How much concrete do I need? Volume, waste and delivery
6 min read · Updated 2026-10-12 · BuildCalc
Concrete is ordered by volume and cannot be topped up once the pour begins, so a concrete estimate is the one calculation worth doing twice. Order too little and you risk a cold joint and a second delivery charge; order too much and you pay for concrete you cannot use. The geometry is easy; the judgement is in the margin and the delivery method.
The volume, converted to the units suppliers use
Length times width times thickness gives the volume in cubic feet, provided the thickness is converted from inches to feet. A ten-by-twelve slab at four inches is forty cubic feet, which is about one and a half cubic yards, since a cubic yard is twenty-seven cubic feet.
Ready-mix is ordered in cubic yards in the United States and cubic metres elsewhere. Break anything complicated into simple boxes — a slab, a footing, a step — work out each and add them. Post holes are cylinders, so use the radius squared times pi times the depth.
Why the margin is not optional
Concrete is lost to uneven subgrade, formwork that bulges under the weight, spillage and the residue in the chute. Five to ten per cent extra is normal. The surplus is never wasted in practice: it fills a small pad, a post base or a couple of spare blocks.
The risk of running short is much worse than the cost of a small surplus. A short pour means stopping mid-slab and starting again later, leaving a cold joint that is weaker and visible, or paying for a second delivery with its own minimum charge and waiting time.
Ready-mix or mix it yourself
Mixing by hand is sensible only for small jobs. An eighty-pound bag yields about six-tenths of a cubic foot, so a single cubic yard is over forty bags and well over a tonne of material to shovel, mix and place before it starts to set. Above a couple of cubic yards, ready-mix is both easier and usually cheaper.
If you do order ready-mix, plan the pour. Have enough people to move and level it, the reinforcement in place, and a finished plan for the surface. Concrete does not wait, and the first thirty minutes after the truck arrives decide the quality of the whole job.
Run your own numbers
Free calculators that answer the “how much” version of this question. They run entirely in your browser — nothing you type is uploaded or stored.
Common questions
How many bags of concrete make a cubic yard?
Around 45 eighty-pound bags, since each yields about 0.6 cubic feet and a cubic yard is 27 cubic feet.
How thick should a slab be?
Four inches is typical for a garden path or shed base, five to six for a driveway carrying vehicles, and thicker footings where the drawings specify.
BuildCalc provides general information and planning estimates only, not professional construction, engineering or electrical advice. Material quantities are illustrative and depend on your own project, products and local codes. See our full disclaimer.