How the concrete math works
- Find the volume in cubic feet. For a slab, length × width × thickness, with the thickness converted to feet (inches ÷ 12). For a round column, π × radius² × height, with the radius in feet.
- Add a margin. Multiply by 1.10 for 10 percent extra, or whatever margin you choose.
- Divide by the bag yield. QUIKRETE's Concrete Mix data sheet lists 0.30 cu ft for a 40 lb bag, 0.375 for 50 lb, 0.45 for 60 lb and 0.60 for 80 lb. Round up to whole bags.
- For ready-mix, divide cubic feet by 27 to get cubic yards.
Example: a 10 × 10 ft patio, 4 inches thick, is 10 × 10 × 0.333 = 33.3 cubic feet. With 10 percent extra that's 36.7 cubic feet, or 36.7 ÷ 0.60 = 61.1, so buy 62 bags of 80 lb mix. That's about 1.36 cubic yards if you'd rather order ready-mix.
Concrete bags chart
Slabs, including 10 percent extra, using QUIKRETE Concrete Mix yields.
| Slab | Cubic yards | 80 lb bags | 60 lb bags | 40 lb bags |
|---|---|---|---|---|
| 4 × 4 ft, 4" | 0.22 | 10 | 14 | 20 |
| 8 × 8 ft, 4" | 0.87 | 40 | 53 | 79 |
| 10 × 10 ft, 4" | 1.36 | 62 | 82 | 123 |
| 10 × 12 ft, 4" | 1.63 | 74 | 98 | 147 |
| 12 × 12 ft, 4" | 1.96 | 88 | 118 | 176 |
| 10 × 10 ft, 6" | 2.04 | 92 | 123 | 184 |
| 12 × 20 ft, 4" | 3.26 | 147 | 196 | 294 |
| 20 × 20 ft, 4" | 5.43 | 245 | 326 | 489 |
Before you pour
- Compact the base and set forms first. Every inch the ground dips below your form adds concrete you didn't plan for.
- QUIKRETE recommends about 6 pints of water per 80 lb bag to start, and at least 1 cubic foot of mixer capacity per 80 lb bag mixed at once.
- Other brands publish their own yields. If your bag lists a different yield, divide the cubic feet above by that number instead.
- Setting posts? The fence post calculator subtracts the post and accounts for the soil cap.