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MasterMath

Mortar Calculator

How much mortar a job takes for rendering, bedding masonry or fixing floor tiles, and how much cement, sand and water go into mixing it.

Currency and number format for

Mortar needed

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Mortar needed—
Bags of cement, 25 kg—
Cement—
Sand—
Water—
Or, bags of ready-mixed mortar—
Volume without allowance—

How this was worked out

    The formula

    volume = area × thickness, and the dry material is 35 % more

    Where it comes from

    There are two parts to this, and the second is the one that surprises people. The volume of mortar is simply the area times the thickness of the layer. But making that volume takes more material than it looks: loose sand has voids between its grains, and mixing it with cement and water fills those voids so the whole thing shrinks. That is why you work with roughly 35 % more dry material than fresh mortar.

    How to work it out by hand

    1. Multiply the area by the thickness in metres
    2. Add the waste allowance
    3. Multiply by 1.35 for the dry material needed
    4. Split that material between cement and sand by the mix ratio

    What is worth knowing

    The mix ratio is not a matter of taste. A mortar too rich in cement is stiffer than the background and ends up cracking or bringing the whole render off; that is why rendering is mixed 1:6 and not 1:3, even though an inexperienced bricklayer tends to add cement "so it grips well". The rule repeated on site is that mortar should never be stronger than the unit it beds. As for water, it goes in gradually with the mixer running: too much water is the fastest way to ruin the strength of a correctly proportioned mortar.

    Frequently asked questions

    How much cement is in a cubic metre of mortar?

    At a 1:4 ratio, around 270 kg, which is nearly eleven 25 kg bags. At 1:6 it drops to about 190 kg.

    What mix ratio should I use for rendering?

    Usually 1:6 by volume, one part cement to six of sand. A richer mix ends up too stiff and tends to crack.

    How far does a bag of ready-mixed mortar go?

    A 25 kg bag gives around 12.5 litres of mix, so roughly eighty bags per cubic metre.

    How much water does mortar take?

    Around 0.6 litres per kilo of cement, but it goes in gradually: the exact point depends on how damp the sand is.

    How long does mortar take to set?

    It starts to stiffen in two to four hours, but does not reach full strength for twenty-eight days. Only mix what you will use in the next hour and a half.

    Is mortar the same as concrete?

    No. Mortar is cement, sand and water; concrete adds aggregate, which is what gives it structural strength. Mortar joins units, concrete forms elements.