Bags of Concrete Calculator
See how many 40, 60 and 80 lb bags a slab or footing needs, side by side, so you can compare bag sizes before you load the cart.
Calculator
For 48.0 cu ft you need about 80 × 80 lb, 107 × 60 lb or 160 × 40 lb bags. Yields are labeled typicals — confirm on your bag.
Bagged mix comes in three common sizes, and the store shelf rarely tells you which one is the fewest trips to the register. This calculator prints all three counts at once for the same slab — 80 lb, 60 lb and 40 lb — using each bag’s labeled yield, so you can weigh price per bag against how many you would have to mix.
Formula
Convert the slab to volume, then divide by each bag’s labeled yield and round up:
cu ft = length_ft × width_ft × (thickness_in ÷ 12)
bags = ceil(cu ft ÷ bag yield) where yield = 0.60 / 0.45 / 0.30 ft³ for 80 / 60 / 40 lb.
Worked example
For 48 cu ft (a 12 × 12 ft slab at 4 in):
- 80 lb: ceil(48 ÷ 0.60) = 80 bags
- 60 lb: ceil(48 ÷ 0.45) = 107 bags
- 40 lb: ceil(48 ÷ 0.30) = 160 bags
Same concrete, very different trips to the mixer — which is why the 80 lb bag wins on any pour of real size.
Yields are labeled typicals
The 0.60 / 0.45 / 0.30 ft³ yields are the widely-published planning figures, but the exact volume printed on your bag can differ by mix and brand — high-early, crack-resistant and fast-setting blends are not identical. Always read the coverage on the bag and treat 80 bags as “buy 82”: a bag that splits, a scoop that spills or a low spot in the grade all disappear into the pour.
Reference table
Labeled bag yields and how many bags fill one cubic yard:
| Bag | Yield | Bags per yd³ |
|---|---|---|
| 80 lb | 0.60 ft³ | 45 |
| 60 lb | 0.45 ft³ | 60 |
| 40 lb | 0.30 ft³ | 90 |