Loose material compacts down — over-order a little so you don't run short.
Concrete for slabs, bases and footings is ordered by the cubic metre. Enter the area and thickness to get the volume, and we'll estimate the all-in ballast and cement for a general-purpose mix if you're mixing on site. For anything over about a cubic metre, ready-mix delivered by volumetric or drum mixer is usually cheaper and easier.
How to use this calculator
- Measure the slab or base area in square metres.
- Set the thickness — 100mm suits a shed base or path, 150mm for a driveway slab.
- Read the volume in cubic metres; over ~1m³, get ready-mix quotes rather than bagging it.
- For self-mix, use the ballast and cement estimate as your shopping list.
How we work this out
We take your area and finished thickness, work out the volume in cubic metres, then convert to weight using a loose delivered density of 2 tonnes per cubic metre for Ready-mix concrete — the figure used across the UK trade. The compaction and wastage allowance is added on top so you don’t run short. Bulk-bag counts assume a standard 850 kg dumpy bag.
Delivered densities and coverage vary with grade, moisture and how hard the material is compacted, so these figures are a working estimate to size your order — always confirm the exact quantity and price with your supplier before buying. Our team reviews the density figures and guidance periodically; this page was last reviewed on 12 July 2026.
Frequently asked
How much ballast and cement per m³ of concrete?
A general-purpose mix takes roughly 1.9 tonnes of all-in ballast and about seven 25kg bags of cement per cubic metre.
Should I mix on site or order ready-mix?
Under about 1m³, bagged or barrowed mixing is fine. Above that, ready-mix delivered by volumetric mixer is usually cheaper, faster and more consistent.
What strength concrete do I need?
C20 suits domestic slabs and paths; C25–C30 for driveways and foundations; C35+ for heavy-duty or structural work.