Every other calculator gives you a volume. Nobody sells volume — they sell bags, or they send a truck. This one tells you which, how many, what it weighs, and when you're about to make an expensive mistake.
For slabs, pads, footings and post holes. Enter the shape, get the volume, the weight, the bag count — and a straight answer on whether to bag it or order it.
Works in metric and US imperial — you choose at the start and can switch at any time. Every figure this calculator uses is published with its source, and the result comes with a printable order sheet. No account, no email, and nothing you type leaves your browser.
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Volume first: length × width × depth for a slab, or the hole shape for footings and posts. The calculator then adds a 10% allowance — spillage, an uneven subgrade and the bit left in the barrow, which is what the trade allows — and converts the result three ways: cubic volume for a ready-mix order, weight so you know what you are handling, and a bag count for the bag size you choose.
A 4 × 3 m slab at 100 mm — a typical domestic slab depth — is 1.2 m³ of concrete. With the 10% allowance that is about 1.32 m³, weighing roughly 3.2 tonnes cured. Bagged, the same job is about 140 twenty-kilo bags. That is why a slab that "sounds like a few bags" turns out to be forty — and why this one is a ready-mix order, not a trolley run.
Plan measurements are easy; depth is where the error hides. Being 25 mm / 1 in out on a 4 × 3 m slab changes the order by around 0.12 m³ — enough to leave you short. Measure the depth at several points, use the deepest, and check access before you order: a truck needs firm ground, about 4 m of headroom, and the chute only reaches so far.
A 20 kg bag of concrete mix yields about 0.009 m³ once mixed, so allow roughly 105–110 bags per cubic metre. A 40 lb bag yields about 0.3 cu ft, which works out near 90 bags per cubic yard. Bag sizes vary — 20, 25 and 30 kg are all standard in metric markets, 40, 60 and 80 lb in the US — so count from the bag size you will actually buy.
Small jobs are bags: below a supplier's minimum load, a bulk delivery means paying a delivery fee and a minimum charge for concrete you won't use. Large jobs are ready-mix: past that point bagged mix costs more and you would lift every bag twice and mix them one at a time. In between, price both — minimum charges often make a small load cost the same as a larger one.
Running out mid-pour leaves a cold joint — a permanent weak line through the slab. This is the one material where slightly over-ordering is clearly the right call: order once, and have the formwork, reinforcement and helpers ready before the truck arrives, because once concrete is on site you have well under two hours.
No. Concrete mix is cement already blended with sand and aggregate — you add water. Cement on its own is just the binder, and its bags differ again: 25 kg across the UK and Europe, 50 kg across much of Asia, and 94 lb in the US. If you are mixing from scratch rather than buying premix, don't reuse premix figures.