Concrete Calculator

Concrete works the same way as any other volume calculation: length by width by depth in metres gives cubic metres. A 3 m by 2 m slab at 100 mm deep is 0.6 cubic metres. Where it gets fiddly is the bag arithmetic. A standard 20 kg premix bag yields roughly 0.009 cubic metres of mixed concrete, so that 0.6 cubic metre slab needs 67 bags. At £3.50 a bag in 2026, that is £234 in mix alone, before delivery.

Above about 0.5 cubic metres it is almost always cheaper to buy ready-mix from a local depot. They charge by the cubic metre (typically £100 to £130 in the UK), deliver in a wagon or mini-mix barrow, and the quality is more consistent than mixing 60 bags by hand in a wheelbarrow. The calculator shows both the bag total and the DIY cement-sand-gravel breakdown so you can compare.

A 1:2:3 mix (one part cement, two sand, three aggregate) is the workhorse for slabs, paths and footings. For post holes, a 1:2:4 mix or even straight Postcrete (a fast-set blend you pour dry, then add water) is fine because the load is compressive. A bag of cement weighs 25 kg in the UK and yields roughly 0.025 cubic metres at standard ratios, so the cement count is the volume divided by 0.025 then rounded up.

Always order an extra 5 to 10% on top of the calculated volume. Concrete is unforgiving: if you run short halfway through a pour you cannot really pause and order more, because the original mix sets and the new pour will not bond cleanly. For the aggregate side, [Gravel Calculator](/gravel-calculator) handles the volume to tonnes conversion.