Work it out by
Unit
Waste allowance (spillage, board losses, off-cuts)
| Mortar | Amount |
|---|
Materials: sand and cement
| Cement (25kg bags) | 0 |
| Building sand | 0 t |
| Sand in bulk bags (0.85t usable each) | 0 |
Add a plasticiser (or a splash of washing-up liquid, the traditional site substitute) at the manufacturer's stated rate for easier working mortar and better frost resistance while it cures. No specific product is recommended here; follow the instructions on whatever you buy.
How this calculator works
Mortar volume can come from two directions, and this tool covers both. By count, it multiplies the number of bricks or blocks you're laying by a mortar allowance per unit: 0.0005 m3 per standard UK brick (215 x 102.5 x 65mm) by default, or about 0.05 m3 per 100 bricks, and around 0.0012 m3 per 440 x 215 x 100mm block, or roughly 0.12 m3 per 100 blocks. Real usage moves with frog fill, how consistent the joints are, and waste, and published figures for a single brick sit anywhere from about 0.0004 to 0.0008 m3; the default here is a deliberate mid-range pick, in the same ballpark as the 0.025 m3 per m2 single-skin rate already used on the brick and block calculator. Both unit figures are editable, in case your bricklayer or merchant quotes a different allowance. By area, it multiplies a wall area in square metres by a mortar rate per square metre, the same logic the brick and block calculator uses on its own mortar step.
Worked example: 500 bricks at a 1:4 mix
500 bricks at the default 0.0005 m3 per brick gives 0.25 m3 of raw mortar. Adding 10% waste brings that to 0.275 m3. Loose dry sand and cement occupy more space than the same materials do once mixed, wetted and packed into a joint, so a 50% bulking allowance scales that up to 0.4125 m3 of dry material to buy. At a 1:4 mix (one part cement to four, five parts total) that splits into 0.0825 m3 of cement and 0.33 m3 of sand. At 0.0175 m3 per 25kg bag, that's 5 bags of cement (rounded up). At a sand density of 1.6 tonnes per cubic metre, that's about 0.53 tonnes of sand, still inside one 0.85-tonne usable bulk bag.
What mix should I use for bricklaying?
A 1:4 cement to sand mix is the standard general-purpose choice for brickwork above the damp-proof course (DPC), the layer near the base of a wall that stops rising damp. Below the DPC, or anywhere more exposed and load-bearing, a stronger 1:3 mix is more usual. Blockwork, especially internal or non-load-bearing, is often laid a little weaker, at 1:5 or 1:6, since a mortar that's too strong for the block can crack the block itself rather than the joint if the wall ever moves. These are typical trade starting points, not a specification; if a structural engineer, building control, or a block or brick manufacturer's data sheet gives a different figure for your job, use that instead.
Building sand or sharp sand?
Building sand (also called soft sand or bricklayers' sand) is the one for mortar. It's finer and rounder than sharp sand, which makes the wet mix workable and easy to spread and tool into a neat joint. Sharp sand is coarser and gritty, and belongs in concrete, screed, or paving bedding, where strength and drainage matter more than smooth working. Using sharp sand in mortar tends to make it harsh and hard to lay; using building sand in concrete weakens it. Keep the two separate.
How many bricks per bulk bag of sand?
As a rough guide, at a 1:4 mix and before waste, a bulk bag of building sand (typically around 0.85 tonnes usable) works out to somewhere around 850 to 900 standard bricks' worth of mortar. That moves with joint width, brick size and mix ratio, so treat it as a ballpark for planning rather than a firm order quantity - use the calculator above for anything you're actually buying.
Waste, plasticiser and buying a bit extra
10% waste suits a straightforward job: mortar left in the tray, board losses, the bit that ends up on the floor rather than in a joint. A fiddly job with lots of cutting or repair work can eat more, so push it up if that sounds like yours. A plasticiser, added at the rate stated on the product, eases working and improves frost resistance while the mortar cures - no brand or price is suggested here, so check what your local merchant stocks.
What this calculator does not check
Nothing here goes beyond the quantities themselves. Wall stability, wall-tie spacing on a cavity wall, and foundation adequacy all sit outside its scope, and taller or load-bearing garden walls often need building control approval before work starts - get that checked separately, with your local authority or a structural engineer, rather than relying on a height rule of thumb.
Related tools
Working out the bricks or blocks themselves, not just the mortar to lay them? The brick and block calculator takes a wall length and height and gives you the count this page's "by bricks or blocks" mode is built to follow on from. The bricks per m2 guide explains where the standard unit rates come from. Pouring a footing or slab underneath the wall instead? The concrete calculator covers that mix separately.