How Much Concrete Do I Need? A Practical Guide

Concrete jobs go wrong in two places: ordering the wrong amount, and mixing it badly. Neither is complicated to get right, but both are miserable to fix once the stuff is setting. Paul recommends doing your volume sums before anything else — concrete waits for no one, and the middle of a pour is the worst possible time to discover you're a bag short.

Bagged vs mix-your-own — which is right for your job?

Just-add-water bags (a 25kg bag makes roughly 12.5 litres) are perfect for posts, small pads and repairs — no measuring, no waste, no stress. Once a job passes about a quarter of a cubic metre, mixing your own from all-in ballast and cement works out far cheaper. Paul has experienced this problem firsthand: what looks like 'a few bags for a shed base' quietly becomes twenty bags at a tenner each, when an 850-litre ballast bag and a few bags of cement would have cost half as much.

How deep should concrete be?

Depth depends entirely on the job. Garden paths want 75–100mm, a patio base 100mm, shed bases and small foundations 100–150mm, and driveways 150mm with mesh reinforcement. Fence posts are different again — no slab at all, just a hole around 600mm deep with the post set in rapid post-mix. Paul's rule after years of garden work: when in doubt, go 25mm deeper — the extra material costs a few pounds, while a cracked, undersized path costs you the whole job.

Working out the volume (and allowing for wastage)

The maths is friendlier than it looks: every square metre takes 10 litres per centimetre of depth. A 10m² path at 10cm deep is 1,000 litres — one cubic metre. Then add 10% for wastage, spillage and uneven ground. Paul suggests measuring twice at the deepest point if the ground slopes: concrete finds every hollow, and it's always the hollow you didn't measure that eats your spare bags.

Mixing concrete properly

Whether it's a mixer or a wheelbarrow, the technique matters more than the muscle. Add water slowly — a mix that's slightly too wet is far weaker than one that's slightly too dry. You're aiming for workable porridge that holds its shape when you slice it with the shovel, not soup. Paul's solution after years of working with concrete: batch your mixes identically (same bucket counts every time) so the colour and strength stay consistent across the whole pour, and never add water to a mix that's started to stiffen — it ruins the strength. Discard it and mix fresh.

Curing — the part everyone rushes

Concrete doesn't dry, it cures — and curing needs moisture. Keep fresh concrete covered or lightly damp for at least 48 hours, off-limits to feet for a day or two, and clear of heavy loads for a week. In Paul's experience, more garden paths are ruined by impatience than by bad mixing: the surface dusts and crumbles because it dried out on day one, and no sealer will properly rescue it afterwards.