Key takeaways
- Concrete volume = length × width × thickness, all converted to feet.
- Divide cubic feet by 27 to get cubic yards (how ready-mix is sold).
- An 80 lb bag yields ≈ 0.60 cu ft, so ~45 bags = 1 cubic yard.
- Under ~1 yard, buy bags; over it, order ready-mix and round up.
Start with the volume
Concrete is sold by volume, so every estimate starts there. Measure the length and width of the pour in feet and the thickness in inches, then convert thickness to feet by dividing by 12.
That cubic-yard figure is what a ready-mix supplier needs. For small jobs you'll convert it to bags instead.
A worked example: a 10 × 12 patio
Say you're pouring a 10 ft × 12 ft patio slab, 4 inches thick. The math: 10 × 12 × (4 ÷ 12) = 40 cubic feet, and 40 ÷ 27 = 1.48 cubic yards. In 80 lb bags that's 40 ÷ 0.60 = 67 bags — a lot of mixing, which is exactly why a pour this size usually goes to ready-mix.
The concrete calculator runs this instantly and shows the bag counts for 40, 60, and 80 lb mixes side by side.
Bags or ready-mix?
The dividing line is about one cubic yard. Below it, bagged mix is cheaper and you skip ready-mix delivery minimums and short-load fees. Above it, ready-mix wins on both cost per yard and labor.
| 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 |
Don't forget the base and the waste
Most slabs sit on 4–6 inches of compacted gravel base. Size that separately with the gravel calculator. And always add 5–10% to the concrete for spillage and an uneven subgrade — round ready-mix up to the next quarter yard.
Frequently asked questions
How do I calculate concrete for a slab?
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 make a cubic yard?
About 45 × 80 lb, 60 × 60 lb, or 90 × 40 lb bags per cubic yard.
When should I order ready-mix?
Above about a cubic yard — it's cheaper per yard and far less labor than mixing dozens of bags.