Plaster Calculator
Plaster is a thin cement and sand layer on walls and ceilings for a smooth, finished surface. It improves appearance, protects masonry, and prepares for paint or tiles. This tool estimates plaster volume and dry materials from surface dimensions, thickness, and mix ratio.
Masonry joints: mortar calculator. Floor screed: screed calculator. Sand–cement mixes: sand and cement calculator.
Plaster mix
Surface area = length × height (exclude large openings if you want a closer net estimate).
Typical guides: internal walls ~10–12 mm, external ~15–20 mm, ceilings often ~6–8 mm.
Default bulk densities: cement 1440 kg/m³, sand 1600 kg/m³ (override below if needed).
Volumes
Surface area 0.00 m²
Plaster volume (placed) 0.00 m³
Dry batching volume 0.00 m³
Materials
Cement (bulk) 0.00 m³ → 0.00 t · Bags: 0
Sand (bulk) 0.00 m³ → 0.00 t
Add roughly 5–10% extra for wastage when ordering.
How this calculator works
Steps:
- Surface area = length × height
- Plaster volume (placed) = area × thickness
- Dry batching volume = plaster volume × 1.33 when enabled
- Cement : sand split by dry volume: each material = dry batching volume × (its part ÷ sum of parts)
- Mass from bulk densities (defaults cement 1440 kg/m³, sand 1600 kg/m³), plus cement bags
Standard plaster mix ratios
Common mixes (dry materials before water):
- 1 : 3 — external walls or high-strength finish coats where specified
- 1 : 4 — standard internal plaster in many specifications
- 1 : 5 or 1 : 6 — leaner mixes for backing or low-strength work
Ratio affects durability, shrinkage, and finish quality—match the job and engineer or manufacturer guidance.
Typical plaster thickness
Indicative bands (always confirm with your spec):
- 10–12 mm — many internal wall coats
- 15–20 mm — external or heavier build-ups where allowed
- 6–8 mm — some ceiling finish scenarios (system-dependent)
Thickness drives volume directly—small errors in thickness change cement and sand a lot over large areas.
Where this calculator is used
- Wall plastering (internal and external)
- Ceiling finishing
- Repair and patching
- Surface leveling before painting or tiling
Example calculation
Wall:
- Length 5 m, height 3 m, thickness 12 mm (0.012 m)
Step 1 — Area = 5 × 3 = 15 m²
Step 2 — Volume = 15 × 0.012 = 0.18 m³
Step 3 — Dry volume = 0.18 × 1.33 ≈ 0.24 m³
For a 1 : 4 mix (total parts 5):
- Cement ≈ 0.24 × (1/5) ≈ 0.048 m³ — about 69 kg at 1440 kg/m³ — on the order of 1–2 × 50 kg bags depending on rounding
- Sand ≈ 0.24 × (4/5) ≈ 0.19 m³
Enter 5, 3, and 12 mm with ratio 1 : 4 to reproduce.
Why dry volume adjustment is needed
Dry mix behaves differently from placed wet plaster because of:
- Voids between sand particles
- Air in the dry mix
- Water demand when mixing
The 1.33 factor (optional to disable) aligns dry cement and sand orders with placed plaster volume in a common planning approach.
Common mistakes to avoid
- Ignoring thickness variation — uneven backgrounds use more material.
- Skipping the dry volume factor — can underestimate orders.
- Wrong mix ratio — weak plaster may crack or peel; strong mixes shrink more.
- No wastage allowance — add roughly 5–10% for spillage and rework.
Practical tips
- Measure area carefully; deduct large openings if you want net quantity.
- Add a small margin for wastage.
- Keep thickness as even as practical.
- Cure and protect coats to limit early cracking.
Two-coat and base/finish work
If base and finish coats differ in thickness or ratio, run the calculator per coat (or per zone) and sum cement and sand for ordering.
Frequently asked questions
How much cement is needed per square meter of plaster?
It varies with thickness and ratio; rough planning figures often cite about 5–8 kg/m² of cement for some typical internal scenarios, but your job may differ. Use this page with your dimensions for a closer number.
Can plaster thickness vary across the wall?
Yes. Dubbing, uneven blockwork, and services chases increase average thickness—consider a higher wastage factor or zone-by-zone measurements.
Is plastering required before painting?
Many painted finishes need a suitable, stable, and sufficiently smooth substrate; cement–sand or proprietary plaster systems are common ways to achieve that on masonry. Follow coating manufacturer requirements.
What happens if plaster is too thick?
Thick layers add shrinkage and weight and can crack or debond if the background or mix is wrong for that thickness.