Concrete Calculator

Work out how much concrete you need for a slab, footing, wall or round column, in cubic yards, cubic feet and premix bags, with extra for waste.

How to use the concrete calculator

Choose Slab, footing or wall for anything rectangular, or Round column for Sonotubes and post holes.
Enter length and width in feet and thickness in inches. A typical patio or shed slab is 4 inches thick; driveways are often 5–6 inches.
Add 5–10% for waste, spillage and uneven ground. The result shows cubic yards for ordering ready-mix and the number of premix bags.

Concrete volume formulas

Slab: cubic yards = length (ft) × width (ft) × thickness (in) ÷ 12 ÷ 27 Column: cubic yards = π × (diameter (in) ÷ 24)² × height (ft) ÷ 27

Dividing the thickness by 12 turns inches into feet, so the result is in cubic feet. There are 27 cubic feet in a cubic yard (3 ft × 3 ft × 3 ft).

Worked example

A 10 × 10 ft slab, 4 inches thick: 10 × 10 × (4 ÷ 12) = 33.3 ft³. Divide by 27 to get 1.23 cubic yards, then add 10% for waste: 1.36 cubic yards, or about 62 bags of 80 lb premix.

How many bags of concrete do I need?

Bag size	Yield per bag	Bags per cubic yard
40 lb	0.30 ft³	90
50 lb	0.375 ft³	72
60 lb	0.45 ft³	60
80 lb	0.60 ft³	45

Yields are typical for standard concrete mix and can vary a little by brand, so check the bag. For jobs over about one cubic yard, a ready-mix truck usually costs less and saves hours of mixing.

Recommended slab thickness

Project	Typical thickness
Walkways, patios, shed floors	4 in
Driveways (cars)	5–6 in
Garage floors	4–6 in
Heavy equipment or RV pads	6–8 in

Thickness and reinforcement requirements vary with soil, climate and local building codes. Check with your local building department or a contractor for structural work.

Frequently asked questions

How much concrete do I need for a 10×10 slab?

At 4 inches thick, a 10 × 10 ft slab needs about 1.23 cubic yards, or 1.36 cubic yards with 10% extra. At 6 inches thick it needs about 1.85 cubic yards before waste.

How many 80 lb bags make a yard of concrete?

About 45 bags, since each 80 lb bag yields roughly 0.6 cubic feet and a cubic yard is 27 cubic feet.

How do I calculate concrete for a round post hole?

Choose Round column, enter the hole diameter in inches and the depth in feet, and set the quantity to the number of holes.

Should I order extra concrete?

Yes. Ordering 5–10% extra covers uneven subgrade, spillage and forms that bow out. Running short in the middle of a pour causes weak cold joints.

Is cement the same as concrete?

No. Cement is the powder that binds concrete together. Concrete is cement mixed with sand, gravel and water.