bags" and nobody can check it. Here is the arithmetic, so you can.

The plaster calculator: one formula

Plaster mortar is measured wet but mixed from dry materials, so you add an allowance. Dry volume = plaster area x thickness x 1.27. Cement = dry volume x 1 / (1 + sand parts) x 1,440 kg. Sand = dry volume x sand parts / (1 + sand parts). Add a wastage margin on top.

In words: find the wet volume of the plaster, increase it to get the dry volume of materials, split it by the mix ratio, and convert cement to kg.

Where each number comes from

• Thickness. IS 1661 (1972), Table 2, lists 10 to 15 mm for single-coat plaster. Two-coat work should not normally exceed 20 mm in total, and no more than 15 mm on in situ concrete soffits (clause 7.3.2). We use 12 mm, which sits inside that range.
• Mix ratio. The same table lists cement-lime-sand mixes of 1:0:3, 1:0:4 and 1:0:6, where the middle number is lime. We use 1:4 and 1:6 (no lime). Which one your wall needs is for your engineer to say.
• Cement density. IS 1661, clause 11.1.2, takes 1 cubic metre of cement as about 1,440 kg.
• The 1.27 factor. This is a commonly used estimating allowance for the dry material needed to make one unit of wet mortar. It is not from IS 1661. Treat it as an assumption and ask your engineer which factor they use.
• Bag size. We assume 50 kg bags.

How much cement and sand per sq m and per sq ft?

Using the formula, here is the quantity for one square metre of plaster. Divide by 10.76 for one sq ft.

Our arithmetic, before wastage. Sand in cubic metres; multiply by 35.3 for cubic feet. Notice that a leaner 1:6 mix uses less cement but slightly more sand, and thicker plaster uses more of everything.

Worked example: one bedroom, 12 ft x 14 ft, 10 ft high

These are our assumptions: 12 mm plaster, 1:4 mix, walls and ceiling plastered, one door of 7 x 3 ft and one window of 4 x 4 ft deducted, 50 kg bags.

The 10 percent wastage is our planning allowance for spillage, rebound and leftovers, not a rule. Real wastage depends on the mason and the site.

How do I scale this to a whole house?

Do the room-by-room measurement above for every room, add the plaster areas, and apply the per sq ft figures. As a shortcut, 1:4 at 12 mm needs about 0.0082 bags per sq ft of plaster, so 1,000 sq ft of plaster area needs about 8.2 bags, or 9 bags with 10 percent wastage. Plaster area means the surface of walls and ceilings, which is a different number from floor area. Ask your contractor for the plaster area on the quantity sheet, and check it against your own sums.

What changes the answer in real life?

• Uneven walls. If the wall is out of line, the plaster is thicker in places. A wall that needs 20 mm in patches will use far more than 12 mm on paper.
• Wet sand. Damp sand takes up more volume (bulking), so a measured box of wet sand holds less real sand. IS 456 (clause 10.2.4) tells site engineers to allow for bulking when batching by volume. Ask your mason how the sand is being measured.
• Water. IS 1661 (clause 11.1.4) gives about 70 percent of the cement's weight as water for a 1:3 mix, with adjustments for the sand and the weather. Ask your mason who controls the water.
• Openings and the way they are counted. Rules for deducting doors and windows vary between contracts. Agree on one method before work starts.

What does it cost?

Cement cost = bags x your dealer's price per bag. Sand cost = volume x your supplier's price per unit. Both prices vary by day and supplier, so we give none here. The house materials guide and the cement types guide help you pick what to ask for.

Does plastering cost extra on a package?

If you build on a per sq ft package, plaster is part of the building scope, so you do not order the bags yourself. Check what your package lists and what is excluded on the packages page, and ask for the finish and thickness in writing. For a whole-house budget, use the cost estimator.

Next step: get your whole-house budget, not just plaster, from the cost estimator.

Sources: BIS, IS 1661:1972 Code of practice for application of cement and cement-lime plaster finishes (Table 2, clauses 7.3.2, 11.1.2, 11.1.4); BIS, IS 456:2000 (clause 10.2.4, bulking). The 1.27 dry-volume factor, 12 mm thickness, 10 percent wastage and room dimensions are our assumptions. Standards are revised; rules and codes change; confirm with your architect or structural engineer.

Frequently asked questions

How much cement is required for plastering per sq ft?

For 12 mm plaster in a 1:4 mix, about 0.41 kg of cement per sq ft, so one 50 kg bag covers roughly 120 sq ft. That uses a 1.27 dry-volume allowance and excludes wastage. Thicker plaster or a richer mix needs more, so confirm the mix and thickness with your engineer.

How much sand is required for plastering per sq ft?

For 12 mm plaster in a 1:4 mix, about 0.0011 cubic metres, or 0.04 cubic feet, of sand per sq ft, before wastage. In a 1:6 mix it is slightly more sand and less cement. Allow for bulking if the sand is damp, and ask how your mason measures it.

What is the plaster thickness and mix ratio for walls?

IS 1661 (1972) lists 10 to 15 mm for single-coat plaster, with mixes such as 1:0:3, 1:0:4 and 1:0:6 (cement, lime, sand). Two-coat work should not normally exceed 20 mm in total. The right choice for your wall is your engineer's decision, so confirm it before work begins.

Why do I need to add 27 percent to the wet volume?

Mortar is measured wet but made from dry cement and sand, which pack down once water is added. Estimators therefore multiply the wet volume by an allowance, here 1.27. That figure is a commonly used assumption, not a value from IS 1661, so ask your engineer which factor they use.

How do I measure the plaster area of a room?

Multiply the room perimeter by the wall height, subtract doors and windows, and add the ceiling area. A 12 x 14 ft room that is 10 ft high with one door and one window comes to 651 sq ft in our example. Agree the deduction method with your contractor first.