Plaster and Render Calculator
Mortar, cement and sand by layer for internal plastering.
Inputs
Result
Formula
| Layer volume | area × thickness ÷ 1000 |
| With waste | volume × (1 + waste%) |
| Total mortar | sum of the layers |
| Cement | total × cement content |
| Sand | total × 1.02 m³ per m³ |
| Bags | ceil(cement ÷ 50) |
100 m² at 12 + 6 + 3 mm with 5% waste and 400 kg/m³: 21 mm total, 2.205 m³ of mortar, 882 kg of cement (18 bags) and 2.249 m³ of sand (3.37 t).
FAQ
Why three coats instead of one thick one?
Because mortar is plastic before it sets, and a thick coat sags under its own weight and debonds from the background. A single 20 mm coat hollows in over 30% of the area; layers of 6-13 mm applied after the previous has set keep it under 5%. This is not fussiness — it is what passes the tap test at inspection.
Should openings be deducted from plaster area?
Under measurement rules, openings under 0.3 m² are not deducted and larger ones are — but the reveals must be added back. For ordering, leaving them in is closer to reality, because reveals, angles and pipe collars are fiddly and consume more per square metre than flat wall.
How much does the substrate change quantities?
A lot. Fair-faced concrete needs only 15 mm in total, blockwork 20 mm, and rubble or old walls 30 mm or more to level. Absorption matters too: AAC sucks water fast, so it needs a bonding agent and pre-wetting, otherwise the mortar dries before it gains strength and dusts off under a fingernail.
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Results are estimating aids only and are not a design basis. Construction and final accounts must follow the drawings, the applicable standards and an approved method statement. Anything affecting structural safety, shoring or falsework must be checked by a suitably qualified engineer.