How much concrete do I need?
A slab, a footing or a set of post holes — work out the volume, then see it as bags of ready-mix or as the cement and ballast for a mix-your-own batch. Running out part-pour leaves a cold joint, so we add a margin. Free, no sign-up.
A guide, not a guarantee. The mix-your-own figures are for a general-purpose 1:5 cement-to-ballast concrete and are approximate — always follow the ratio on your cement bag for the job. Bagged concrete is convenient for small pours; once you're past about a cubic metre, a mixer with cement and ballast is cheaper, and past two cubic metres a ready-mix truck (barrowed or pumped) is usually the answer. Add reinforcing mesh and a sub-base where the job needs it. Nothing you type leaves your browser.
Volume first, then bags or a mix.
Work out the volume. For a slab or footing it's length times width times depth. For post holes it's the area of the round hole times its depth, times the number of holes. We add your margin on top, because you can't top up concrete once it's starting to go off — running short mid-pour leaves a weak cold joint.
Bagged, for small jobs. Pre-mixed concrete just needs water. A 25 kg bag makes roughly 0.011 cubic metres, so about 90 bags fill a cubic metre. We divide your volume by the bag's yield and round up. Bags are easy for a few post holes or a small base, but they get expensive — and heavy to carry — fast.
Mix your own, for bigger pours. A general-purpose concrete is about one part cement to five parts all-in ballast. Per cubic metre that's roughly six to seven 25 kg bags of cement and about 1.7 tonnes of ballast. We give an approximate figure — always follow the ratio printed on your cement bag, as it varies with the strength you need.
Know when to phone the truck. Past about a cubic metre, hiring or buying a mixer and using cement and ballast beats bag after bag. Past roughly two cubic metres, ready-mixed delivered by truck — barrowed or pumped — is usually cheaper and far less work than mixing by hand, and it all goes off together.
What this leaves out. A compacted sub-base under a slab, reinforcing mesh or rebar, a damp-proof membrane, shuttering and any expansion joints are all separate. Get the depth and the base right — that's what stops a slab cracking.
Keep every job and its details in one place.
Stead remembers your projects, what you built and when, and what's due next — so the next job starts with the numbers already to hand.