Work out the concrete for a slab, base, footing trench or post holes, as ready-mix m³ or cement and ballast to mix yourself.
Good for: drives, slabs, bases
Slabs and trenches are area × depth. Round post holes are π × radius² × depth × the number of holes.
We add your waste allowance and round up to the next 0.25 m³, which is how ready-mix is usually sold.
Dry materials take up about 54% more room than the finished concrete, so we multiply the volume by 1.54, then split it by the mix ratio. Cement is 1,440 kg/m³, all-in ballast 1,750 kg/m³, sharp sand 1,600 kg/m³ and gravel 1,700 kg/m³.
These are estimates for ordering materials, not a structural design. Check site conditions, and anything unusual, with a qualified builder or engineer.
About 13 × 25 kg bags for a 1:2:4 mix, 16 for 1:1.5:3, and 9 for 1:3:6, before waste.
1:1.5:3 for driveways and anything that takes vehicles, 1:2:4 for paths, shed bases and general slabs, and 1:3:6 for footings, haunching and fence posts.
Sand and gravel ready-blended in one bulk bag. You add only cement and water, so it's the simplest way to buy concrete materials for mixing by hand.
Up to about half a cubic metre, mixing by hand or with a hired mixer is fine. Above about 1 m³, ready-mix is usually cheaper once you count the time, and the pour is more even.