How it works
- Round hole volume = 3.1416 × radius² × depth
- Slab volume = length × width × thickness
- Cubic yards = cubic feet ÷ 27
- Bags = volume, plus extra, ÷ what one bag makes, rounded up
Example: four holes, each 12 inches across and 36 inches deep. One hole is 3.1416 × 0.5² × 3 = 2.36 cubic feet. Four holes are 9.42 cubic feet, or 0.35 cubic yards. With 10 percent extra that is 10.37 cubic feet. An 80 lb bag makes 0.60 cubic feet, so you need 18 bags.
How much one bag makes
| Bag size | Makes about | 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 |
These are the yields printed on standard premixed concrete bags. A few specialty mixes differ, so check the bag.
How deep and how wide
The International Residential Code puts deck footings at least 12 inches below undisturbed ground. Where the ground freezes, they also need to reach below the frost line so frozen ground cannot lift them. That depth can be 48 inches or more in cold areas. The width depends on the load and your soil. Your local building department sets both numbers, so ask before you dig.
Bags or a truck
Once a job gets past about 1 cubic yard, which is 45 bags of 80 lb mix, ordering ready mix is usually less work than mixing by hand. Suppliers often have a minimum order, so call and compare.
Common questions
How many 80 lb bags of concrete make a cubic yard?
It takes 45 bags of 80 lb mix to make one cubic yard, because each bag makes about 0.60 cubic feet. With 60 lb bags it takes 60.
How many bags of concrete do I need for a 12 inch by 36 inch hole?
The hole holds 2.36 cubic feet. That is 4 bags of 80 lb mix, or 5 if you add 10 percent extra.
How many bags of concrete for a 10x10 slab?
A 10 by 10 foot slab 4 inches thick is 33.3 cubic feet, or 1.23 cubic yards. That is 56 bags of 80 lb mix before any extra, so most people order ready mix for a slab that size.
How deep should deck footings be?
At least 12 inches below undisturbed ground under the International Residential Code, and below the frost line where the ground freezes. Your building department can tell you the exact depth for your area.
Should I subtract the post from the hole volume?
For deck footings, no, because the post sits on top of the concrete. If you are setting a post inside the hole, the post takes up some space, but the extra concrete is small and useful as a margin.
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Sources
- Quikrete Concrete Mix data sheet (yield per bag)
- International Residential Code 2021, Chapter 5: decks (R507)
Facts on this page were last checked against these sources in October 2026.