Concrete is bought by the cubic metre, but the numbers you start from are almost always simpler: a length, a width and a depth in millimetres. Get the shape right and the volume follows from one multiplication - the trickier part is settling on a sensible thickness for the job at hand, then deciding whether bags or a ready-mix lorry make more sense for that amount. This guide walks through both, with a full worked example, before handing over to the concrete calculator for slabs, footings, trenches and post holes, where you can plug in your own dimensions directly.
From dimensions to cubic metres
Take a simple shed base: 3m long, 2m wide, poured 100mm thick. Convert the depth to metres first (100mm is 0.1m), then multiply the three figures together: 3 × 2 × 0.1 = 0.6 cubic metres (m3). That's the raw concrete needed to fill the space, before any margin gets added on top - covered further down. A driveway, a path or a strip footing use exactly the same three-figure multiplication - only the numbers going in change.
Typical depths for common jobs
How thick a pour needs to be depends on what sits on top of it and how much load it carries, and none of the figures below are fixed rules - a heavier vehicle, soft ground, or a building control requirement can all push a depth up. As a starting point: a shed or greenhouse base is typically around 100mm; a driveway or other heavy-use slab typically wants 150 to 200mm; a fence post is usually set in a surround of concrete roughly a third of the post's length in the ground; and a garden path typically runs 75 to 100mm. Take all four as a planning figure rather than a spec to build to without checking your own ground and, for anything load-bearing, local regulations.
Typical depths, job by job (figures are typical, not fixed)
| Job | Usual depth |
|---|---|
| Shed or greenhouse base | ~100mm |
| Driveway / heavy-use slab | 150-200mm |
| Fence post surround | ~1/3 of post in ground |
| Garden path | 75-100mm |
Bags or ready-mix: which makes more sense
For small volumes, bagged concrete is the more practical option - nothing to book in advance, no lorry needing access to the site, and you buy exactly what the job needs. As the volume climbs, though, the effort of barrowing and turning bag after bag adds up fast, and a ready-mix delivery becomes the easier route, even though it usually needs booking ahead and clear vehicle access. There's no fixed cut-off, but as a rough guide: under about 0.5 m3, bags are usually the practical choice; once you're ordering 1 m3 or more, ready-mix tends to involve far less manual effort and gives a more even pour throughout, since the whole load arrives already combined rather than being made up bag by bag. Between those two points is a judgement call based on how much lifting and turning you're prepared to do yourself.
How many bags per cubic metre
Bagged concrete comes two ways: ready-to-use bags where the cement and aggregate are already blended and only need water stirred in, or plain cement bought separately and combined with sand and gravel on site. For the ready-blended type, a 25kg bag typically yields somewhere around 0.011 m3 once mixed - the same working assumption the concrete calculator on this site uses by default - a workable planning figure that varies a little by brand, so always check the yield printed on the bag you're actually buying. Working from that figure, one cubic metre needs roughly 90 bags (1 ÷ 0.011 = 90.9), and the 0.6 m3 shed base above needs 0.6 ÷ 0.011 = 54.5, rounded up to 55 bags, before any margin is added. Combining plain cement with sand and gravel yourself needs the ratio getting right rather than just tipping a bag in: a leaner mix such as 1:6 (cement to aggregate) suits non-structural fill, while a richer mix suits anything carrying real weight. The concrete calculator works out bag counts and material weights for whichever ratio you pick.
Add an ordering margin
Whichever way you're buying, order a bit more than the raw figure rather than exactly to it. A pour rarely comes out to the precise millimetre it was planned on paper, and running short with wet concrete already going off is a far worse position than having a little left over, so building in 5 to 10% headroom on the calculated volume is a sensible habit for most domestic jobs. On the shed base example, 0.6 m3 plus 8% is 0.6 × 1.08 = 0.648 m3, which at the same 0.011 m3 per bag works out to 0.648 ÷ 0.011 = 58.9, rounded up to 59 bags rather than the bare 55.
Concrete strength grades: C20, C25, C35
Concrete is also graded by strength, written as a C number (for example C25) relating to how much load it can take once cured. As a broad, hedged guide for domestic DIY: C20 suits general, non-structural work like paths, shed bases and bedding-in slabs; C25 to C30 covers most ordinary domestic footings and driveway slabs; and C35 or above is reserved for heavier-duty or more exposed situations, such as ground bearing a structure or facing harsh weather and de-icing salt. Treat these bands as rough guidance for casual jobs only - anything supporting a building, wall or other structure should be specified by whoever handles the structural design, or checked with building control, since ground conditions and load change what grade is actually needed.
FAQ: pouring and curing concrete
Can you pour concrete in the rain? Light drizzle onto a pour that's already placed is usually manageable and won't ruin the job. Heavy or prolonged rain is a different matter - it can wash cement paste out of the surface and weaken it, so treat that as a no-go; if rain is forecast, cover a fresh pour with plastic sheeting or wait for a drier spell.
How long does concrete take to cure? As a typical rule of thumb, a pour is firm enough to walk on carefully after about 24 to 48 hours, but full strength takes much longer - around 28 days is the commonly quoted figure, though this varies with temperature, mix and how the surface is protected while it cures. Treat both timeframes as typical rather than fixed, especially in cold weather when curing slows down.
Most slabs also sit on a compacted layer of hardcore or gravel first, laid and firmed down before any concrete goes in - that base course is ordered separately, and the aggregate calculator converts its size into tonnes to order.