Plaster and Render Calculator
Mortar, cement and sand by layer for internal plastering.
Quick answer
With the default inputs: Plaster area 100 m² · Base coat 12 · Middle coat 6 etc.Total mortar 2.205 m³
Edit the inputs below to recalculate.Inputs
Result
Total mortar
2.205m³
| Total thickness | 21.0mm |
| Base coat mortar | 1.260m³ |
| Middle coat mortar | 0.630m³ |
| Finish coat mortar | 0.315m³ |
| Cement | 882kg |
| Cement bags | 18bags |
| Sand | 2.25m³ |
| Sand mass | 3.37t |
| Mortar per m² | 0.0221m³/m² |
Total plaster thickness is usually 15-20 mm; beyond 25 mm you need mesh or reinforcement or it hollows and cracks. Layering is not about saving material — a single 20 mm coat is heavy enough to shear its own bond. Use 1:3 for the base and 1:2.5 or a gauged mix for the finish; less cement near the face actually trowels better.
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Enter a target result; the other inputs stay fixed and this one is worked out.
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).
How to work it out
- Fix the total thickness first: 18 mm for ordinary plastering, 25 mm for high-grade, and anything over 35 mm needs mesh or a reinforced build-up.
- Split into coats: base 10-15 mm to bond and level, middle 5-8 mm to level further, finish 2-3 mm for appearance and trowelling. The total is never applied in one pass.
- Cost each coat separately: volume = area × that coat's thickness, and the three add up to the total mortar.
- Convert by mix: 1:3 or 1:2.5 cement mortar for the base, 1:2.5 or 1:2 for the finish (stronger but shrinks more, so thick finish coats crack); allow about 400 kg of cement per m³.
- Add 5-8% waste for droppings, edges and reveals around openings; take the upper figure in rooms with many openings.
Where it goes wrong
- Applying 20 mm in one pass — anything over 8 mm slumps and cracks; the code limits each coat to 5-7 mm for cement mortar, and coats are not red tape.
- One mix for all three coats — the base needs bond (more cement), the finish needs low shrinkage (less cement or some fly ash); all 1:2 gives widespread crazing.
- Over 35 mm without mesh — thick plaster pulls itself off under its own weight and shrinkage, so mesh or a bagged render build-up is mandatory.
- Not wetting the substrate — aerated concrete and lightweight block absorb water fast and rob the mortar of the water it needs to hydrate, which always ends in debonding.
Material and cost
Internal plaster at 18 mm with labour 18-30 CNY/m², external 25-40 including mesh and arises. Cement mortar 320-420 CNY/m³, bagged plaster 400-550 CNY/t.
On site
- Set datums and screed rules before plastering at 1.2-1.5 m centres; plastering by eye guarantees more than 4 mm out of flat.
- Fix 300 mm wide mesh or alkali-resistant scrim across every change of substrate — concrete to block, beam to wall. It is the single most effective anti-crack measure.
- Form arises at openings in 1:2 mortar at least 2 m high and 50 mm wide each side, or the corner spalls the first time something is carried through.
Basis
- GB 50210-2018 Code for acceptance of building decoration, Chapter 4 (plastering)
- JGJ/T 220-2010 Technical specification for plastering mortar
- GB 50327-2001 Code for construction of residential decoration, Chapter 4
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.
Cite this page
Use this format in reports, method statements or coursework. The link is permanent.
Building Calculator. Plaster and Render Calculator. 2026-10-25. https://jianzhujisuan.com/en/plaster-coat
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.