Concrete Bag Calculator (40, 50, 60 & 80 lb bags)
Most small pours (pads, steps, post bases, short walks) are done with premixed bags. The question is always how many. Enter your slab dimensions and this calculator converts the volume to bag counts using the manufacturer's published yield per bag.
How the math works
Volume (cu ft) = length (ft) × width (ft) × thickness (in) ÷ 12. Divide by 27 for cubic yards. Then divide the volume by the bag yield and round up, because you can't buy part of a bag.
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 50 lb | 0.375 cu ft | 72 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
Source: QUIKRETE Concrete Mix (No. 1101) data sheet. Sakrete and other brands publish nearly identical figures for standard 4,000 psi mix. High-strength, fast-setting and crack-resistant mixes can yield slightly differently; check the bag.
Bags needed by slab size (no waste added)
| Slab | Thick | Cu ft | Cu yd | 40 lb | 60 lb | 80 lb |
|---|---|---|---|---|---|---|
| 4 × 4 ft | 4 in | 5.33 | 0.2 | 18 | 12 | 9 |
| 4 × 4 ft | 6 in | 8 | 0.3 | 27 | 18 | 14 |
| 5 × 5 ft | 4 in | 8.33 | 0.31 | 28 | 19 | 14 |
| 5 × 5 ft | 6 in | 12.5 | 0.46 | 42 | 28 | 21 |
| 4 × 8 ft | 4 in | 10.67 | 0.4 | 36 | 24 | 18 |
| 4 × 8 ft | 6 in | 16 | 0.59 | 54 | 36 | 27 |
| 6 × 6 ft | 4 in | 12 | 0.44 | 40 | 27 | 20 |
| 6 × 6 ft | 6 in | 18 | 0.67 | 60 | 40 | 30 |
| 8 × 8 ft | 4 in | 21.33 | 0.79 | 72 | 48 | 36 |
| 8 × 8 ft | 6 in | 32 | 1.19 | 107 | 72 | 54 |
| 10 × 10 ft | 4 in | 33.33 | 1.23 | 112 | 75 | 56 |
| 10 × 10 ft | 6 in | 50 | 1.85 | 167 | 112 | 84 |
| 10 × 12 ft | 4 in | 40 | 1.48 | 134 | 89 | 67 |
| 10 × 12 ft | 6 in | 60 | 2.22 | 200 | 134 | 100 |
| 12 × 12 ft | 4 in | 48 | 1.78 | 160 | 107 | 80 |
| 12 × 12 ft | 6 in | 72 | 2.67 | 240 | 160 | 120 |
Tips before you buy
- Add 5–10%. Uneven subgrade eats concrete fast. A ½ in low spot across a 10 × 10 slab is another 4 cu ft, about 7 extra 80 lb bags.
- 4 inches is the norm for walks and patios. Use 5–6 in for anything that will carry a hot tub, a mower trailer or a vehicle.
- Past about 1 cubic yard (45 × 80 lb bags), compare pricing with a ready-mix truck. See the bagged vs ready-mix break-even calculator.
- An 80 lb bag is cheaper per cubic foot, but 60 lb bags are easier on your back if you're mixing in a wheelbarrow.
Frequently asked questions
How many 80 lb bags of concrete make a yard?
45. Each 80 lb bag yields about 0.60 cu ft, and a cubic yard is 27 cu ft (27 ÷ 0.60 = 45).
How many bags of concrete do I need for a 10x10 slab?
At 4 inches thick a 10×10 slab is 33.3 cu ft: 56 × 80 lb bags or 75 × 60 lb bags before waste. Add 5–10% for waste.
How many 60 lb bags are in a yard of concrete?
60. A 60 lb bag yields about 0.45 cu ft, and 27 ÷ 0.45 = 60.
Is it cheaper to use 80 lb or 60 lb bags?
Usually 80 lb bags, because the price per cubic foot is lower. Compare shelf prices: divide the price by the yield (0.60 or 0.45 cu ft).
Last reviewed October 2026 by Jim Frith. These are planning estimates; confirm quantities with your supplier and follow local codes.