Deck Footing Concrete Calculator
Six 12" footings, 48" deep, come to 5.5 × 80 lb bags each, about 33 bags total. Set your footing count, diameter and depth below. Each footing rounds to the nearest half bag because you pour them one hole at a time.
Tube volume (minus an embedded post) plus any bell base, rounded to a half bag per footing. Bell math assumes a 12" bell height added below the tube; your form prints its own. Depth is the concrete column, not the wood post above it.
Tube forms, flared bell bases, post bases and a mixing tub for the pour: deck footing forms and hardware on Amazon (sponsored)
Bags per footing, at a glance
| Footing | Volume (48" deep) | 80 lb bags each |
|---|---|---|
| 8" footing | 1.40 ft³ | 2.5 |
| 10" footing | 2.18 ft³ | 4 |
| 12" footing | 3.14 ft³ | 5.5 |
| 14" footing | 4.28 ft³ | 7.5 |
| 16" footing | 5.59 ft³ | 9.5 |
These are straight tubes with no post deduction and no bell, at 48" deep. Shorter or deeper footings scale straight with depth. Working form by form instead of by project? The sonotube calculator is the same math per tube: a sonotube is the cardboard form, a footing is the whole below-grade pier you pour into it.
How many footings does a deck need?
That is set by your beam and joist spans, not the deck's square footage, so treat the counts below as a starting point and confirm them against a span table or your plans. Footing bags assume the 12" × 48" default above.
| Deck size | Typical footings | 80 lb bags (12" × 48") |
|---|---|---|
| 8 × 8 ft | 3 | 17 |
| 10 × 10 ft | 3 | 17 |
| 12 × 12 ft | 3 | 17 |
| 12 × 16 ft | 4 | 22 |
| 12 × 20 ft | 4 | 22 |
| 16 × 20 ft | 6 | 33 |
Once the footings cure and the posts are up, size the slab or pad portions with the slab calculator and check bag yields on the yields chart. If the whole set of footings adds up past a cubic yard, run bags vs ready-mix first.
Pouring footings that pass inspection
- Depth first: the bottom of the footing goes below frost line. That is the entire reason it exists; a shallow footing heaves and racks the deck.
- Bell where the soil is soft: a flared base spreads the load. In firm soil a straight tube is often fine; your inspector or engineer makes that call.
- One continuous pour per footing: fill and rod each tube in one go so there is no cold joint mid-pier.
- Set the hardware wet: drop the post base or anchor bolts in while the top is workable, aligned to a string line, so the beam lands where the plan says.
- Rebar is cheap insurance: 2 to 4 vertical bars in a taller pier; count them with the rebar calculator if your plan calls for them.
FAQ
How many bags of concrete per deck footing?
A 12" tube footing 48" deep takes 5.5 × 80 lb bags. Smaller 8" footings run about 2½ bags each; 16" footings about 9½. Add a flared bell base or embed the post and the count climbs. The tool rounds each footing to the nearest half bag because you batch and pour them one hole at a time.
What size sonotube for a deck footing?
Common practice is a tube about 3× the post width, so 10" or 12" tubes for 4×4 and 6×6 posts, but the real number is a bearing-area calculation from the load on that post and your soil. Your building department sets it. The sonotube calculator is the form-by-form version of this tool if you already know your tube size and height.
How deep do deck footings go?
Below your local frost line, which runs from about 12" in the mild South to 48" or more across the northern tier. Frost depth, footing diameter, and whether you need a flared base are load-and-soil calls your building inspector or a structural engineer makes, not a rule of thumb. Pull the permit and get the number before you dig.
Do deck footings need a flared or bell base?
Sometimes. A bell (a wider frustum at the bottom, like a Bigfoot form) spreads the post load over more soil so the footing does not punch down in soft ground. Whether you need one, and how wide, comes from the tributary load on that post and the soil bearing capacity. Switch the flared-base option on to add that volume once an engineer or your code official has specified it.
Should I mix bags or order ready-mix for footings?
Footings are one of the better cases for bags: each hole is a small, separate pour, so cold joints between batches are not an issue the way they are on a monolithic slab. Once the total climbs past about a cubic yard (roughly 45 × 80 lb bags), price a ready-mix short load against the bags and the mixing day.