the symbol ofStrengthFind my product →Enter the wall area and the plaster thickness you're applying — we'll work out how much Ready-Mix Plaster you need and how many 40 kg bags to buy, with a wastage allowance built in.
Internal walls are typically plastered at 10–12 mm; rougher external walls at 15–20 mm. Thicker plaster uses more material per m².
Supplied in one standard pack.
Extra for spillage, mixing loss & uneven surfaces.
Consumption is estimated at about 2.2 kg of plaster per m² per mm of thickness, so a 40 kg bag covers roughly 15–20 sq.ft at 10–12 mm — matching the product datasheet. Actual usage varies with surface roughness, wall undulation and workmanship; always confirm against the datasheet and allow for wastage.
Most plaster calculators online work out cement-sand ratios for site-mixed plaster. This one is different: it estimates SUMO Ready-Mix Plaster — a factory-blended 40 kg bag you just add water to — so the answer comes out in bags, not in cement bags plus sand volumes plus a dry-volume factor.
Consumption is calibrated to the product brochure: about 2.2 kg per m² for every millimetre of thickness, which lands a 40 kg bag at 15–20 sq.ft of wall at the common 10–12 mm internal thickness.
Enter the wall area to plaster (or length × height) in sq.ft or m².
Pick the plaster thickness — 10–12 mm is typical for internal walls, 15–20 mm for external or uneven masonry.
Keep ~10% wastage for mixing losses and floor droppings.
Read off kilograms and 40 kg bags to buy.
The 2.2 kg/m²/mm constant is calibrated to the brochure coverage of the product — 15–20 sq.ft per bag at 10–12 mm — not a generic mortar density.
| Thickness | Typical use | Consumption | Coverage per 40 kg bag |
|---|---|---|---|
| 10 mm | Internal walls, even masonry | ≈ 22 kg/m² | ≈ 1.8 m² · 20 sq.ft |
| 12 mm | Internal walls, standard | ≈ 26 kg/m² | ≈ 1.5 m² · 16 sq.ft |
| 15 mm | External walls | ≈ 33 kg/m² | ≈ 1.2 m² · 13 sq.ft |
| 20 mm | External / uneven, built in coats | ≈ 44 kg/m² | ≈ 0.9 m² · 10 sq.ft |
Thicker external builds are usually applied in two coats — the total bag count stays as calculated for the full thickness.
Bag-versus-sand price comparisons miss where site-mix plaster leaks money. The honest comparison counts these:
Sand logistics — ordered loose, stored in the open, walked over, rained on. 10–20% of a sand heap never reaches the wall.
Ratio drift — every hand-batched mix is a slightly different plaster; the weak batches crack and become rework.
Screening labour — site sand needs sieving; the bag arrives graded.
Speed — no batching station, no waiting on the mixer between floors. Plasterers plaster.
Check the wall plumb before quoting a thickness — a wall leaning 10 mm needs a 20 mm average coat, not 12 mm, and the bag count doubles quietly.
External or thick builds go on in two coats; the calculator’s total for the full thickness still holds.
Mesh the junctions — where masonry meets RCC columns and beams, embedding mesh in the coat is commonly recommended to stop the classic junction crack.
Cure it like any cement plaster, per the datasheet — a cracked coat is the most expensive way to save water.
400 sq.ft = 37.2 m².
37.2 × 26.4 kg/m² ≈ 981 kg → + 10% wastage ≈ 1,079 kg.
1,079 ÷ 40 = 27.0 → whole bags.
About 15–20 sq.ft at the common 10–12 mm internal thickness — ~20 sq.ft at 10 mm, ~16 sq.ft at 12 mm. At external thicknesses (15–20 mm) it drops to 10–13 sq.ft per bag.
Ready-mix removes the biggest waste sources of site mixing: no sand piles losing material to wind and rain, no ratio drift between batches, no over-ordering loose sand. You buy exactly the bags the wall needs — which is what this calculator tells you.
Internal walls on even blockwork: 10–12 mm. External walls or uneven masonry: 15–20 mm, applied in coats. Thickness is the whole cost driver — going 12 → 15 mm is 25% more material.
Yes — it is a cement-based plaster and needs water curing like any cement plaster. Follow the curing schedule in the product datasheet on the Ready-Mix Plaster page.
Ready-Mix Plaster is made for masonry backgrounds including AAC block walls — see the datasheet for substrate preparation. For the blockwork itself, pair it with Block Jointing Mortar.