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How the mortar calculator works
Mortar is cement and sand mixed with water, and what you buy is a ratio rather than a product: 1:4 means one part cement to four parts sand, by volume. So the calculator starts with the volume of mortar the job takes, bulks it up to a dry volume — dry sand and cement occupy more room than the mortar they make, the same ×1.33 allowance this site applies to brickwork mortar — and then splits that dry volume by the ratio. From there it weighs the materials at the bulk densities the site uses throughout, cement at 1,440 kg/m³ and sand at 1,600 kg/m³, and rounds up to whole 25 kg bags.
The mix ratio is the whole calculation
The ratio decides everything. At 1:3, one part in four of the dry mix is cement; at 1:6 it is one part in seven, so the same volume of mortar takes markedly less cement and more sand. The four mixes here run from the richest to the leanest, and 1:4, 1:5 and 1:6 are the same ratios the site's brick calculator offers for brickwork, so the two pages use one figure rather than two. Which mix the work actually needs is a specification question, not an arithmetic one: this page sizes whichever mix you choose, and leaves the choice to your spec or your supplier's product data.
By area or by volume
If you can measure the job, work by area. A patio 4 m × 3 m with a 40 mm bedding course is 0.48 m³ of mortar, and the thickness box arrives pre-filled with a typical figure for the job you picked — 40 mm for a bedding course, 10 mm for pointing, 50 mm for a screed, 15 mm for a render. Those are planning estimates and every one can be typed over, which matters because thickness moves the order in a straight line. If the quantity is already quoted by volume, such as a screed priced by the cubic metre, switch to volume and enter it directly. The mix arithmetic is identical either way.
Water, plasticiser and buying sand
Water is the part of a mortar mix that is gauged by eye. The calculator prints a starting figure at a water/cement ratio of about 0.5 — the same guidance the concrete calculator gives — but the real test is workability, and a plasticiser or a little hydrated lime lets you reach it with less water. For the sand, buy loose or in bulk once the job is more than a few bags: it is far cheaper per tonne than the equivalent 25 kg bags. Cement is best bought fresh and used within a few weeks of opening rather than stored over a season. Everything on this page is a planning estimate — check your own measurements and the product data before you order.
Mortar FAQs
How much sand and cement do I need?
It depends on the volume of mortar and on the mix. For 0.48 m³ of 1:3 mortar — a 12 m² paving bed at 40 mm — the calculator works out roughly 230 kg of cement and 766 kg of sand: 10 bags of cement and 31 bags of sand, plus about 115 litres of water. The mix ratio decides the split, so a leaner 1:6 mix puts far more sand and less cement into the same volume of mortar. These are planning estimates — always check the product data before ordering.
What mix ratio should I use?
A mortar mix is written as cement : sand, by volume, and the calculator offers 1:3, 1:4, 1:5 and 1:6. More cement gives a richer, stronger, dearer mix; more sand gives a leaner, cheaper one. The 1:4, 1:5 and 1:6 mixes here are the same ratios the site's brick calculator offers for brickwork mortar, so the two pages cannot disagree about the same material. Which mix your job requires is a specification question — check the spec for the work, or the product data of any pre-blended mortar you are comparing against, rather than taking a default from a calculator.
Should I measure the job by area or by volume?
Use area when you can measure the job itself: length × width × thickness gives the volume of mortar for you. Use volume when the quantity is already quoted that way, such as a screed priced by the cubic metre, or when you are mixing to a known volume in a gauge box. Both paths then run exactly the same mix arithmetic, so only the volume input changes.
How much water goes into a mortar mix?
The calculator prints a starting figure at a water/cement ratio of about 0.5 — the same guidance the site's concrete calculator gives — so 25 kg of cement takes roughly 12.5 litres. Treat it as guidance only: mortar is gauged by eye until it is workable, and a plasticiser or a little hydrated lime lets you reach that workability with less water. Too much water weakens the set, so add it gradually rather than all at once.
How much mortar does a 25 kg bag of cement make?
As a planning estimate, a 25 kg bag of cement makes roughly 0.065 m³ of 1:4 mortar. That follows from the figures this site uses throughout: cement at 1,440 kg/m³ makes 25 kg about 0.017 m³, four parts of sand take the dry mix to about 0.087 m³, and the ×1.33 dry-to-wet allowance brings the mixed mortar back to roughly 0.065 m³. Check the bag and the sand you actually buy, because both vary.
Is this the same as the brick calculator's mortar figure?
It uses the same underlying figures, but it answers a different question. The brick calculator starts from a wall: its mortar is what is left of the brick module once the brick itself is counted, so it gives you the mortar that laying that wall consumes. This page starts from a volume or an area and a mix, which is the shape of the question for pointing, a paving bed, a screed or a render. The cement and sand densities, the dry-mix bulking factor and the 25 kg bag sizes are shared between the two pages.