How deep do deck footings need to be? (frost depth, diameter, and concrete volume)

Deck footings must extend below the local frost depth, full stop. A footing that sits above the frost line will heave when the ground freezes, pushing the post up and leaving it lower when it thaws — over and over, every winter, until the deck is visibly racked and the ledger connection is compromised. The frost depth varies significantly by geography: 0" in the deep South and Florida; 12″–20″ in the mid-South and Pacific Coast; 30″–42″ across the Midwest, Mid-Atlantic, and Northeast; and 48″–80″ in the northern Great Lakes, New England, and northern Plains. Minnesota, Wisconsin, North Dakota, and Maine commonly require 60″ or more. The USDA Frost Depth Map published by the American Wood Council gives the design value for your county.

The working formula is straightforward: footing depth = local frost depth + 6" of clearance below undisturbed soil. The 6" accounts for the gravel drainage bed at the bottom of the hole and keeps the concrete off the disturbed zone at the bore bottom. If your frost depth is 36", dig to 42" and pour to within 6" of grade.

How footing diameter is calculated

Footing diameter is determined by the weight of the deck, not by the height of the post or the size of the tube form. The calculation runs through three numbers:

Footing area required  =  Tributary load / Soil bearing capacity

Tributary load (lb)    =  Tributary area (sq ft)  ×  Design load (psf)
Design load            =  Live load + Dead load
                          (40 psf live + 10 psf dead = 50 psf, typical residential per IRC R301.5)

Soil bearing           =  1,500 psf (conservative for undisturbed soil)
                          2,000 psf (firm, compacted native soil)

Footing diameter       =  2 × sqrt(Footing area / π)

The American Wood Council’s Prescriptive Residential Deck Construction Guide (DCA 6) includes prescriptive footing-diameter tables keyed to tributary area and soil bearing capacity — most code officials accept these tables directly. The IRC Section R507.3 references the same approach.

Worked example: middle beam post on a 12 × 16 deck

A post sitting mid-span under the main beam of a 12 × 16 deck carries a tributary area of roughly 6 × 8 = 48 sq ft (half the span in each direction). Here’s the math:

Tributary area:        6 ft × 8 ft = 48 sq ft
Design load:           40 psf live + 10 psf dead = 50 psf  (IRC R301.5)
Tributary load:        48 × 50 = 2,400 lb
Soil bearing:          1,500 psf (conservative)

Footing area needed:   2,400 / 1,500 = 1.60 sq ft

Footing radius:        sqrt(1.60 / π) = sqrt(0.509) = 0.714 ft = 8.56"
Footing diameter:      2 × 8.56" = 17.1"

→ Round UP to the next standard tube-form size: 18".
  Use an 18" diameter tube form for this post.

Corner posts on the same deck carry roughly half the tributary area (one quarter of each adjacent bay), so they can step down to a 12" or 14" footing. Interior posts under a second beam point in the other direction — more load, bigger footings. Always run each post separately.

Frost heave: what it looks like and how footings resist it

When saturated soil freezes, it expands. A straight cylindrical footing sitting in that soil has nothing to grip: the freezing dirt grabs the sides of the concrete and pushes upward. In a single freeze–thaw cycle, that lift is typically 1⁄8" to 1⁄4" — small in isolation, cumulative over a decade. After enough cycles, the post is noticeably high on one side, the decking pitch is wrong, and the ledger flashing may be compromised.

A bell-bottom footing (flared at the base) resists uplift better than a straight tube because the wider base has soil bearing down on it from above. A standard Quikrete tube form is straight-sided; a Bigfoot System form or hand-cut bell adds the mechanical lock against frost pull-out. In sandy or clay soils that shift, the bell-bottom is worth the extra digging.

Post-to-footing connection: stand-off brackets

Never embed a wood post directly in concrete. Water travels easily up the end grain of lumber but poorly across the face grain — a post set in concrete creates a permanent reservoir against the most vulnerable surface of the wood. Even pressure-treated lumber will rot at the concrete interface within 10–15 years in most climates.

