Concrete Calculator

How this calculator works

Each mode reduces to the same job: turn a shape into a volume. A slab is length × width × depth. A footing or trench is the same, just narrow and deep rather than wide and shallow, so width is entered in millimetres instead of metres. A post hole is a cylinder, π × radius² × depth, multiplied by however many holes you're filling. Every depth and width figure you enter in millimetres is converted to metres before the volume maths runs, so mixing units (metres for the long dimension, millimetres for the narrow ones) is deliberate and safe here.

Choosing a wastage allowance

The 10% default covers the concrete lost to an uneven sub-base, spillage during barrowing, and formwork that isn't perfectly true. For a level, well-boxed slab you can often drop to 5%. For a hand-dug trench footing on soft or uneven ground, push it up toward 15 to 20%, since trench sides rarely come out as neat as the number you measured.

Bagged, ready-mix, or mixed on site

For a handful of post holes or a very small patch, pre-mixed 25kg bags (just add water) are the least hassle, at the highest cost per cubic metre. The default yield of 0.011 m³ per bag is typical for a standard 25kg bag; check the bag for the manufacturer's figure and adjust it here. For anything over about a cubic metre, a ready-mix delivery is usually cheaper and far less physical work; the figure in this calculator is what to quote a supplier, though most apply a minimum order and a part-load surcharge below roughly 4 m³, so ask when you book. In between, mixing your own from bagged cement and loose or bulk-bag ballast is the traditional DIY route and what the bottom card works out.

Understanding the ballast to cement ratio

UK ratios are conventionally written cement first: 1:5 means one part cement to five parts all-in ballast (combined sand and gravel), measured by loose volume, for example five buckets of ballast to one of cement. Because dry cement and ballast settle and compact once water and mixing bring them together, you need more loose dry material than the finished concrete volume: this calculator uses a 50% bulking allowance by default (the finished volume needs 1.5 times that much loose material), a figure builders' merchant guides typically place between 40 and 65%, so adjust it if your supplier quotes differently. The cement figure then converts by volume to bag count using a standard bagged cement density, and the ballast figure converts to tonnes using a typical all-in ballast density, both editable above if you know the exact products you're buying. A 1:5 general-purpose mix at these defaults works out to a cement content in the same range concrete engineers normally specify for ordinary mixes, which is a useful sanity check if you're comparing against a merchant's own ready-reckoner.

What this calculator does not check

This tool gives you a volume and a materials list, nothing more. It does not check ground bearing capacity or whether compaction and a sub-base are needed before you pour, whether the slab needs reinforcing mesh or the footing needs rebar, minimum foundation depths (these depend on soil type, frost, and nearby trees, and are set by building control, not by a fixed number), or damp-proofing requirements under a slab that will have a building on it. For anything structural, or any footing supporting a permanent building, check with a structural engineer or your local building control before you dig.

Related tools

Ordering loose sand, gravel or hardcore by the tonne for the sub-base under a slab, rather than as part of the concrete mix itself? The aggregate calculator handles that. Building a wall on top of a footing from this page? The brick and block calculator works out the bricks or blocks and the mortar. Setting fence posts instead, and want bagged postcrete quantities rather than a wet mix? See the fence calculator.