Concrete Calculator: Yards and Bags
Enter the size of your slab, patio, or footings to find how much concrete to order, in cubic yards for a ready-mix truck or in bags for smaller pours.
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How to calculate concrete
- Measure the length and width in feet and the thickness in inches.
- Convert the thickness to feet by dividing by 12 (4 inches = 0.333 ft).
- Multiply length × width × thickness to get cubic feet.
- Divide cubic feet by 27 to get cubic yards.
- Add 5–10% for uneven ground, spills, and form spread.
- For bags, divide cubic feet by the bag yield: 0.6 cu ft for an 80 lb bag, 0.45 for 60 lb, 0.3 for 40 lb.
Concrete needed for common slabs (4" thick)
| Slab size | Cubic yards | 80 lb bags |
|---|---|---|
| 10′ × 10′ | 1.23 | 56 |
| 12′ × 12′ | 1.78 | 80 |
| 10′ × 20′ | 2.47 | 112 |
| 20′ × 20′ | 4.94 | 223 |
Before adding waste. Above about 1 cubic yard, a ready-mix delivery is usually cheaper and much easier than mixing bags.
Questions
Concrete Calculator FAQ
How many 80 lb bags of concrete make a yard?
About 45 bags. One cubic yard is 27 cubic feet, and an 80 lb bag yields about 0.6 cubic feet.
How thick should a concrete slab be?
Patios and walkways are usually 4 inches thick. Driveways should be 4–5 inches, and 6 inches for heavy trucks or RVs.
Should I buy bags or order ready-mix?
Bags make sense for post holes and small pads under about 1 cubic yard. Beyond that, ready-mix is usually cheaper per yard and gives a stronger, more consistent pour.