The correct detail is a stand-off post base — a Simpson ABU, ABA, or equivalent bracket anchored in the wet concrete with a J-bolt, with a minimum 1" standoff that keeps the post base above the concrete surface. The air gap lets the end grain dry between rains. The bracket transfers the load; the gap protects the wood.

• Use hot-dipped galvanized (ZMAX, G185) brackets for above-grade installs. ACQ-treated lumber corrodes standard zinc coatings.
• Never bury the bracket. Even galvanized hardware set in soil contact corrodes through in 3–5 years.
• Set J-bolts while the concrete is still wet, templated to the bracket hole pattern. Drill-and-epoxy anchors work but add a step and a cost.
• Minimum post burial in the bracket: 1" minimum engagement at the bracket base before the standoff begins. Follow the bracket manufacturer’s installation instructions for bolt pattern and structural screws.

Footing spacing: where posts go

Footings live under posts, and posts are where the beam needs support. The beam span determines post spacing, and the beam span is dictated by the joist span that runs perpendicular to it. Typical residential deck post spacing ranges from 6 to 10 feet on center; 8 feet is the most common default because it keeps beam sizing manageable (doubled 2×10 or 2×12 for most spans) without requiring a footing at every joist.

The upstream calculation — joist size, joist span, and therefore beam location — is covered in the deck joist span and spacing guide. Start there before sizing footings.

Concrete volume per footing

Use the cylinder formula to estimate how much concrete a footing takes:

Volume (ft³)  =  π × r² × depth

r = radius in feet (diameter / 2 / 12)
depth = total footing depth in feet

Example: 18" diameter × 36" deep footing
  r     = 18 / 2 / 12 = 0.75 ft
  depth = 36 / 12     = 3.0 ft
  Vol   = π × (0.75)² × 3.0
        = 3.1416 × 0.5625 × 3.0
        = 5.30 ft³

Bags of 80 lb concrete (yield = 0.60 ft³/bag):
  5.30 / 0.60 = 8.83 bags → buy 9 bags per footing

Add 20% for drilled holes (walls are never perfectly round):
  9 × 1.20 = 10.8 → order 11 bags per footing to be safe

For a deck with four footings at that size, that’s 44 bags of 80 lb concrete (4 footings × 5.30 ft³ = 21.2 ft³ ÷ 27 = roughly 0.8 cubic yards total). Bags are the right call at this volume — a short-load ready-mix truck typically makes sense for pours above roughly 1 cubic yard (about six of these footings or more). See the concrete yardage guide and the bags-per-footing guide for the full breakdown.

Common mistakes

• Measuring footing depth from the bottom of the tube form instead of from finished grade. If you backfill later, the footing is shallower than it looks.
• Not accounting for a 4–6 inch gravel drainage bed at the bottom of the hole. Dig 6 inches deeper than your target footing depth, add the gravel, then pour.
• Using a tube form that is too small because it was the only one in stock. Footing diameter is not the same as tube diameter — order the right form or upsize.
• Skipping the frost line in mild climates. A 12" frost depth still heaves a footing set at 10".
• Setting posts directly in concrete. End grain in concrete contact is a 10-year countdown regardless of treatment level.
• Forgetting the J-bolt template. Setting the bracket after the concrete cures is possible with epoxy anchors, but it adds cost and a cure-time delay — set the bolts wet.

Related

• Deck joist span and spacing — the upstream calculation
• How many bags of concrete for a footing or pier
• Concrete yardage guide — when to call ready-mix
• BuilderCalc — full deck project system on iOS + Android

Disclaimer: This article is for general educational reference only. All structural decisions must be verified against your local adopted building code and reviewed by the building official or a licensed structural engineer before construction. Local codes, soil, snow, wind, and seismic conditions vary. Always obtain required permits. Footing diameters above are based on 1,500 psf soil bearing (conservative); expansive, loose, or engineered soils require professional review.