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Concrete Calculator
How much concrete for a slab, a footing or a run of post holes — and roughly how much cement, sand, aggregate or ballast to buy for it. Enter your dimensions and it works out live. Concrete is unforgiving, so treat every figure below as a starting estimate, not gospel.
Raw volume 0.00 m³ + 10% wastage.
Mix ratio by volume: 1 : 2 : 4 (cement : sand : aggregate). Add clean water gradually to a workable mix — roughly 0 litres as a guide (about 0.5 water-to-cement by weight). Too much water badly weakens concrete, so err dry and add slowly. These are guide quantities — for anything structural, order ready-mix or check with an engineer.
Quantities assume dry material densities of about 1.6 t/m³ for sand and ballast and 1.5 t/m³ for coarse aggregate, with a dry-to-wet shrinkage factor of ~1.55 (dry ingredients settle down when mixed and cured). Bag and tonne counts are rounded up. Real yields vary with materials, compaction and how you mix — always confirm with your supplier before ordering.
Who this is for
Anyone pricing or planning a small concrete pour — a shed or greenhouse base, a garden path, strip footings for an extension or garden wall, fence and gate post holes, or a driveway sub-base. It gives you a volume to order and a rough shopping list of materials if you are mixing on site rather than ordering ready-mix.
How much concrete do I need?
Concrete is ordered by volume in cubic metres (m³). For a slab or footing, multiply length (m) × width (m) × depth (m) — so a 4 m × 3 m base at 100 mm deep is 4 × 3 × 0.1 = 1.2 m³. For post holes, work out the volume of one cylinder (π × radius² × depth) and multiply by the number of holes. This tool does that maths for you, then adds a wastage margin because dig depths are never perfectly even and you always lose a little to spillage. Ordering a touch over beats running short mid-pour.
What's the difference between C20, C25 and C30 concrete?
The number is the compressive strength in newtons per square millimetre (N/mm²) after 28 days — higher means stronger. C20 is a general-purpose mix for paths, shed bases and non-structural work. C25 is the common choice for footings, foundations and domestic driveways. C30 is a harder-wearing mix for heavy hardstanding and anything carrying serious load. Stronger mixes use more cement relative to the aggregate, which is why the material ratios change. If a structural design or building control specifies a grade, follow that — don't downgrade to save on cement.
How many 25kg bags of cement per m³ of concrete?
As a rough guide, a cubic metre of concrete needs anywhere from about 12 bags of 25kg cement for a weaker C20 mix up to the low-20s for a strong C30. These are nominal hand-mixing quantities batched by volume, and they deliberately run higher than factory ready-mix, which is designed to strength and typically uses less cement. This tool estimates from your chosen ratio and rounds up, so it errs on the side of not leaving you short. For any sizeable or structural pour, order ready-mix by the cubic metre — it removes the guesswork and gives a consistent, certified strength.
Need aggregate or a wall to go with it?
Need loose gravel, MOT Type 1 sub-base or decorative aggregate for the area around your pour, rather than mixed concrete? Our Gravel & Aggregate Calculator works it out in tonnes and bulk bags. Pouring a footing for a new wall? Our Brick & Block Calculator works out the bricks, blocks and mortar to go on top once the footing's cured.
Does this tool store my details?
No. There is no sign-up and nothing is uploaded. Every calculation happens in your browser — the dimensions you type never leave your device and nothing is saved once you close the page.
Please note: this tool gives a guide or estimate only. It may not be 100% accurate and is not professional advice — always check the figures before you rely on them. Concrete quantities are especially sensitive to materials, mix method and site conditions — always confirm with your ready-mix supplier or builders' merchant before ordering, and use an engineer for anything structural. Everything you type stays in your browser — nothing is uploaded or saved.
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