Mortar mix calculator

What this calculator does

This mortar mix calculator turns a target mortar volume and a chosen cement-to-sand ratio into a concrete shopping list: kilograms of cement, kilograms of dry sand, litres of mixing water, and the number of cement bags you actually need to buy. It is meant for jobsite estimation — bricklaying, blockwork, plastering, screeding, repointing, small footings — where you do not want to wing it and you do not want to drag a wheelbarrow back to the merchant for two extra bags. Inputs accept the most common European 25 kg and 50 kg cement bag formats, mix ratios from a strong 1:3 down to a soft 1:8 pointing mix, water-to-cement (w/c) ratios from 0.35 to 0.80, and a configurable waste percentage from 0 % to 30 %. Outputs are computed instantly in your browser; nothing leaves the page.

The math is intentionally transparent: the wet mortar volume you enter is multiplied by 1.33 to convert to dry volume (the classic site rule that accounts for void closure when sand grains lock into the cement paste), then by (1 + waste%/100) for waste, then split between cement and sand by the ratio parts. Cement mass uses a bulk density of 1440 kg/m³ and sand uses 1600 kg/m³ — both conservative averages for ordinary Portland cement powder and dry building sand. Water demand is the cement mass times the w/c ratio. Bags are rounded up because you cannot buy a fraction of a bag.

How to read the results

The big KPI on the left is cement in kilograms — usually the deciding cost line. The "bags needed" badge below it is the practical number you'll quote to the builders' merchant. To its right are sand in kilograms (sand is sold by the tonne or the bulk bag, so divide by 1000 if you are quoting in tonnes), water in litres, and a duplicate of the bag count for visual symmetry on wider screens. Below the KPIs, a horizontal stacked bar shows the volumetric split between cement and sand — useful as a sanity check that you picked the right ratio. A 1:4 mix should look roughly 20 % cement / 80 % sand by volume; a 1:8 pointing mix is closer to 11 % / 89 %. If the bar looks wrong, double-check the dropdown.

Pitfalls and assumptions to know

• The 1.33 dry-volume factor is a heuristic. It works well for ordinary medium-grade builders' sand (0–4 mm). Very fine sand can push the factor closer to 1.40; sharp coarse sand can drop it to 1.25. For a critical structural pour, weigh a known volume of your actual sand and recompute.
• Sand types matter more than people think. Sharp sand (angular, coarse) gives a strong, slightly harsh mix used for screeds and concrete. Builders' sand (soft, rounded, often called "soft sand" or "bricklayer's sand") flows better under a trowel and is the default for laying bricks and blocks. Masonry sand is graded specifically for ASTM mortars. Plastering sand is finer still. Substituting one for another changes workability, strength, and water demand.
• High w/c weakens mortar. Anything above 0.60 starts to cost compressive strength fast. The default 0.55 is a sensible compromise; 0.45–0.50 yields a much stronger mortar but is harder to trowel. Drop to 0.35–0.45 only with a plasticiser or a richer cement content.
• Cure damp for at least 7 days. Freshly placed mortar wants to lose water by evaporation, which arrests hydration and leaves you with a dusty, weak surface. Keep masonry damp by misting (or covering with damp burlap or polythene) for the first week — longer in hot, dry, or windy conditions.
• Hot weather changes everything. Above 25 °C, mist the substrate and the mortar regularly, work in smaller batches, and consider mixing in the shade. Above 30 °C, a retarder may be needed.
• Mortar is not concrete. This calculator is for mortar (cement + sand + water). Concrete adds gravel/coarse aggregate, typically in proportions like 1:2:4 or 1:3:6 (cement : sand : gravel). Use the dedicated concrete-volume calculator for slabs, beams, and columns.
• ASTM Type S/N/M standards. In North America, mortars are classified by minimum compressive strength: Type M (≥17 MPa, structural / load-bearing below grade), Type S (≥12 MPa, exterior at or above grade), Type N (≥5 MPa, the everyday choice for above-grade non-load-bearing work), Type O (≥2.5 MPa, interior non-bearing). The European labelling system (EN 998-2: M2.5, M5, M10, M15, M20) is conceptually similar. A 1:4 ratio with ordinary Portland cement gets you broadly into Type S / M10 territory; 1:6 lands near Type N / M5.
• Batch consistency beats batch precision. A site-mixed mortar that varies wildly from batch to batch will crack and weather unevenly. Pick a method (a builder's bucket, a fixed shovel count) and stick to it for the whole job.

Variations and related mixes

• Lime mortar (1:1:6 cement : lime : sand). Adding hydrated lime improves workability, water retention, and bond — and lets the mortar flex slightly with thermal movement. Mandatory for repointing historic brickwork, where modern Portland-rich mortars are too hard and trap moisture against the masonry.
• Hydraulic lime (NHL) mortar. For repointing fragile or pre-1900 masonry, a softer NHL 2 or NHL 3.5 mortar is correct; Portland cement will damage the surrounding stone or soft brick.
• Gypsum plaster. Different beast: gypsum sets by hydration of calcium sulphate, not by cement chemistry. Used for interior plaster finishes where speed and finish matter more than weather resistance.
• Polymer-modified mortars. Bagged products with added latex/acrylic for tile-fixing, waterproofing, or tile grout. Trade flexibility and bond strength for cost and shorter pot life.
• Dry pre-mixed mortar bags. Convenient — just add water — and great for small jobs (under 0.2 m³). Cost per cubic metre is roughly 2–3× site-mixed, so reserve for snags, repairs, or anywhere you cannot stage a sand pile.
• Ready-mix delivered mortar. Above ~1 m³, a truck-delivered "silo mortar" with a continuous mixer on site beats hand mixing on speed, consistency, and back pain. Worth pricing once you exceed about 25 bags of cement.
• Masonry vs render vs screed. Masonry mortar (this calculator's default) bonds bricks/blocks. Render is essentially the same recipe applied as an exterior coat (often 1:5 or 1:6 with lime added). Screed is a richer, drier mix (1:3 or 1:4) used to level floors before tiling.

Quick reference

Always mock up a small test batch before a large pour. The numbers here are a planning tool, not a substitute for a specifier's mix design on engineered work.