Concrete Footing Calculator
Estimate the volume of concrete needed for your footings. Enter the footing dimensions and quantity to calculate total volume in cubic yards and the number of 80lb bags required.
Understanding Concrete Footings
Concrete footings are the foundation elements that transfer the weight of a structure to the ground. They are typically wider than the walls or columns they support, spreading the load over a larger area of soil to prevent settling and structural failure. Properly sized and installed footings are critical to the stability and longevity of any building.
The size of a footing depends on the weight it must support and the bearing capacity of the soil beneath it. Soft soils require wider footings to distribute the load, while dense, compacted soil can support narrower footings. Always consult local building codes and, for significant structures, a structural engineer.
Types of Footings
- Spread Footings (Isolated Footings): Individual pads of concrete that support a single column or post. These are commonly used for deck posts, fence posts, and steel columns. They distribute the point load over a wider area of soil.
- Strip Footings (Continuous Footings): Long, narrow footings that run beneath load-bearing walls. This is the most common type of footing for residential construction. They create a continuous base that distributes the wall load evenly along the length of the foundation.
- Pier Footings: Deep, narrow columns of concrete that extend down to stable soil or bedrock. They are used when the surface soil is too weak to support spread or strip footings. Pier footings are common in areas with expansive clay or high water tables.
Footing Sizing Guide
As a general rule, residential footings should be at least twice the width of the wall they support. Here are common sizing guidelines:
- One-story homes: 12-inch wide footings, 6 inches deep minimum
- Two-story homes: 15 to 18-inch wide footings, 7 to 8 inches deep
- Three-story homes: 22 to 24-inch wide footings, 8 to 10 inches deep
Why Frost Line Depth Matters
The bottom of your footings must be placed below the frost line—the maximum depth at which the ground freezes in winter. When water in the soil freezes, it expands and can push footings upward, a process called frost heave. This can crack foundations, damage walls, and compromise the entire structure. Frost line depths range from a few inches in the deep South to over 4 feet in northern states. Always check your local building code for the required frost depth in your area.
Frequently Asked Questions
How deep should footings be?
Footings should extend below the frost line in your area, which varies by region. In the northern United States, frost lines can be 36 to 48 inches deep, while in southern states they may be only 12 inches. Most building codes require footings to be at least 12 inches below undisturbed soil and below the frost line. Always check your local building code for specific requirements before digging.
How wide should a footing be?
A general rule of thumb is that footings should be twice the width of the wall they support. For a standard 8-inch concrete block wall, the footing should be at least 16 inches wide. For a 6-inch wall, the footing should be at least 12 inches wide. The exact width depends on the structural load and soil bearing capacity. For critical structures, consult a structural engineer who can evaluate your specific soil conditions.
How many bags of concrete do I need for a footing?
An 80-pound bag of pre-mixed concrete yields approximately 0.6 cubic feet of cured concrete. To determine how many bags you need, calculate the total volume of your footing in cubic feet and divide by 0.6. For example, a footing that is 16 inches wide, 8 inches deep, and 20 feet long has a volume of about 17.8 cubic feet, requiring approximately 30 bags of 80-pound concrete mix. For large projects, ordering ready-mix concrete by the cubic yard is more cost-effective.