Concrete Calculator — Volume, Bags & Ballast Needed

How much concrete do I need?

Multiply length (m) × width (m) × thickness (m) to get cubic metres. A 3 m × 2 m shed base at 100 mm thick needs 3 × 2 × 0.1 = 0.6 m³ of concrete. The calculator above adds a wastage allowance and converts the volume into whatever you're actually buying: 25 kg ready-mix bags, or cement and ballast for hand mixing.

For quick estimating: one cubic metre of concrete needs about 108 × 25 kg ready-mix bags — which shows why bagged concrete only makes sense for small jobs. Above roughly half a cubic metre, hand mixing from ballast and cement or ordering a ready-mixed delivery is far cheaper and easier.

Bags, hand mixing, or ready-mixed delivery?

Ready-mix bags (25 kg) suit small jobs — fence posts, repairs, a small pad. Just add water. But at over a hundred bags per cubic metre, larger jobs get expensive and exhausting fast.

Hand mixing from ballast and cement is the standard DIY approach for shed bases and paths. A general-purpose C20 mix uses 1 part cement to 5 parts all-in ballast — about 350 kg of cement (14 bags) and 1.75 tonnes of ballast per cubic metre. You'll want a cement mixer for anything over a few barrow loads.

Ready-mixed delivery wins for anything over about 1 m³ — driveways, garage floors, large bases. The truck delivers exactly your volume, mixed to specification. Most suppliers have minimum loads and part-load charges, so get your volume right before ordering — that's what this calculator is for.

How thick should a concrete slab be?

A garden shed base needs 100 mm; paths can be 75–100 mm; a domestic driveway 100–150 mm; and a garage floor 150 mm. Every slab needs a compacted sub-base of MOT Type 1 (typically 100 mm) underneath it — concrete poured straight onto soil will crack as the ground moves. Work out the sub-base layer with our MOT Type 1 calculator.

Frequently asked questions

How many 25 kg bags of concrete are in a cubic metre?
About 108 bags. Each 25 kg bag yields roughly 0.011 m³ (11 litres) of mixed concrete.

How much ballast and cement do I need for 1m³ of concrete?
For a C20 general-purpose mix: about 1.75 tonnes of all-in ballast and 350 kg (14 × 25 kg bags) of cement.

What concrete mix should I use for a shed base?
C20 (1 part cement : 5 parts ballast) is the standard general-purpose mix for shed bases, paths, and most domestic slabs. For driveways carrying vehicles, C25 with a slightly higher cement content gives extra strength.

How long does concrete take to dry?
You can usually walk on it after 24–48 hours, but concrete takes about 28 days to reach full strength. Keep it damp (cover with polythene) for the first few days in hot weather to prevent surface cracking.

Do I need a sub-base under concrete?
Yes — 100 mm of compacted MOT Type 1 under the slab spreads loads and prevents cracking from ground movement. Pouring onto soft or unprepared ground is the most common cause of slab failure.

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