This is one of the most common home concrete jobs, a small patio or shed floor. Here is the full answer for the normal 4-inch thickness, worked out slowly so you can copy the same steps for any size.
For a 10 × 10 slab at 4 inches thick: order 1.36 cubic yards of ready-mix, or buy 62 bags of 80 lb concrete mix. (Exact need before the safety extra: 1.23 cubic yards / 56 bags.) At 6 inches thick: 2.04 cubic yards, or 92 bags.
Going bigger? At some point bags stop making sense, mixing 62 bags is already a full day of hard work. Our Concrete Cost Calculator compares both prices for your exact size.
Step 1: Volume in cubic feet 10 ft × 10 ft × (4 ÷ 12) ft = 33.33 cubic feet. (Thickness is in inches, so divide by 12 to change it to feet.)
Step 2: Change to cubic yards concrete is sold by the cubic yard, and 1 cubic yard = 27 cubic feet. 33.33 ÷ 27 = 1.23 cubic yards exact.
Step 3: Add 10% extra ground is never perfectly flat and some concrete spills. 1.23 × 1.1 = 1.36 cubic yards to order. Running short mid-pour is the one thing you must not do.
Step 4: If you buy bags instead one 80 lb bag makes 0.6 cubic feet. With the extra: 36.67 ÷ 0.6 = 61.1, so 62 bags. (Without the extra it would be 56 bags, too risky.)
All at 4 inches thick, amounts to order (with the 10% extra included):
| Slab size | Ready-mix to order | 80 lb bags to buy |
|---|---|---|
| 8 × 8 ft | 0.87 yd³ | 40 bags |
| 10 × 10 ft | 1.36 yd³ | 62 bags |
| 10 × 12 ft | 1.63 yd³ | 74 bags |
| 12 × 12 ft | 1.96 yd³ | 88 bags |
| 10 × 20 ft | 2.72 yd³ | 123 bags |
| 20 × 20 ft | 5.43 yd³ | 245 bags |
4 inches is normal for patios, walkways and shed floors. Use 6 inches if cars will drive or park on it. At 6 inches this slab needs 2.04 cubic yards or 92 bags (80 lb).
62 bags is a lot of mixing, most people find ready-mix easier at this size, and with delivery it often costs about the same. Our Concrete Cost Calculator compares with your local prices; at example prices ($150 a yard + $75 delivery) ready-mix comes to $279 against $341 for bags.
Same steps: 10 × 10 × (5 ÷ 12) = 41.67 cubic feet = 1.54 cubic yards exact, 1.70 yards to order, or 77 bags of 80 lb.