Concrete is measured by volume in cubic yards, then converted to bags for small pours or a ready-mix delivery for large ones. Find the volume, divide by 27 for cubic yards, and add a waste factor.
Worked example
A 10 ft × 10 ft slab at 4 inches thick: 10 × 10 × (4 ÷ 12) = 33.3 cubic feet; ÷ 27 = 1.23 cubic yards. Add 5–10% waste → about 1.3 yards.
Bags or ready-mix?
Bagged concrete is convenient for small jobs but adds up fast. Yields per bag:
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 40 lb | ~0.30 cu ft | ~90 |
| 60 lb | ~0.45 cu ft | ~60 |
| 80 lb | ~0.60 cu ft | ~45 |
Our 33.3-cubic-foot slab needs 33.3 ÷ 0.60 = 56 eighty-pound bags before waste — roughly a full pallet to mix by hand, a strong sign to order ready-mix instead. The rule of thumb: past about 1 cubic yard, a ready-mix truck is usually cheaper and more consistent.
Frequently asked questions
How many 80 lb bags make a cubic yard? About 45.
When should I order a truck instead of bags? Generally above 1 cubic yard — mixing 45-plus bags by hand risks cold joints.
How much extra should I order? 5–10% over your calculated volume.