How Much Concrete Do I Need?

Multiply length by width by depth, all in metres, and you have the volume in cubic metres. A slab 4 metres by 3 metres at 100 millimetres thick is 4 × 3 × 0.1, which is 1.2 cubic metres. Post holes are the same idea using the area of a circle: a 300 millimetre hole 600 millimetres deep holds about 0.042 cubic metres, so twenty of them come to 0.85 cubic metres. Always add five to ten per cent, because the ground is never as flat as you think.

C20, mixed 1:2:4 by volume with cement, sharp sand and 20 millimetre aggregate, is the general purpose mix for shed bases, paths and non-structural slabs. C25 suits a domestic driveway or a garage floor. C35 is for heavy duty use and exposed external work. For anything carrying a building — foundations, footings, a structural slab — take the specification from an engineer or building control rather than from a website.

Around £105 to £150 per cubic metre delivered in 2026 including VAT, for a standard C20 to C25 mix. Below about three cubic metres most Northern Ireland plants add a part-load or short-load charge of £60 to £120, because the lorry makes the same trip either way. That surcharge is why one cubic metre delivered can cost nearly as much as two, and why small pours are often cheaper mixed on site.

Under about two cubic metres, mixing on site from cement, sharp sand and aggregate usually works out cheaper, even after hiring a mixer. Above that, ready-mix wins on both price and quality: it is batched by weight rather than by shovel, it arrives at a consistent strength, and two cubic metres is roughly eighty barrow loads, which is a very long day for two people. Bagged pre-mix only makes sense below about a quarter of a cubic metre.

About 108. Each 20 kilogram bag of pre-mixed concrete yields roughly 0.0093 cubic metres once mixed, so a cubic metre takes a shade over a hundred bags — well over two tonnes to carry, open and mix by hand. That arithmetic is the quickest way to see whether a job belongs in bags at all. For anything above a quarter of a cubic metre, price the loose materials or the mixer lorry first.

Yes. Ready-mix, bagged materials and aggregates are all standard-rated, and Northern Ireland merchants and batching plants quote domestic customers inclusive of VAT at 20 per cent. Trade counters often quote you the price excluding VAT without saying so, which is the usual reason a merchant price looks twenty per cent better than the figures here. Ask which it is before you compare. Delivery is also worth confirming separately, because aggregate and sand prices are frequently quoted collected rather than delivered, and a tonne of stone is not something you fetch in a car.