What this calculator does
Plaster and screed are ordered by area and thickness, but mixed by volume. This calculator converts between the two, applies the standard 1.33 dry-to-wet expansion for mortar, and returns the bag count and sand volume for the mix ratio you choose.
How to use it
- Enter the area to be plastered and the finished thickness.
- Choose the mix: 1:3 for a hard finish, 1:4 for general rendering and 1:6 for a light screed.
- The dry volume allows for the water that evaporates, so it is what you mix.
- Add a waste allowance and check the coverage against the supplier's datasheet.
Governing formula
V_wet = A·t ; V_dry = 1.33·V_wet ; cement = V_dry × kg/m³(mix) Where:
-
A= Surface area [m²] -
t= Thickness [m] -
1.33= Dry-to-wet volume expansion factor for mortar [–]
Derived Equations:
bags = ⌈cement_kg / 50⌉ Worked example — 120 m² of wall at 12 mm in a 1:4 mix
The figures below were produced by running this page's own engine with the example inputs (Surface area (m²), Thickness (mm), Mix ratio, Waste allowance (%)). If the calculator returns something different for the same numbers, that is a bug, not a rounding difference.
| Wet volume | 1.44 m³ |
|---|---|
| Dry volume to mix | 1.92 m³ |
| Cement bags (50 kg) | 15 no |
| Sand volume | 1.53 m³ |
| Mix ratio | 1:4 |
| Assessment | 120 m² × 12 mm needs ≈ 1.44 m³ (1.92 m³ dry) → 15 bags of cement and ≈ 1.53 m³ of sand at 1:4. |
- The wet volume is 120 × 0.012 = 1.44 m³.
- Dry volume with the 1.33 factor is about 1.92 m³.
- A 1:4 mix needs roughly 380 kg of cement per m³, so about 730 kg — 15 bags.
Assumptions & limitations
Engineering calculation. Volume, dry-volume expansion and cement/sand quantities for three standard mixes. Mix proportions assume 50 kg cement bags and well-graded sand.
- Mix proportions are nominal; cement content per m³ varies with the sand grading and the mix design.
- Assumes a one-coat application; two-coat plaster needs separate quantities for each coat.
- No allowance for surface preparation, curing wastage or admixtures.
This tool supports professional engineering work; it does not replace a sealed design. Confirm every result against the applicable standard and a licensed engineer before it is built.
References & standards
- BS EN 998-1 — specification for mortar for masonry.
- BS EN 1992-1-1 — concrete construction, tolerances and execution.