Concrete Footing Calculator

Footings must extend below the local frost depth to prevent heaving. Minimum depths by region: Southern US (no frost) 12 inches, Mid-Atlantic 24–30 inches, Midwest 36–42 inches, Northern states and Canada 48–60+ inches. Always check your local building code — engineered footings may be required for load-bearing structures regardless of frost depth.

A footing should be at least twice as wide as the wall or post it supports. For an 8-inch CMU wall, the footing should be 16 inches wide. For a 6×6 post, a 24-inch square pad is common for residential decks. Width also depends on soil bearing capacity — weak or expansive soils require wider footings to spread the load.

A footing is the widened base at the bottom of a foundation that distributes the structure's load to the soil. A foundation is the full below-grade structural system — it includes the footings plus any stem walls, piers, or slabs above them. The footing is always the lowest element and is always wider than the wall or column it supports.

Most building codes require horizontal rebar in spread and strip footings. A common minimum is two #4 bars (1/2 inch) running the length of the footing. Vertical dowels tie the footing to the wall above. For pad footings under posts, a grid of #4 bars at 12-inch spacing is typical. Always check local code — unreinforced footings are only permitted for very light, non-structural applications.

Residential footings typically use 3,000 PSI (20 MPa) concrete, which is the minimum for most building codes. Increase to 3,500–4,000 PSI for footings in freeze-thaw climates, footings in contact with sulphate-bearing soils, or footings supporting heavy loads. Ask your ready-mix supplier for a mix with low water-cement ratio if durability is a concern.

Order 5–10% more concrete than calculated. Excavated trenches and holes are rarely perfectly dimensioned — soft spots, over-excavation, and form variation all consume extra concrete. For small pours (under 1 yard), round up to the nearest quarter yard. Most ready-mix companies have a minimum order of 1 cubic yard, so consider combining footing pours with any slab work scheduled on the same day.

Bagged concrete (60 lb or 80 lb) is practical for up to 10–15 bags per session. An 80 lb bag yields about 0.60 ft³. For 4 footings at 2 ft × 2 ft × 1 ft each (16 ft³ total), you would need 27 bags — manageable for a weekend project. For larger foundations, ready-mix is faster and produces more consistent concrete. The break-even point is typically around 0.5–1 cubic yard.