Concrete Calculator — Bags & Cubic Yards for a Slab
A 10×10 ft concrete slab at 4 inches thick needs about 1.24 cubic yards of concrete — roughly 56 bags of 80 lb, 74 bags of 60 lb, or 111 bags of 40 lb mix before waste. One cubic yard is 27 cubic feet, or about 45 bags of 80 lb concrete.
Enter your slab dimensions to get the concrete volume in cubic yards and cubic feet, plus exactly how many bags you need for your chosen bag size — with a waste allowance included.
How to use this calculator
- Measure the length and width of the slab in feet.
- Enter the thickness in inches — 4 inches is standard for patios and walkways, 5–6 inches for driveways.
- Choose the bag size you plan to buy (80 lb bags are most common for slabs).
- Keep a 5–10% waste allowance for spillage and uneven subgrade.
The formula
Concrete volume is calculated in cubic feet, then converted to the cubic yards used for ready-mix or the bags used for smaller jobs.
Volume (cubic feet) = length (ft) × width (ft) × thickness (in) ÷ 12.
Cubic yards = cubic feet ÷ 27. Order ready-mix by the cubic yard.
Bags = cubic feet ÷ bag yield, rounded up. An 80 lb bag yields about 0.6 cu ft, a 60 lb bag 0.45 cu ft, and a 40 lb bag 0.30 cu ft.
Worked example
A 10 ft × 10 ft slab, 4 inches thick, using 80 lb bags with a 5% waste allowance.
Volume = 10 × 10 × (4 ÷ 12) = 33.33 cubic feetWith 5% waste = 33.33 × 1.05 = 35 cubic feetCubic yards = 35 ÷ 27 ≈ 1.30 cubic yardsBags = 35 ÷ 0.6 = 58.3 → 59 bags
A 10×10 slab at 4 inches needs about 1.30 cubic yards — roughly 59 bags of 80 lb concrete.
Reference tables
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
| 50 lb | 0.375 cu ft | 72 |
| 40 lb | 0.30 cu ft | 90 |
| Slab size | Cubic yards | 80 lb bags |
|---|---|---|
| 4 × 8 ft | 0.40 | 18 |
| 10 × 10 ft | 1.24 | 56 |
| 12 × 12 ft | 1.78 | 80 |
| 20 × 20 ft | 4.94 | 222 |
Frequently asked questions
How many 80 lb bags of concrete are in a yard?
An 80 lb bag yields about 0.6 cubic feet, and a cubic yard is 27 cubic feet, so you need 45 bags of 80 lb concrete per cubic yard (60 bags of 60 lb, or 90 bags of 40 lb).
How many bags of concrete for a 10x10 slab?
A 10×10 slab at 4 inches thick is about 1.24 cubic yards (33.3 cu ft). That is roughly 56 bags of 80 lb, 74 bags of 60 lb, or 111 bags of 40 lb concrete before waste.
How thick should a concrete slab be?
Four inches is standard for patios, sheds, and walkways. Use 5–6 inches for driveways or anywhere vehicles will park, over a compacted gravel base.
Should I buy bags or order ready-mix concrete?
Bagged concrete is cost-effective up to roughly 1 cubic yard. Beyond that, ordering ready-mix by the truck is usually cheaper and far less labor.