Concrete Bags Calculator
Find how many bags of concrete you need — enter your slab, footing or column dimensions and the bag size to get the bag count for 40, 60 and 80 lb bags, plus an estimated cost.
How many bags of concrete do I need?
Start with the volume of your slab in cubic feet — length × width × thickness, all in feet — then divide by the yield of the bag you're buying. An 80 lb bag of concrete mix yields about 0.60 cubic feet, a 60 lb bag about 0.45, and a 40 lb bag about 0.30. The calculator above does this from your dimensions, rounds up, and adds a waste allowance so you don't come up short.
bags = (length × width × thickness ÷ 12, in cu ft) ÷ bag yield
Bag yield and coverage
| Bag size | Yield | Covers at 4″ | Bags per yd³ |
|---|---|---|---|
| 80 lb | 0.60 cu ft | ≈ 1.8 sq ft | 45 |
| 60 lb | 0.45 cu ft | ≈ 1.35 sq ft | 60 |
| 40 lb | 0.30 cu ft | ≈ 0.9 sq ft | 90 |
Buying a specific brand? See the Quikrete and Sakrete calculators, which use the same yields. For bigger pours, the concrete calculator compares bags against ready-mix and also handles footings and round post holes.
Frequently asked questions
How many bags of concrete do I need?
Work out the volume in cubic feet (length × width × thickness, all in feet), then divide by the bag yield: about 0.60 cu ft for an 80 lb bag, 0.45 for a 60 lb bag, and 0.30 for a 40 lb bag. The calculator does this from your dimensions and rounds up, including a waste allowance.
How many 80 lb bags of concrete make a yard?
About 45. A cubic yard is 27 cubic feet and an 80 lb bag yields roughly 0.60 cu ft, so 27 ÷ 0.60 ≈ 45 bags. For 60 lb bags it is about 60 per yard, and for 40 lb bags about 90 per yard.
How much does an 80 lb bag of concrete cover?
An 80 lb bag of standard concrete mix yields about 0.60 cubic feet of finished concrete. That covers roughly 1.8 square feet at 4 inches thick, or 1.2 square feet at 6 inches. The calculator converts your area and thickness into the exact bag count.
How many bags of concrete for a fence post?
For a typical 4-inch post in a 10-inch-wide hole 2 feet deep, plan on roughly one to two 50–60 lb bags per hole. Wider or deeper holes need more. Enter the hole as a round column above (diameter and depth) to size it exactly, and remember to subtract the post itself for a close figure.