How Much Concrete Do I Need for a Slab?
Work out concrete volume for any slab in three steps: measure, convert the thickness, and divide by 27. Includes worked examples for a patio, garage, and driveway, plus bag counts.
Ordering concrete comes down to one number: cubic yards. Suppliers sell ready-mix by the cubic yard, and bagged concrete is rated by how many cubic feet each bag yields. This guide shows the exact calculation, with worked examples you can check against our Concrete Calculator.
The one rule to remember: measure everything in feet, then divide the total cubic feet by 27 to get cubic yards, which is the unit every ready-mix supplier quotes.
The Slab Volume Formula
Concrete volume is length × width × thickness. The only trap is units. Measure length and width in feet, but thickness is usually in inches, so divide it by 12 first:
Then convert to cubic yards, which is what suppliers quote. One cubic yard is 27 cubic feet:
Worked Example: 10 × 10 ft Patio, 4 Inches Thick
A common backyard patio pour:
- 10 × 10 × (4 ÷ 12) = 10 × 10 × 0.333 = 33.3 cubic feet
- 33.3 ÷ 27 = 1.23 cubic yards
- With a 10% waste factor: 1.36 cubic yards (order 1.5 yards)
More Common Slab Sizes
| Project | Size | Volume | +10% Waste |
|---|---|---|---|
| Small patio | 10 × 10 ft × 4" | 1.23 yd³ | 1.36 yd³ |
| Shed base | 10 × 12 ft × 4" | 1.48 yd³ | 1.63 yd³ |
| Two-car garage | 24 × 24 ft × 4" | 7.11 yd³ | 7.82 yd³ |
| Driveway | 10 × 40 ft × 6" | 7.41 yd³ | 8.15 yd³ |
Bags or Ready-Mix?
Bagged concrete makes sense for small pours. Each bag yields a fixed volume once mixed:
- 80 lb bag ≈ 0.60 cubic feet, requiring 45 bags per cubic yard
- 60 lb bag ≈ 0.45 cubic feet, requiring 60 bags per cubic yard
For the 10 × 10 patio above (33.3 cubic feet), that is about 56 bags of 80 lb mix before waste. Anything above roughly 1.5 to 2 cubic yards is usually cheaper and far less work as a ready-mix delivery. A standard truck carries 9 to 11 cubic yards.
Why You Should Always Add a Waste Factor
Subgrades are never perfectly level, forms flex, and some concrete always stays in the mixer or wheelbarrow. Running short mid-pour creates a cold joint, which weakens the slab. Add 10% for typical slabs, and consider more if your excavation depth varies.
Skip the Manual Math
Our Concrete Calculator runs this exact calculation live, including bag counts for four bag sizes, truckload estimates, waste factor, and cost, and handles slabs, footings, columns, walls, and stairs. If you first need the area of an irregular space, start with the Square Footage Calculator.