Sand and Cement per m3 of Mortar

Sand and cement mortar calculation

Mortar mixes get written as cement:sand by volume. A 1:3 mix is rich and goes into render and skim coats. 1:4 is what you'd normally use for masonry laying, meaning bricks and blocks. Go leaner with 1:5 or 1:6 when the fill isn't structural. As a rough reference, one 50 kg cement bag plus 150 kg sand and around 22 L water gives you roughly 120 L of fresh mortar at 1:3. Pick washed medium sand for render and coarse sand for laying. Say you're rendering 30 m² at 2 cm thick: that comes to 0.6 m³ of mortar, so figure about 5 bags of cement and 0.75 m³ of sand for a 1:3.

Applications and standards

Bricklayers, contractors and DIY builders lean on this to size up materials for render (reboco), bonding coat (chapisco), masonry laying and floor settling beds. The Brazilian standard NBR 13281 sorts masonry and rendering mortars by strength, water retention and density. Stick to it and the mortar's class will actually match the job (M5, M10, M15, M20).

FAQ

Why does 1 bag + 3 bags sand not give 4 bags volume? Sand drops into the gaps between cement grains, so the mixed volume ends up smaller than the sum, somewhere around 60-70% of it.

Can I add lime? Yes. A 1:1:6 (cement:lime:sand) gives you a render that's easier to work and less brittle, which is why it shows up a lot in old building restoration.

How long can I use mixed mortar? Give yourself up to 2.5 hours after mixing. Past that the set has kicked in, and throwing water back in just weakens it.