The concrete calculator estimates the volume of concrete for a slab, patio, footing, wall or round post hole, in cubic yards for ready-mix delivery and in bags for premixed concrete you mix yourself. A small allowance for waste is included because forms bulge, ground is uneven and some concrete is always left in the mixer.
How to use it
- Choose a slab (also use this for footings and walls) or a round column.
- Enter the dimensions. Thickness is usually given in inches.
- Enter how many identical pours you need, such as the number of fence post holes.
- Keep 5–10% extra, more if the ground is uneven.
The formulas
Column: π × (diameter ÷ 2)² × height (ft) ÷ 27
A 10 × 10 ft patio 4 inches thick is 10 × 10 × 0.333 = 33.3 cubic feet, or 1.23 cubic yards. With 10% extra, that’s about 1.36 cubic yards.
How many bags of concrete?
Premixed concrete bag yields are printed on the bag. Common figures for standard concrete mix are about 0.60 cubic feet for an 80 lb bag, 0.45 cubic feet for a 60 lb bag and 0.30 cubic feet for a 40 lb bag, which the calculator uses. Check your brand’s label, especially for fast-setting or high-strength mixes. For jobs larger than about one cubic yard, ready-mix delivery is usually easier than mixing dozens of bags by hand.
Typical thicknesses
- Walkways and patios: commonly 4 inches.
- Driveways: often 4–6 inches, thicker for heavy vehicles.
- Footings and post holes: follow local building code and the post manufacturer’s instructions; frost depth varies widely across the US.
Frequently asked questions
How many 80 lb bags make a cubic yard?
About 45, since a cubic yard is 27 cubic feet and each 80 lb bag yields about 0.6 cubic feet.
How many 60 lb bags make a cubic yard?
About 60 bags, at roughly 0.45 cubic feet per bag.
Does this include rebar or gravel base?
No. It covers concrete only. Use the gravel calculator for a compacted stone base.