Plaster cement and sand from mix ratio

Ratio parts, bulking and bags — batching sand-cement plaster for the wall

The ratio is a site convention counted in volume, not a laboratory recipe. One-to-four means one bucket of cement gauged against four of sand, however big the bucket — which is why the split happens in volume parts and the conversion to mass comes afterwards, through each material’s own loose bulk density.

The bulking factor is where the dry heap and the finished coat are reconciled. Water fills the voids between sand grains and the trowel compacts what the mixing slumped, so a coat always consumes more loose material than its own measured volume. The factor carries that whole loss in one number, and trial-gauging on site outranks any default.

The wall itself decides how honest the thickness input is. A background out of plumb is dubbed out through the plaster, high-suction blockwork drinks the mix as it is laid on, and a raked joint takes its own filling — so the figure entered here is the average thickness the wall will really take, not the specification’s nominal coat.

Masses and bags are the answer because heaps and buckets are not how cement arrives. The engine reports cement and sand in kilograms and the cement again as bags at the size you buy, unrounded — a demand just past a whole bag is worth seeing exactly before the order is placed.

What the figure excludes is everything that is not the undercoat mix: the gauging water, plasticiser and any additive, the gypsum setting coat that skins the work, and beads, stops and mesh at edges and junctions. Those are their own lines — and whether this mix suits this background is the specification’s ruling, never this page’s.

When this calculation is used

• Ordering cement and sand for a plastering package once wall areas are measured and the specification names a ratio.
• Converting a specified one-to-four undercoat into a merchant order of bags and a tipped load of sand.
• Checking a plasterer’s materials allowance by reproducing it from area, thickness, ratio and a declared bulking factor.
• Re-costing the same walls when the specification leans the mix from one-to-four to one-to-five.

Worked example

The pack’s declared vector: fifty square metres of wall at the default thirteen millimetres, a one-to-four mix, a bulking factor of one point three three, the usual loose densities for cement and sand, and fifty-kilogram bags.

Three figures come back: the cement mass, the sand mass, and the cement counted again in bags. Read the bag count’s fraction before ordering — and read the sand beside the cement, because the tipped load is the bulkier half of the same ratio and the easier one to under-order.

Every figure in this example is produced by the certified engine when the calculator loads, checked against the signed pack’s declared test vectors; nothing on this page stores an answer. Lean the ratio from one-to-four to one-to-five and watch the cement fall while the sand climbs — the wall is unchanged, and only the recipe moved.

What each input represents

Assumptions and limits

• This is nominal volumetric proportioning, not a designed mix: it batches what a named ratio consumes and makes no claim about the strength or suction behaviour of the result.
• The thickness is an average over the whole area; a wall needing heavy local dubbing-out deserves its own area at its own thickness rather than a padded average.
• One bulking factor stands for the sand actually on site at its delivered moisture; very wet or bone-dry sand moves it, which is what trial-gauging is for.
• The result is undercoat cement and sand only: gauging water, plasticiser, the gypsum setting coat, and beads and stops at edges are separate lines.

What the guards protect against

• A bulking factor outside the narrow band plastering practice actually uses is refused, and the pack ships the refusal as a declared test vector — a factor below the band usually means the wet-to-dry conversion has been forgotten entirely, not measured better.
• A combined thickness beyond forty millimetres is beyond a single measured plaster system and is refused; builds that heavy are dubbing-out and backing-coat work that deserves its own take-off per coat.
• The area, ratio parts, densities and bag mass must all be strictly positive — the split divides by the ratio sum and the bag count divides by the bag, so a zero anywhere is refused before it becomes a nonsense order.

Provenance

Nominal volumetric mix proportioning (ACI 211.1 tradition; BS EN 998-2)

Wet coat volume as area times combined thickness, grossed to loose dry volume by the bulking factor; that volume split by the ratio parts, each part converted to mass through its bulk density, and the cement divided into bags, reported unrounded.

A batching convention to be verified against the project specification and trial gauging; mix suitability for the background is never determined here. The signed pack carries its own citation, and the page reports the verification state of the release it mounted.