Deck Footing Calculator — Size & Concrete Bags
Enter tributary area, soil bearing capacity, and frost depth to size a deck post footing and its concrete.
Footing Diameter
24 in
Required: 22.11 in · rounded up to nearest standard tube size
This tributary area/soil combination needs a footing larger than any standard form tube (24"). Consult a structural engineer for a custom or engineered footing.
Footing Depth
36 in
Deeper of frost depth and the IRC 12" minimum
Concrete Volume
9.42 ft³
0.35 yd³
Concrete Bags Needed
21 × 60 lb
or 16 × 80 lb bags
How deck footings are sized
A footing's job is to spread a post's load over enough soil that the pressure stays under the soil's bearing capacity. The IRC's approach (Section R507.3, "Deck footings") ties footing size to three numbers: how much deck area the post carries (tributary area), the standard residential deck design load (40 psf live + 10 psf dead, per R301.5), and the soil's allowable bearing pressure — 1,500 psf by default absent a geotechnical report (R401.4.1).
This calculator applies that load/bearing relationship directly — required footing area = tributary area × 50 psf ÷ soil bearing — then converts the area to a round diameter and rounds up to the nearest size you can actually buy. It does not reproduce the IRC's own R507.3.1 lookup table number-for-number (that table folds in extra conservatism from the code committee's own rounding choices), so treat the result as a solid sizing estimate — confirm against your adopted local code, and involve a structural engineer for anything unusual.
Standard footing diameters
Concrete form tubes (Sonotube and similar) are stocked in a handful of standard diameters. The calculator always rounds your required diameter up to one of these:
| Standard diameters |
|---|
| 10", 12", 16", 18", 20", 24" |
If your required diameter exceeds the largest standard size, the footing needs engineered design rather than an off-the-shelf tube.
Frost depth and concrete volume
IRC Section R403.1.4 requires footings to reach below the locally established frost line, or at least 12 inches below grade — whichever is deeper. Frost depth varies enormously by climate, from as little as 12 inches along the Gulf Coast to 48 inches or more across the northern US and Canada, so check with your local building department rather than guessing.
Concrete volume is calculated from the rounded footing diameter (what you'll actually pour) and the footing depth: volume = π × radius² × depth. Bag counts use the standard published yields for pre-mixed concrete: a 60 lb bag yields about 0.45 ft³ and an 80 lb bag yields about 0.6 ft³ of mixed concrete, both rounded up so you don't come up short mid-pour.
Frequently asked questions
- What size footing does a deck post need?
- It depends on how much deck area that post carries (its tributary area) and your soil's bearing capacity. On the IRC's default 1,500 psf soil, a post carrying 80 sq ft of tributary area needs roughly a 22-inch-diameter footing, which rounds up to the next standard 24-inch form tube. Stronger soil or a smaller tributary area allows a smaller footing.
- How do I find a post's tributary area?
- Multiply half the beam span on each side of the post by half the joist span on each side (or the post spacing along the beam). For evenly spaced posts, a simple estimate is beam span × post spacing — the rectangle of deck each post is responsible for holding up.
- How deep does a deck footing need to be?
- Per IRC R403.1.4, footings must extend below the locally established frost line, or at least 12 inches below grade, whichever is deeper. Frost depth varies enormously by region — from as little as 12 inches along the Gulf Coast to 48+ inches across the northern US — so check with your local building department before digging.
- How many bags of concrete does one deck footing need?
- It depends on the footing's volume (diameter and depth). As a reference, a standard 60 lb bag yields about 0.45 cubic feet of mixed concrete, and an 80 lb bag yields about 0.6 cubic feet — this calculator converts your footing's volume into both bag counts, rounded up.