Concrete Footing Size for Deck Posts: Load Calculation, Soil Bearing, and Frost Depth

The permit inspector flagged the deck: footings weren’t deep enough for the frost line, and the post spacing put one corner footing under a load it wasn’t sized to carry. Two re-pours and a schedule delay later, the homeowner is unhappy and the contractor is eating the cost. Both problems are preventable with a footing calculation that takes fifteen minutes to run correctly the first time.

Concrete footing size for deck posts isn’t guesswork — it’s determined by three numbers: the load the post carries, the soil bearing capacity, and the frost depth for your location. Here’s how to calculate each and combine them into a footing spec that passes inspection.

Step 1: Calculate the Load Each Post Must Carry

Deck post loads come from two sources: dead load (the deck structure itself) and live load (people and furniture). The IRC uses 15 psf dead load and 40 psf live load for residential decks, giving a total design load of 55 psf.

Each post carries the load from its tributary area — the deck area that drains to that post. For a rectangular deck with uniformly spaced posts, the tributary area for an interior post is the product of half the span between posts in each direction.

The edge posts at midspan carry twice the load of the corner posts. This is the source of the common sizing mistake — using one footing diameter for all posts when the midspan edge posts may require a larger diameter.

Step 2: Determine Soil Bearing Capacity

The soil bearing capacity determines how large a footing area is needed to distribute the post load without excessive settlement. Bearing capacity varies significantly by soil type. IRC Table R401.4.1 establishes presumptive soil bearing capacities:

• Rock: 4,000 psf (usually controls on rock; footings can be small)
• Gravel, well-graded gravel and sand: 3,000 psf
• Sandy gravel and gravel: 2,500 psf
• Fine sand, clayey sand: 2,000 psf
• Clay (stiff): 2,000 psf
• Clay (soft) or silt: 1,500 psf

If you don’t know your soil type, 1,500 psf is the conservative default. If you have soft, organic, or wet soil, a geotechnical assessment may be required before footing design can proceed — those soils can have bearing capacity below 1,000 psf, which IRC Table R401.4.1 doesn’t cover.

Step 3: Calculate Required Footing Area and Diameter

The required footing base area is the post load divided by the allowable soil bearing capacity:

For a round sonotube footing (the standard for deck posts), convert the required area to diameter:

Standard sonotube diameters available at most lumber yards: 8, 10, 12, 14, 16, 18, and 24 inches. Always round up to the next available size — rounding down produces an undersized footing regardless of how close you are to the next smaller size.

Step 4: Frost Depth — Where the Bottom of the Footing Must Be

Frost heave is the other footing failure mode. Water in the soil expands when it freezes, and a footing placed above the frost line will heave upward during freeze-thaw cycles, damaging the deck structure.

The IRC requires that footings for attached structures extend below the frost line. Local frost depths by region:

• Deep South (Zone 0): 0–6 inches (some counties exempt from frost depth requirements for freestanding decks)
• Mid-Atlantic, Midwest: 24–36 inches
• Northern tier (Minnesota, Wisconsin, Michigan UP, Maine): 48–60 inches
• Mountain West: varies dramatically by elevation — 36 inches at 5,000 ft, 60+ inches at 8,000 ft

Your local building department publishes the frost depth for the jurisdiction. The footing bottom must be at or below that depth — not the frost line itself but the footing base. A 48-inch frost line means the bottom of your sonotube sits at 48 inches below finished grade at minimum.

Concrete Volume Calculation for Sonotubes

Once you have the diameter and depth, calculate the concrete volume for the sonotube using the cylinder volume formula:

For a 12-inch (1 ft) diameter sonotube at 36 inches (3 ft) depth:

A 60-lb bag of concrete yields about 0.45 cu ft. For this sonotube: 2.36 / 0.45 = 5.2 bags → round up to 6 bags per footing. For a deck with 12 posts at this size, order 72 bags with a 10% buffer = 80 bags. The concrete yardage calculator handles this math for mixed footing sizes when your post count and dimensions vary.

Inspections and What the Inspector Is Checking

Most jurisdictions require a footing inspection before concrete is poured. The inspector verifies:

• Hole diameter matches the approved drawings
• Hole bottom is at or below the required frost depth
• Soil at the bottom is undisturbed and not saturated (soft, wet soil requires rebar or a footing pad)
• The sonotube is positioned correctly relative to the ledger and post locations

Schedule the inspection before the pour, not after. Pouring footings without inspection and then having the inspector require excavation is an avoidable problem. In most jurisdictions, the footing inspection is required whether or not the deck requires a full permit.