Plaster and screed volume, cement bags and sand
Converting area and thickness into wet volume, dry volume and the bag count you will actually order.
The engineering question
Plaster is ordered by volume but measured by area. How many bags and how much sand does a given area and thickness need?
Worked example
The figures below were produced by running the Plaster & Screed Volume Calculator with the same inputs — during the build, from its authoritative engine.
| Wet volume | 1.44 m³ |
|---|---|
| Dry volume to mix | 1.9152 m³ |
| Cement bags (50 kg) | 15 no |
| Sand volume | 1.5322 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. |
- Wet volume is area × thickness.
- Dry volume allows 1.33× for the water that evaporates — mix the dry volume, not the wet.
- Cement content per cubic metre depends on the mix ratio, so 1:3 uses more cement than 1:6.
Sanity check
If your dry volume equals your wet volume, the 1.33 expansion factor has been missed and the mix will be weak.
Tools for this problem
These are the EngiMetric calculators that compute the numbers in this topic. Each one shows its formula, its units and its limits rather than returning a bare result.
References & standards
- BS EN 998-1 — specification for mortar for masonry
- BS EN 1992-1-1 — execution of concrete structures
Related problems
Blockwork quantities: blocks, mortar and the waste you actually incur
Block counts per square metre, opening deductions, mortar volume and a realistic waste allowance.
Taking off concrete volumes without mis-ordering the pour
Where concrete volume actually goes — including the parts that are habitually forgotten — and how wastage and batching affect the order.
Also: Civil & Structural