Concrete Footing Size Chart: How Big Should Your Footings Be?

Footing size is one of the most commonly underspecified elements in residential construction. Under-sized footings settle, tip, or fail — causing damage that’s extremely expensive to repair. This guide covers how to size footings correctly for common residential applications.

Quick Answer: Footing area = Load ÷ Soil bearing capacity. For residential work, assume 1,500 psf if soil type is unknown. Always extend below your local frost line — the most common and most expensive footing mistake is going too shallow.

What Is a Footing?

A footing is a widened concrete base that spreads a structural load over a larger soil area. The purpose: reduce the pressure on the soil below the bearing capacity of that soil.

Without a footing, a post carrying 5,000 pounds concentrated in a 4×4 area would exert far more pressure per square inch than most soils can support. The footing spreads that load over a larger area.

The Core Sizing Principle

Footing Area = Load ÷ Soil Bearing Capacity

Soil bearing capacity for common soil types:

• Clay (soft): 1,000–2,000 psf
• Clay (medium): 2,000–4,000 psf
• Sand (loose): 2,000–3,000 psf
• Sand (dense): 3,000–6,000 psf
• Gravel: 4,000–8,000 psf
• Bedrock: 10,000+ psf

If you don’t know your soil type, use 1,500 psf as a conservative default for residential work.

Deck Footing Sizes

Deck footings support specific posts that carry tributary areas of the deck. IRC table R507.3.1 provides footing diameter requirements based on load area and soil capacity.

Deck Post Footing Diameter (Round Footings)

For 1,500 psf soil bearing (conservative), 1-story deck:

Tributary Area	Footing Diameter
Up to 25 sq ft	10 inches
Up to 50 sq ft	14 inches
Up to 75 sq ft	18 inches
Up to 100 sq ft	20 inches

Tributary area = the area of deck each post supports. For a simple rectangular deck with 4 posts, divide total area by 4.

Use our Deck Footing Calculator to calculate concrete volume per footing.

Foundation Wall Footing Sizes

For residential bearing wall foundations, the IRC specifies minimum footing widths based on the number of stories supported:

Stories Supported	Footing Width	Footing Thickness
1 story	12” wide	6” thick
2 stories	15” wide	7” thick
3 stories	23” wide	8” thick

These are minimum IRC requirements. Local amendments may require larger footings. Always check with your local building department.

Footing Depth: The Frost Line

Footings must extend below the local frost depth — the depth to which the ground freezes in your region. Frost heave can lift a footing and everything above it.

Approximate frost depths:

• Deep South (FL, south TX, AZ): 0–12 inches
• Southeast (NC, TN, GA): 12–18 inches
• Mid-Atlantic: 24–36 inches
• Great Lakes: 36–48 inches
• Upper Midwest (MN, ND): 42–60 inches
• Northern New England: 48–72 inches

Check your specific frost depth with your local building department — this is the single most common footing error.

Concrete Volume for Footings

Strip Footings

Volume = Length × Width × Depth

For a 60-foot perimeter house, 16” wide × 8” deep footing:

• 60 × (16/12) × (8/12) = 53.3 cubic feet = 1.97 cubic yards

Round Column Footings

Volume = π × r² × Depth

For a 12” diameter, 42” deep footing:

• π × (0.5)² × 3.5 = 2.75 cubic feet ≈ 5 bags of 80 lb concrete

Use our Concrete Footing Calculator for all footing configurations.

Frequently Asked Questions

Do I need rebar in residential footings? Most building codes require at minimum 2 continuous horizontal bars in all footings. Common specifications: 2–3 #4 bars running the length of the footing, with wire ties at corners.

What’s the minimum footing size for a fence post? Fence posts typically don’t require engineered footings. A 10-inch diameter hole, 24–30 inches deep (below frost), filled with concrete provides adequate support for standard wood or vinyl fence panels.

Can I pour a footing in cold weather? Yes, with precautions. The concrete mix temperature must be above 50°F when placed. Use heated water or add calcium chloride accelerant (not in concrete with rebar — it causes corrosion). Protect the fresh pour from freezing for at least 7 days.

What happens if my footing is too shallow? Frost heave lifts the footing, which can crack the foundation, rack door and window frames, and damage finishes. The cost of properly deep footings is trivial compared to the cost of fixing frost heave damage.

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See also: Concrete Calculation Methodology — footing volume formulas, bag yield tables, and IRC R403.1 references. Also see the Frost Depth Reference for all 50 states.