How to calculate plaster and render for a wall

1. Measure the net area, face by face

Multiply the length by the height of each wall and subtract doors and windows. If both faces of a wall are plastered, count both: the area doubles.

• Deduct only the openings that are not plastered inside their reveals; the reveals themselves are small extra areas.
• Ceilings and returns are separate surfaces: add them.

2. Choose the thickness

A single cement coat is often 10 to 20 mm. A wall that is not plumb or is very rough takes more, and above about 20 mm the render is normally applied in several coats. The thickness is the biggest lever on the quantity: 25 mm needs two thirds more mortar than 15 mm.

3. Add a waste margin

Rough masonry absorbs mortar, corners need extra and part of the mix is lost while working. A margin of 10 to 25 % is common; use more for a very uneven background.

4. Convert the mortar into cement and sand

Mortar volume = net area × thickness × (1 + waste). Cement and sand follow from the mix: kilograms per m³ of mortar multiplied by the volume. The calculator starts from a cement dosage of 350 kg/m³ and 1,400 kg of sand per m³, as editable defaults; your site practice and the supplier’s data sheet take precedence.

Worked example: a 12 m × 2.6 m wall

Gross area 31.2 m², minus 4 m² of openings = 27.2 m² net, one face, 15 mm, 15 % margin.

• Mortar: 27.2 × 0.015 × 1.15 ≈ 0.469 m³.
• Cement: about 164 kg, that is 5 bags of 35 kg.
• Sand: about 657 kg.

What this does not cover

Gypsum and ready-mixed plasters have their own consumption per m² and per mm, printed on the bag: use that figure for them. Primer, mesh, drying time and prices are not included, and no unit price is assumed.