Mortar, sand & cement

Every mortar calculator answers the wrong question

They all give you tonnes of sand and bags of cement. That is what you need once, on the phone to the merchant, and it is completely useless from the moment the wagon leaves.

What you need at half seven, standing at the mixer, is how many shovels. So this one works out both.

The mistake this exists to kill

One bag of cement to six shovels of sand is not a 1:6 mix. It is not even close.

A ratio is by volume. And cement is sold by weight. A 25kg bag of cement is about 17 litres, and six builder's shovels of damp sand is about 30 litres. So one bag to six shovels is nearer 1:1.7 by volume: a mix roughly three and a half times stronger than intended.

Which sounds harmless, and is not. Over-strong mortar is harder than the brick it is holding, so when the wall moves the brick cracks instead of the joint. On soft or handmade brick it takes the faces off. And it is expensive, in cement you did not need to buy.

The other direction is worse and just as common: somebody who knows a bag is not a whole part guesses low and under-gauges the wall.

So the tool does the conversion and shows it: for a 45 litre load at 1:6, about nine shovels of sand to a shovel and a half of cement, which is 0.43 of a bag. Not one bag. Not half a bag by eye.

Buckets are easier to get right than shovels

A builder's bucket is about 15 litres and it is the same 15 litres every time. A shovel is however much you happened to pick up, and a shovel of wet sand is not a shovel of dry sand.

If a job matters, gauge it with buckets. The tool gives both.

Three ways in

From wall area, if you know the square metres. From a brick count, if you are working off a pallet order. Or straight from a volume if you have already worked it out: including from the brick calculator, which shares the same engine so the two pages cannot disagree with each other.

Blockwork uses roughly a third of the mortar brickwork does per square metre, because a 440 × 215 block has far less joint area than a brick covering the same wall. That surprises people ordering sand for a job with both in it.

The mixes, and why the tool will not pick one

• 1:6: general brickwork and blockwork above damp course, sheltered to moderate exposure.
• 1:5: more exposed work, parapets, anything taking weather from all sides.
• 1:4: below damp course, retaining, and anything in contact with the ground.
• 1:1:6: the traditional gauged mix. More workable, more forgiving of movement.
• 1:2:9: softer still, for old or handmade brick where a hard mortar would damage it.

Which one a job wants depends on exposure and on what is being built, and that is not a decision a web page should make for you. What the tool does is say what each is conventionally used for, and flag the two cases people get wrong: going below ground with a general mix, and using a strong cement mortar on soft old brick when the answer is a lime mix rather than a stronger one.

Fifteen per cent waste, and that is not generous

Mortar is not a brick. A brick you did not use is still a brick tomorrow. Mortar that stiffens on the spot board is gone, and a batch that has gone off gets thrown rather than retempered, whatever anybody tells you.

Add the bit left in the drum, the bit that goes over the side of the board, and the batch you mixed at four o'clock and did not get through. Under ten per cent is optimistic.

The American side is a different table, not a conversion

Switch the market at the top and every figure changes, because the units are different objects. A British brick gives 60 to the square metre. An American modular brick gives 6.75 to the square foot, and converting the British number lands on 5.57, which is a fifth light. Nothing about that is a rounding problem.

So the American path runs off the Brick Industry Association tables rather than off our metric arithmetic with a conversion bolted on. Technical Notes 10 gives 675 modular brick and 5.5 cubic feet of mortar per 100 square feet at a 3/8 inch joint, and a quarter more at a half inch. Concrete block is 112.5 per 100 square feet whatever its thickness, because the face size does not change and only the bed joint gets wider.

The output changes too, and this is the part a units toggle could never have handled. British practice batches cement, lime and sand on site to a ratio, so the answer is tonnes and bags and then shovels per mixer load. American practice buys pre-blended masonry cement and adds sand, so the answer is bags and cubic feet of sand, and there is no ratio to gauge. Which type of masonry cement, N, S or M, is set by ASTM C270 against the exposure and what is being built, and this tool does not choose that any more than it chooses a British mix.

Brick also carries the same three-tier naming that lumber does: nominal, specified and actual. A modular brick is called 4 by 2 2/3 by 8 and specified at 3 5/8 by 2 1/4 by 7 5/8, the difference being the 3/8 joint. If you have met that on a 2x4 measuring 1.5 by 3.5, it is the same habit.

What the numbers rest on

Stated plainly, because sources disagree on it. Cement and lime are taken to fill the voids between the sand grains rather than adding to the volume, so a batch yields roughly the volume of sand that went in. Sand is quoted damp, as delivered, because that is how it is bought and how it is mixed.

Shovel and bucket figures are rules of thumb. They are there to get a batch approximately right at the mixer, not to replace gauging by volume properly on work where it matters.