Plastering a wall needs two numbers: how much wet mortar covers the area, and how much dry material that wet mortar comes from. The second step is the one most guides skip, and skipping it leaves you short at the merchant. The method below covers both.
Step 1: wet volume
Wet volume = wall area x plaster thickness. Measure the wall in square metres and the thickness in metres. For a 10 square metre wall plastered 12 mm thick: 10 x 0.012 = 0.12 cubic metres of wet mortar. Subtract doors and windows from the wall area first, or you will buy mortar for holes.
Thickness matters more than people expect. Internal plaster is often 12 mm, but rough or uneven walls can need 15 to 20 mm. Measure the thickest spots rather than assuming the wall is flat, and add a little extra for waste.
Step 2: dry volume
Dry mortar takes up more volume than wet mortar because the dry mix contains air gaps that water fills. The standard allowance is to multiply wet volume by 1.33. For our example: 0.12 x 1.33 = 0.16 cubic metres of dry material. If you skip this factor, you will come up about a quarter short.
This 1.33 factor is a planning rule used across masonry guides. It is approximate, not exact, because sand grading and moisture change the real figure. It is far better than ignoring the difference, and rounding up covers the rest.
Step 3: split by mix ratio
For a 1:4 cement-to-sand mix, cement is 1/5 of the dry volume and sand is 4/5. Cement: 0.16 / 5 = 0.032 cubic metres. At about 1,440 kg per cubic metre, that is about 46 kg of cement, roughly two 25 kg bags. Sand: 0.16 x 4/5 = 0.128 cubic metres, about 0.13 cubic metres.
Mix ratios are genuinely debated: some guides use 1:4 for internal walls, others 1:6. A richer mix (more cement) is stronger but costs more and can crack if it dries too fast. A leaner mix is cheaper but weaker. For internal plaster, 1:4 to 1:6 is the usual range; for external render exposed to weather, richer mixes and sometimes a waterproofing additive are common. Follow local practice for your wall type.
Step 4: add waste and check
Add 5 to 10% for spillage, sticking to the mixer, and uneven walls. Our example becomes about 50 kg of cement and 0.14 cubic metres of sand. Buy cement in whole bags, so round up to the next bag. It is cheaper to return one unopened bag than to stop work halfway through a wall, because a cold joint between old and fresh plaster shows forever.
Ceilings, second coats, and waste
Ceilings need more care than walls: plaster can sag if applied too thick in one pass, so ceilings are often done in two thinner coats with drying time between them. Budget the same total thickness, but expect higher waste, around 10 to 15%, because overhead work drops more material.
Second coats also change the maths. A base coat (scratch coat) plus a finish coat means two volume calculations, often at different thicknesses and sometimes different mixes. Calculate each coat separately and add them, rather than doubling a single-coat figure, since the base coat is usually thicker and rougher than the finish.
Try it faster with the tool
You can follow every step above by hand, but the free Render and Plaster Mortar Calculator — Dry Mortar Kilos and Bags by Area and Layer Thickness does the arithmetic for you. Enter your numbers and it gives the answer right away, so you can check your hand calculation or skip straight to the result.
Quick tips
- Wet volume = area x thickness.
- Dry volume = wet volume x 1.33.
- Round cement up to whole bags and add 5-10% waste.