What this calculator works out
Mortar is the bedding and jointing material that holds brick, block and paving together, a mix of cement, sand and water in a ratio chosen for the job rather than one fixed recipe. Ordering it by guesswork tends to go one of two ways: run short partway through a wall with the mixer still turning, or over-order cement that then sits going hard in a shed. This calculator takes a bricklaying, blocklaying or paving job, or a mortar volume you already know, and works out the finished mortar volume, then the cement bags, sand and water needed to mix it, with a US premix bag equivalent alongside.
Four job types cover the jobs people actually price at home: a brick count or a wall area for bricklaying, a block count or a wall area for blockwork, an area and bed depth for bedding paving slabs, or a mortar volume typed straight in when it's already known. Whichever route in, the same pipeline of steps turns it into a shopping list.
How the maths works
Bricklaying and blocklaying both start from a per-square-metre figure. A standard single-skin, half-brick (102.5mm) wall in stretcher bond with 10mm joints takes 60 bricks and about 0.022 m³ of mortar per square metre, the figures Wickes' own building material quantities guide uses and independent mortar calculators corroborate. Standard 100mm blockwork with 10mm joints takes 10 blocks and about 0.012 m³ of mortar per square metre, the same source's blockwork figure. Entering a brick or block count instead of an area simply divides that per-m² mortar figure by the bricks or blocks it covers, so either route lands on the same answer for the same wall.
Whichever job produces it, that's the wet, finished mortar volume, the actual space it fills once mixed and in place. A 10% waste allowance is added by default, the same convention this site's Concrete and Paving Calculators use, since spillage, an uneven joint and mortar left in the mixer at the end of the day are all normal on a real job. From there, two more steps turn wet volume into a shopping list: bulking, then the mix ratio split.
Dry materials against finished mortar: the 1.33 bulking factor
Loose, dry sand and cement take up noticeably more space than the mortar they end up making. Sand alone holds roughly a quarter of its volume as air between the grains; water fills those gaps as the mix is worked, and the finished, compacted mortar shrinks down accordingly. The standard construction-estimating allowance for that shrinkage is a factor of 1.33: for every cubic metre of finished mortar wanted in place, buy and mix 1.33 cubic metres of dry sand and cement. Skipping this step is one of the most common ways a mortar order comes up short, since the bare wet-volume figure looks deceptively small next to the pile of dry material it actually takes to make it.
That dry volume then splits by the chosen mix ratio. A 1:4 mix is 1 part cement to 4 parts sand by volume, five parts in total, so cement gets a fifth of the dry volume and sand the other four fifths; a 1:6 mix spreads the same dry volume across seven parts instead, with cement's share shrinking accordingly. Cement's volume converts to kilograms and then 25kg bags using a bulk density of 1,440 kg/m³, and sand's volume converts to tonnes using 1.6 t/m³ and then whole bulk bags at the common 850kg size, both figures already used elsewhere on this site by the Concrete and Paving Calculators.
Working the default example through: 10 m² of half-brick wall at 0.022 m³/m² is 0.22 m³ of wet mortar. With the 10% waste allowance on, that's 0.242 m³ to mix. Bulked by 1.33, that's 0.322 m³ of dry materials. Split 1:4, that's 0.064 m³ of cement and 0.257 m³ of sand; at 1,440 kg/m³, the cement comes to about 93kg, rounding up to 4 bags of 25kg; the sand comes to about 0.41 tonnes, comfortably inside a single 850kg bulk bag.
Which mix ratio for which job
Mortar is deliberately mixed weaker than the brick, block or stone it's holding together, so that if anything eventually cracks under movement, it's the mortar joint that gives rather than the masonry unit itself, since a cracked joint is far cheaper and easier to rake out and repoint than a cracked brick. A 1:3 mix, cement-rich and strong, suits engineering brick, chimneys and anything below the damp course where the mortar is more exposed to water and needs real compressive strength. 1:4 is the everyday default for ordinary above-ground domestic brickwork, the ratio this calculator starts on. 1:5 suits standard concrete blockwork and softer facing bricks, and 1:6 suits very soft old bricks and repointing, where an overly strong mix risks doing exactly the damage the weaker-mortar principle is meant to avoid. A mortar plasticiser, or traditionally lime, entrains tiny air bubbles through the mix that improve workability and help frost resistance without materially changing the sand and cement split shown here; product dosages vary, so follow the manufacturer's own instructions on the bag.
