Concrete Calculator
Find the cubic yards of ready-mix or the number of bags you need for a slab, patio or pad.
How to use this concrete calculator
Enter the slab's length and width in feet and its thickness in inches. The waste allowance covers spills, uneven ground and forms that bow out a little. 5–10% is normal. The result shows cubic yards for a ready-mix order and the number of premixed bags in the three common sizes.
How thick should a slab be?
| Project | Typical thickness |
|---|---|
| Sidewalk, patio, shed floor | 4 inches |
| Hot tub pad, garage floor | 5–6 inches |
| Driveway for cars | 4–5 inches |
| Driveway for heavy trucks or RVs | 6 inches or more |
Local building codes may set a minimum. Check before you pour anything that needs a permit.
Bag yields
| Bag size | Approx. yield | 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 |
Yields vary slightly by brand, so check the bag. For anything over about 1 cubic yard, ready-mix delivery is usually less work and often cheaper than mixing 45+ bags by hand.
The math behind it
Cubic feet = length × width × (thickness ÷ 12). Multiply by 1 + waste%, then divide by 27 for cubic yards, or by the bag yield for the number of bags.
Example: a 10 × 10 ft slab, 4 inches thick, is 33.3 cubic feet. With 10% waste that's 36.7 cubic feet, or 1.36 cubic yards. In 80 lb bags, that's 62 bags.
Before you pour
- Compact a 4-inch gravel base under the slab so it doesn't crack or sink.
- Check the forms are square by measuring both diagonals. They should match.
- Plan control joints about every 8–10 feet on a 4-inch slab.
Often used together
Quick answers
How many bags of concrete are in a yard?
About 45 bags of 80 lb, 60 bags of 60 lb, or 90 bags of 40 lb make one cubic yard.
How much concrete do I need for a 10 × 10 slab?
At 4 inches thick, a 10 × 10 ft slab needs about 1.23 cubic yards, or about 1.36 yards with 10% extra.
Is it cheaper to buy bags or ready-mix concrete?
For small jobs under about 1 cubic yard, bags are usually cheaper and simpler. For larger pours, ready-mix delivery usually saves money and a lot of hard work.