Mortar cement and sand from mix ratio

What the joints swallow: gauging a mortar order from a compacted volume

The volume is a handover point, and this calculator is deliberately incurious about its origin. Whether it came from a bed-and-perpend take-off, a per-unit allowance or a measured pour of test panels, the figure entered is the wet, compacted volume in place — and everything downstream is the conversion from that finished state back to the loose materials that produce it.

The dry-volume factor is that conversion. Loose cement and sand occupy more space in the heap than the dense mortar they become, so the compacted volume is expanded first, and only then divided. Unlike a free multiplier, the pack fences this factor inside the narrow band mortar practice actually supports, and refuses anything outside it.

The ratio is gauged in parts by volume — the bucket-count language of the scaffold, where the size of the bucket cancels out. The default gauge, one part cement to five of sand, is a common general-purpose convention and nothing more: the specification’s ratio, entered as two numbers, always wins.

Mass is how the order leaves the merchant. Each material’s volume share becomes kilograms through its own loose bulk density — a heap property, entered as delivered — and the cement is restated as bags at whatever mass your market bags it in, with the fraction kept so a demand just past a whole bag is seen before it is rounded.

The split is strictly two materials. Gauging water, plasticiser and admixtures sit outside it, and so does lime: a lime-gauged or masonry-cement mortar is a different recipe with a third part this arithmetic does not have. Whether any mix suits the units and the exposure is the specification’s ruling, never this page’s.

When this calculation is used

• Turning a bricklaying package’s mortar demand into a merchant order of bags and a tipped load of sand.
• Checking a gang’s materials allowance by reproducing it from the same volume, gauge and factor.
• Re-pricing the order when the specification leans the gauge richer or leaner for the exposure.
• Splitting a season’s pointing and repair work, summed as one volume, into what the yard should hold.

Worked example

The pack’s declared vectors cover the gauges a site actually meets: a single cubic metre of compacted mortar at the default one-to-five gauge and the default dry-volume factor; half a cubic metre at a richer one-to-four with the factor eased slightly lower; and two cubic metres at a leaner one-to-six, all in fifty-kilogram bags at the usual loose densities.

In every one of these vectors the sand mass dwarfs the cement mass — the tipped load is always the bulk of the order, and the easiest half to under-buy. Read the bag figure with its fraction: each vector returns a bag count that is not a whole number, and the fraction is exactly what you weigh before deciding whether the last bag is bought or borrowed. On the two-cubic-metre lean gauge, the sand line climbs into tonnes.

As everywhere on these pages, the example is computed, not quoted: the certified engine produces each figure at load and the results are checked against the vectors the signed pack declares. The refusals are declared with the same care — a dry-volume factor straying outside the working band is turned away on either side.

What each input represents

Assumptions and limits

• This is nominal volumetric gauging: it batches what a named ratio consumes and claims nothing about the strength, bond or durability of the result.
• The entered volume is wet and compacted in place; frog and perforation fill, joint efficiency and droppings all live inside the figure handed to this page, per the reader’s own take-off method.
• One dry-volume factor stands for the sand actually in the heap on the day, at its delivered moisture — which is what trial gauging with that sand is for.
• The order is cement and sand only: water, plasticiser and admixtures are separate lines, and a lime-gauged or masonry-cement mortar is a different recipe entirely.

What the guards protect against

• A dry-volume factor below the working band is refused, and the pack ships that refusal as a declared test vector — the band is the typical range mortar dry-to-wet conversion supports, and a factor outside it is something to verify against trial-mix data, not to force through.
• The same guard closes the top of the band, and that refusal is a declared vector too: the fence turns away implausible expansion in both directions, not just one.
• The volume, both ratio parts, both densities and the bag mass must each be strictly positive — every one of them sits under or inside a division somewhere in the chain, and the engine refuses the zero rather than letting it surface as an infinite or empty order.

Provenance

Nominal volumetric proportioning (ACI mix-design tradition; European masonry-mortar standards; Chudley and Greeno)

The compacted mortar volume expanded to loose dry volume by the dry-volume factor; that volume split in proportion to the cement and sand parts; each share converted to mass through its loose bulk density; and the cement restated as bags at the entered bag mass, fraction kept.

A batching convention, to be verified against the project specification and the mortar actually gauged on site; whether a mix suits the units and the exposure is a specification matter this page never decides. The signed pack carries its own citation, and the page reports the verification state of the release it mounted.