Water: a rough guide, not a spec
Water is the one ingredient this calculator can only estimate loosely, because the right amount depends on how wet or dry the sand already is and how workable the bricklayer wants the mix. A commonly quoted starting range is 10 to 15 litres per 25kg bag of cement in a general-purpose mortar; this calculator uses the 12.5 litre midpoint as a rough guide figure. Add water gradually while mixing rather than all at once, aiming for a consistency that holds a furrow when a trowel is drawn across it, neither sloppy nor crumbly.
US aside: premix mortar bags
US suppliers commonly sell mortar pre-blended in 80lb bags rather than site-mixed from separate cement and sand. Quikrete's Mortar Mix (product 1102, Type N), a widely stocked pre-blended masonry cement and sand product, publishes a yield of 0.6 cubic feet of mixed mortar per 80lb bag, a figure confirmed across several US retailer listings for that product. This calculator converts the same waste-adjusted wet mortar volume straight into that bag count, so the US aside is comparing like with like against the UK cement-and-sand figures rather than a separate estimate.
Questions people ask
What's the difference between mortar, concrete and cement?
Cement is the fine grey powder that binds everything else together once mixed with water; on its own it's never used to build with. Mortar is cement, sand and water, mixed to bed and joint brick, block and stone. Concrete is cement, sand, a coarser aggregate (gravel or crushed stone) and water, mixed to be poured and cured into a solid structural mass like a slab, footing or post base. Mortar is deliberately weaker than the masonry it joints; concrete is engineered to be strong in its own right. This site's Concrete Calculator covers pours; this one covers bedding and jointing.
What mortar mix ratio should I use for my job?
1:4 (cement to sand, by volume) is the everyday default for ordinary above-ground domestic brickwork. Use a stronger 1:3 for engineering brick, chimneys or anything below the damp course; use a weaker 1:5 for standard blockwork and softer bricks, and 1:6 for very soft old bricks or repointing, since mortar is meant to be weaker than the unit it's joining so any cracking happens in the joint rather than the brick.
Do I need a plasticiser?
It's not compulsory, but a mortar plasticiser (or traditionally lime) is common practice: it entrains tiny air bubbles that make the mix easier to work with a trowel and improve its resistance to frost damage while it cures. It doesn't materially change the cement and sand quantities this calculator shows, just how the mix behaves and lasts. Follow the specific product's dosage instructions rather than a general rule of thumb.
How long does mortar stay workable?
Roughly 90 minutes to two hours after mixing for a typical general-purpose mortar, sometimes called its working or "pot" time, though this shortens in hot weather and lengthens in cold. Once it starts to stiffen it shouldn't be re-tempered with more water to keep using it, since that weakens the cured joint; mix only as much as can realistically be used within that window.
How much mortar do I need per square metre of brickwork or blockwork?
About 0.022 m³ per m² for a standard single-skin, half-brick wall with 10mm joints, and about 0.012 m³ per m² for standard 100mm blockwork with 10mm joints. Both figures assume ordinary metric brick and block sizes; a different brick or block size, or a wider or narrower joint, shifts the true figure slightly.
Why does the dry materials volume need to be more than the finished mortar volume?
Loose sand holds a lot of air between its grains, roughly a quarter of its own volume; water fills those gaps as the mix is worked, and the finished mortar compacts down as a result. The standard allowance for that shrinkage is a factor of 1.33: buy and mix about a third more dry material than the finished, in-place mortar volume you're aiming for.
Figures on this page are typical, published values for ordinary domestic brick, block and paving work, checked against at least two independent sources named above, not a structural specification. Real bulk densities, bag yields and joint widths vary by product and supplier, so treat the figures here as a solid ordering estimate and confirm exact quantities with your supplier before a large order.