Key takeaways
- Concrete volume = length × width × thickness, all in feet.
- There are 27 cubic feet in a cubic yard — the unit ready-mix is sold in.
- One 80 lb bag yields ≈ 0.60 cu ft, so about 45 bags fill a cubic yard.
- Below ~1 cubic yard, bags win; above it, order ready-mix and round up.
How to calculate concrete
Concrete is measured by volume. A cubic yard is 27 cubic feet of concrete — the standard unit ready-mix trucks deliver in. To size a pour, find its volume in cubic feet, then convert to cubic yards for delivery or to bags for a small job.
Thickness is entered in inches because slabs are specified that way, so it's divided by 12 to convert to feet before multiplying.
Worked example: a 10 × 12 patio slab
For a 10 ft × 12 ft slab poured 4 inches thick: 10 × 12 × (4 ÷ 12) = 40 cubic feet, which is 40 ÷ 27 = 1.48 cubic yards. In bags that's 40 ÷ 0.60 = 67 × 80 lb bags. Sixty-seven bags is a lot of mixing — past about a yard, ready-mix is the easier and cheaper call.
Bags per cubic yard by bag size
| 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 |
Yards or bags?
Below about 1 cubic yard, bags are usually cheaper and easier — and they skip ready-mix delivery minimums and short-load fees. Above that, ready-mix delivery is the better call. Either way, add 5–10% for spillage and an uneven subgrade, and round ready-mix up to the next quarter yard. For aggregate base under the slab, size it with the gravel calculator.
Frequently asked questions
How much concrete do I need?
Length × width × thickness in feet gives cubic feet; divide by 27 for cubic yards. A 10×12 slab at 4″ ≈ 1.48 cubic yards.
How many bags in a yard?
About 45 × 80 lb bags, 60 × 60 lb bags, or 90 × 40 lb bags per cubic yard.
How much concrete for a 24×24 garage slab?
A 24×24 slab at 4″ is 192 cubic feet ≈ 7.1 cubic yards — order ready-mix rather than mixing ~320 bags.
Bags or ready-mix — which is cheaper?
Under ~1 cubic yard, bags. Over it, ready-mix is cheaper per yard and far less labor, though suppliers charge a short-load fee below a 1-yard minimum.
How thick should the slab be?
4″ for patios and walkways; 5–6″ for driveways and anything carrying vehicles. Going 4″→6″ adds 50% more concrete.
How much extra should I order?
Add 5–10% for spillage and subgrade variation; round ready-mix up to the next quarter yard.
Bag yields follow standard manufacturer data — Quikrete lists an 80 lb bag of concrete mix at about 0.60 cu ft. The 27 cu ft per cubic yard conversion is exact.
Last reviewed June 2026