Concrete Calculator (UK)
Last updated: 12 August 2026
Work out how much concrete you need for a slab, base or footing. Get the volume in cubic metres plus what to buy — 25kg bags for small jobs, or ballast and cement for site-mixing larger pours.
How the calculation works
Volume = length × width × depth. We convert your depth from millimetres to metres, multiply the three together for cubic metres, then add your wastage allowance for uneven ground and spillage.
For buying: a 25kg bag of ready-to-use concrete yields about 0.011 m³ mixed (roughly 90 bags per cubic metre). For site-mixing a C20-type mix, allow around 2 tonnes of ballast and twelve 25kg bags of cement per cubic metre. Over about 2 m³, priced ready-mix delivery usually wins.
Concrete mix ratios and strengths
| Grade | Mix (cement : ballast) | Typical use |
|---|---|---|
| C15 | 1 : 8 | Blinding, kerb bedding |
| C20 | 1 : 6 | Shed bases, garden slabs, footings |
| C25 | 1 : 5 | Driveways, garage bases, foundations |
| C30/C35 | 1 : 4 | Heavy-duty slabs, commercial hardstanding |
How thick should the slab be?
Shed and garden bases: 100mm. Driveways: 100–150mm over a compacted MOT Type 1 sub-base. Garages and heavy loads: 150mm or more, usually reinforced with A142/A193 mesh. Always check building regulations for structural work.
Frequently asked questions
How many 25kg bags of concrete are in a cubic metre?
A 25kg bag of ready-to-use concrete mix yields roughly 0.011 cubic metres once mixed, so you need approximately 90 bags per cubic metre. For anything over about half a cubic metre, site-mixing with ballast and cement or ordering ready-mix is usually far cheaper.
How much concrete do I need for a 3m x 3m shed base?
At 100mm deep, a 3m x 3m base is 0.9 cubic metres of concrete. With 10% wastage that is about 90 x 25kg bags of ready-to-use mix, or roughly 2 tonnes of ballast and 12 bags of cement if site-mixing.
How thick should a concrete slab be?
As a general guide in the UK: 100mm for sheds and light garden bases, 100-150mm for driveways over a compacted sub-base, and 150mm or more for garages or heavy loads. Always check building regulations for structural work.
What concrete mix should I use for a garden base?
A C20 equivalent mix, roughly 1 part cement to 6 parts ballast, suits most garden bases and shed slabs. Driveways and structural work typically need C25 or stronger.
Is ready-mix cheaper than bags of concrete?
Above roughly half a cubic metre, site-mixing with ballast and cement or ordering ready-mix delivery is usually much cheaper than 25kg bags, which typically cost two to three times more per cubic metre of finished concrete.
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Estimates are for guidance only. Material dimensions, densities and coverage vary by supplier — always confirm quantities before ordering, and consult a qualified professional for structural work.