Concrete is sold by the cubic yard (ready-mix) or by the bag (small pours). To find how much you need, work in cubic yards first, then convert to bags if you’re mixing on site.
The formula
Multiply length × width × thickness — all in feet — then divide by 27 to get cubic yards. For a 4-inch slab, thickness is 0.33 ft.
Worked example
A 12 × 10 ft slab at 4 inches: 12 × 10 × 0.33 ÷ 27 = about 1.5 cubic yards — roughly 74 × 80-lb bags with a little waste.
Bags per cubic yard
One cubic yard is about 45 × 80-lb, 60 × 60-lb, or 90 × 40-lb bags. Each 80-lb bag yields roughly 0.6 cubic feet, and a yard is 27 cubic feet.
Add waste
Always add 5–10% for spillage, uneven subgrade and over-excavation. Order short and the pour stops halfway.
Open the concrete calculator → to get the exact yards and bags for your slab, footing or column.