Concrete Calculator

Concrete is ordered before you can see whether you got it right. Order short and you are left with a cold joint and a panicked second delivery; order long and you are paying to dispose of set material. This calculator works out the volume of a slab, footing, column or stair, converts it into the units a supplier will quote in, and breaks the volume down into the cement, sand, aggregate and water you need for the mix you selected.

It handles rectangular slabs and footings, round and square columns, and staircase steps. Results are given in cubic metres, cubic feet and cubic yards, because the same pour is quoted differently in different countries — ready-mix plants in metric markets sell by the cubic metre, while suppliers in the United States quote cubic yards.

One scope note before you start: the stair option measures the stepped profile of the flight only. The waist slab underneath it — the sloping slab that carries the steps — is a separate volume and has to be calculated as a slab and added. It is easy to miss, and missing it under-orders the concrete by a third or more on a typical staircase. The same applies to any landing.

Why the calculator multiplies by 1.54

Concrete is ordered as a wet volume — the space it will fill — but cement, sand and aggregate are bought as dry materials. Dry materials do not pack to the same volume as the wet concrete they become: the aggregate settles, the cement and sand fill the gaps between stones, and the water occupies space that then leaves as the mix cures.

The correction factor is conventionally 1.54, meaning you need about 1.54 cubic metres of loose dry material to make 1 cubic metre of finished concrete. Without it, every quantity would come out roughly a third too low. It is a rule of thumb rather than a physical constant, which is another reason to round generously when ordering.

Choosing a grade

The grade describes the compressive strength in newtons per square millimetre, and for the lower grades it also fixes the ratio of cement to sand to aggregate. The ratios this calculator applies are the standard nominal mixes:

• M5 (1 : 5 : 10) — bedding, fill, non-structural levelling
• M7.5 (1 : 4 : 8) — lean concrete, blinding under foundations
• M10 (1 : 3 : 6) — mass fill, kerb backing, light footings
• M15 (1 : 2 : 4) — general footings, floors, slabs on grade
• M20 (1 : 1.5 : 3) — reinforced slabs, beams and columns; the default for most domestic structural work
• M25 and above — design mix

M25 and everything above it is where the nominal table stops being useful. These grades are specified as design mixes: the proportions are calculated and tested against the materials actually being delivered to your site, not read off a chart. That is why the calculator asks you to enter a ratio rather than guessing one for you — take it from your engineer's mix design. Historic tables sometimes list M25 as 1 : 1 : 2, and it is a reasonable ballpark, but concrete at that grade is doing real structural work and the mix should come from someone who has tested the aggregate.

Nominal mixes like the rest are fine for domestic and small commercial work. For anything structural beyond that, a designed mix specified by your engineer and verified by cube tests will be both safer and, often, cheaper — nominal mixes deliberately over-specify cement.

Worked example

A slab 20 feet by 20 feet at 4 inches thick is 400 square feet at one third of a foot, giving 133.33 cubic feet — which is 3.775 cubic metres or 4.94 cubic yards. At M20, that needs 31 bags of 50 kg cement, 1.59 cubic metres of sand, 3.17 cubic metres of coarse aggregate and 761 litres of water.

The same slab at M15 — a reasonable choice for a slab on grade with light loading — drops the cement to 24 bags, because M15 uses 1 part cement to 6 parts aggregate where M20 uses 1 to 4.5. On a large pour that difference is worth real money, which is exactly why the grade should be specified by someone qualified rather than chosen by habit.

Note how modest the sand and aggregate volumes are next to the cement count. Only about 1.06 of those 3.775 cubic metres is cement; the rest of the volume is sand, stone and water. That is why cement dominates the cost even though it is the smallest ingredient by volume.

Wastage, over-order and the practical margin

The default 5% wastage allowance covers spillage, an uneven sub-base and the small amount left in the chute. Raise it if your excavation is not flat, if the pour is small, or if you are mixing on site rather than taking ready-mix — in those cases 10% is more realistic.

For a ready-mix delivery, find out the minimum load and the part-load charge before you order. Many plants have a three cubic metre minimum and charge a surcharge below it, which can make two small pours more expensive than one larger one. If your slab is close to the boundary between two truck sizes, it is usually worth ordering the smaller load and mixing a little extra by hand rather than paying for a part-load of the larger.

Curing is not optional

Concrete gains strength by hydrating, and it only hydrates while there is water available. A slab that dries out in its first week can lose a large part of its design strength even though it looks perfectly fine. Keep the surface damp for at least seven days — a week for ordinary work, longer for structural elements or hot weather — by covering it with polythene, hessian or a curing compound.

In hot climates that matters more, not less. High temperatures accelerate the loss of moisture and increase the risk of plastic shrinkage cracking, so pour early in the day, keep the mix as cool as you reasonably can, and start curing the moment the surface will take it.