What Is a Concrete Footing?
A concrete footing is a structural element poured at the base of posts, columns, or walls. Its job is to transfer the loads from the structure above down into stable soil. If the footing is undersized, the soil bearing capacity is exceeded and settlement or structural failure occurs. If it is oversized, you waste money on concrete. Correct footing sizing is one of the most critical steps in any construction project.
The Two Key Variables: Load and Soil Bearing Capacity
Every footing design starts with two numbers:
- Total Load (lbs): The sum of all forces the footing must support — dead load (the weight of the structure), live load (occupants, snow, furniture), and wind load.
- Soil Bearing Capacity (PSF): How many pounds per square foot the soil can safely support before compressing or shifting. Average residential soil is rated at 1,500–2,000 PSF. Sandy or clay-heavy soils can be as low as 1,000 PSF; dense gravel or bedrock can exceed 3,000 PSF.
Footing Area Formula
The required footing area is simply the total load divided by the soil bearing capacity:
Required Area (sq ft) = Total Load (lbs) ÷ Soil Bearing Capacity (PSF)
Example: A deck post carries 8,000 lbs total load. Soil bearing capacity is 1,500 PSF.
8,000 ÷ 1,500 = 5.33 sq ft → footing must be at least 5.33 sq ft in area
For a square footing: √5.33 = 2.31 ft → round up to a 28-inch × 28-inch footing.
Frost Depth: Why Your Footing Must Go Below the Frost Line
In cold climates, the soil freezes and expands in winter, then thaws and contracts in spring. This is called frost heave, and it can lift an entire structure if the footing is not below the frost line. Building codes require footings to be placed below the local frost depth in your region.
| Region / State | Approximate Frost Line Depth |
|---|---|
| Florida, Texas (Gulf Coast) | 0 inches (no freeze) |
| Georgia, Tennessee | 6–12 inches |
| Virginia, North Carolina | 12–18 inches |
| Ohio, Indiana, Illinois | 24–36 inches |
| Michigan, Wisconsin | 36–48 inches |
| Minnesota, North Dakota | 48–60 inches |
| Alaska (Fairbanks) | 72+ inches |
Always check with your local building department for the exact frost depth requirement in your municipality before pouring any footing.
Standard Deck Post Footing Sizes
The IRC provides prescriptive footing sizes for residential decks based on the load area (the deck area each post supports) and the number of stories:
| Deck Load Area per Post | Footing Diameter (Round) | Footing Size (Square) |
|---|---|---|
| Up to 36 sq ft | 10 inches | 10 × 10 inches |
| 37–48 sq ft | 12 inches | 12 × 12 inches |
| 49–64 sq ft | 14 inches | 14 × 14 inches |
| 65–80 sq ft | 15 inches | 15 × 15 inches |
Footing Thickness Rule of Thumb
The thickness of the footing should be at least equal to the distance the footing projects beyond the post or column it supports, but never less than 6 inches. For a standard 6×6 post (5.5 inches actual) on a 20-inch diameter footing: Projection = (20 – 5.5) / 2 = 7.25 inches → footing must be at least 7.25 inches thick. Most residential projects use 8–12 inch thick footings.
Use our free Concrete Calculator to calculate exactly how many cubic yards and bags of concrete you'll need for your footings once you have your dimensions.