How do I work out how much concrete, mortar and sand a job needs?

Two things go wrong with masonry quantities. People forget that dry materials shrink when mixed, so they order to the sum of the parts and come up short. And they measure a hole as if it were the size they intended to dig rather than the size they actually dug, which on soft ground is always bigger. Both errors run in the same direction, which is why running out is far more common than having spare.

Step by step

1. Reduce the job to a volume in cubic metres.A slab or base: length times width times depth, all in metres. A footing: length times width times depth. A post hole: it is roughly a cylinder, so radius squared times 3.14 times depth, less the volume of the post itself. Add every element up.
2. Allow for the bit you cannot see.An excavation is never the size of the drawing. Ground gives way at the edges, the base is not level, and a trench in soft ground bells out. Ten per cent is the minimum allowance; on a hand-dug footing in loose soil, twenty is not unreasonable.
3. Apply the yield factor to loose materials.Cement particles are far finer than sand, so they occupy the voids between grains rather than adding to the pile. A useful working figure is that the finished wet volume is about two thirds of the loose volume of ingredients. So one cubic metre of concrete needs roughly one and a half cubic metres of loose materials.
4. Convert to bags using the printed yield.Bagged concrete and mortar mixes state an approximate yield in litres per bag on the sack. A thousand litres is a cubic metre. Divide your volume by the yield to get the bag count — never estimate from the weight.
5. Estimate mortar for brickwork separately.Mortar for laying is estimated per unit rather than by volume: as a rough guide, around one cubic metre of mortar lays somewhere in the region of a thousand bricks in a half-brick wall, which works out at roughly a tonne of sand and the cement to go with it. Blocks use more mortar per square metre than bricks because the joints are longer.
6. Estimate pointing by area and joint size.Repointing consumes far less than people expect — you are filling a shallow recess, not the whole joint. A useful method is to point a square metre from a measured batch and scale up from what it actually took.
7. Buy cement in whole bags and gauge by bucket.Cement comes in fixed bag sizes, so the practical approach is to work out the number of bags and then gauge sand against it by bucket to keep the ratio right, using the same bucket every time.
8. Round up, and buy the extra bag.Surplus sand keeps for ever and surplus cement is a few pounds. Running out halfway through a pour costs you a permanent cold joint, and running out halfway through an elevation of pointing costs you a colour change.

Tips

• Sand is sold both by the bulk bag and by the tonne, and the two are not the same thing — bulk bag capacities vary by supplier, so ask for the weight or the volume rather than counting bags.
• Damp sand bulks up dramatically — a heap of damp sand can occupy a fifth more volume than the same sand dry. This is why volume measures for sand are approximate and why you gauge by consistency.
• For a slab, work in millimetres of depth and be honest: 100mm across a base that is 20mm over-dug in the middle is a lot more concrete than the arithmetic says.

Common mistakes

• Adding the ingredient volumes together to get the mix volume — The cement disappears into the voids between the sand grains, and the sand partly disappears into the voids between the aggregate. The finished volume is substantially less than the sum, and ordering to the sum leaves you short.
• Measuring the hole you meant to dig — Excavations are always bigger than intended, particularly by hand and in soft ground. Measure what is actually there, at several points, and take the worst.

Questions people ask