Concrete Calculator

What this concrete calculator does

Pour a slab, footing, or patio and the first question is always the same: how much concrete do I actually need? This calculator turns your length, width, and thickness into the volume of concrete required — in cubic feet and cubic yards — and then tells you how many 60 lb or 80 lb bags of ready-mix that works out to. It updates the moment you change a number, and the diagram redraws so you can see the slab you're describing.

How the math works

Volume is simply length × width × thickness, with everything converted to feet (so a 4 inch slab counts as 0.33 ft deep). That gives cubic feet; dividing by 27 gives cubic yards, the unit ready-mix trucks are ordered in. For bags, we assume a 60 lb bag yields about 0.45 cu ft and an 80 lb bag about 0.6 cu ft of set concrete, then round up because you can't buy half a bag. A 10% waste allowance is added by default to cover spillage, uneven subgrade, and over-excavation.

How to use it

1. Enter the length and width of your slab or footing in feet.
2. Set the thickness in inches — 4 in is standard for a patio, 6 in for a driveway.
3. Adjust the waste percentage if you want more or less cushion than the default 10%.
4. Read the cubic yards to order ready-mix by the truck, or the bag counts to buy ready-mix at the store.

FAQ

Should I order by the yard or buy bags?

Under about half a cubic yard, bags are usually easier and cheaper. Above that, a ready-mix truck saves a lot of mixing and back-breaking work.

Why add 10% waste?

Forms flex, the ground is rarely perfectly level, and some concrete always sticks to the wheelbarrow. A little extra beats running short in the middle of a pour.

What thickness should a slab be?

4 inches suits patios and walkways; 5–6 inches is common for driveways and anything that carries vehicle weight.

Are these numbers exact?

They're planning estimates, not professional engineering advice — always confirm the bag yield printed on the packaging and consult a pro for structural work.