Home & construction
Concrete calculator
Concrete is ordered by volume and sold by the bag or the truck. Get the volume right, add the allowance every contractor adds, and you will not be short at the wrong moment.
Uneven subgrade, spillage and settling. 10% is standard; go higher for hand-dug trenches.
Volume, not area
The commonest concrete mistake is calculating the slab area and forgetting that thickness is the third dimension — and that it is usually quoted in inches while everything else is in feet.
The two divisions convert inches to feet, then cubic feet to cubic yards. A 20 by 10 foot patio at 4 inches thick works out at 2.47 cubic yards, which surprises people expecting a smaller number from a modest patio.
Bags or truck
An 80 lb bag of pre-mix yields about 0.6 cubic feet, so a cubic yard takes 45 bags. Mixing 45 bags by hand is a long, heavy day, and keeping the mix consistent across that many batches is genuinely difficult.
| Volume | Practical choice |
|---|---|
| Under 0.25 cu yd | Bags, mixed in a wheelbarrow or tub |
| 0.25–1 cu yd | Bags with a hired mixer, or a trailer of ready-mix |
| Over 1 cu yd | Ready-mix truck — usually cheaper per yard and vastly faster |
Ready-mix suppliers often set a minimum load, commonly one cubic yard, and charge a short-load fee below it. Ask about the minimum before assuming the truck is the cheaper option for a small pour.
Why the allowance is not optional
Subgrade is never perfectly flat. A trench dug by hand is wider than the string line at the bottom in places, forms bulge slightly under pressure, and some concrete is always lost to the barrow and the chute. Ten percent covers ordinary work; hand-dug footings in soft ground can absorb noticeably more.
The asymmetry matters. Surplus concrete is a nuisance you can dump into a spare form or a post hole. Running out mid-pour creates a cold joint, a permanent line of weakness where the first batch has begun to set before the second arrives.
Thickness and reinforcement, briefly
Four inches is the usual minimum for a residential slab carrying foot traffic. Driveways carrying cars typically want five to six inches, and anything taking heavier vehicles more again. Below three inches, concrete cracks under almost any load regardless of the mix.
Reinforcement, control joints, subbase preparation and curing all affect whether a slab lasts thirty years or cracks in its first winter, and none of them are a volume calculation. Anything structural — foundations, retaining walls, anything holding a building up — should be designed by someone qualified and checked against local building codes.
Common questions
How many bags of concrete make a cubic yard?
About 45 of the 80 lb bags, 60 of the 60 lb bags, or 90 of the 40 lb bags. That is a lot of mixing by hand, which is why most people switch to ready-mix somewhere around the one cubic yard mark.
How thick should a concrete slab be?
Four inches, or roughly 100 mm, is the common minimum for patios, sheds and footpaths. Driveways taking cars usually want five to six inches, more if heavier vehicles are involved. Thickness interacts with subbase quality and reinforcement, so for anything structural, follow local code or an engineer rather than a rule of thumb.
How much does concrete weigh?
Around 2400 kg per cubic metre, or about 4000 lb per cubic yard — two tons. This matters for access as much as anything: a wheelbarrow holds roughly 0.1 cubic yard and weighs about 180 kg loaded, which is why long barrow runs are a serious consideration.
How long before I can walk on it?
Typically 24 to 48 hours for foot traffic and about seven days before light vehicle loads, though concrete keeps gaining strength for weeks and is conventionally tested at 28 days. Cold weather slows the process substantially. Keep the surface damp or covered while it cures — concrete that dries too fast is weaker than concrete that cures slowly.