Concrete Footing Calculator

Why Footings Must Go Below the Frost Line The most critical requirement for any post footing is depth — specifically, the footing must extend below the local frost line (also called the frost depth). The frost line is the maximum depth at which soil water freezes during winter. When soil freezes, it expands slightly. When it thaws, it contracts. A footing that sits above the frost line will heave (move up and down) with freeze-thaw cycles, which damages the structure above: cracking concrete, racking frames, and causing deck boards to buckle.

Frost Depth Varies Dramatically Frost depth is zero in Florida and Hawaii. It reaches 6 inches in coastal California. It rises to 30–36 inches across the Midwest and Mid-Atlantic. In northern Minnesota and most of Canada, frost depth can reach 48–72 inches. Your local building department will have the required footing depth for your area — always check before digging.

Round vs. Square Footings Round tube form footings (Sonotube is a common brand) are the most popular for deck posts. They are easy to form, resist soil pressure well, and produce a smooth cylinder of concrete. Square footings are used where a broader bearing area is needed for heavier loads or soft soils. The volume formulas are simple: π × r² × h for round, w² × h for square (all in consistent units).

Concrete Weight and Mix Cured concrete weighs approximately 150 lbs per cubic foot. That means a 12-inch diameter footing 36 inches deep weighs about 118 lbs — easy for two people to manage with pre-mixed bags. Standard pre-mixed bag yields: 40-lb bag = 0.30 cu ft, 60-lb bag = 0.45 cu ft, 80-lb bag = 0.60 cu ft. Always order at least 10% extra to account for slightly oversized holes, spillage, and variations in mixing water.

Post Size and Load For typical deck posts supporting normal residential loads, 10-inch diameter footings are usually code-minimum. For heavy beams, multi-story applications, or large point loads, 12-inch or 16-inch footings are common. Your local building code or a structural engineer’s calculation will specify the required size.

Mixing Tips Always mix concrete in a clean container and follow the water ratio on the bag — too much water weakens the mix significantly. For tube forms, pour in layers of 6–8 inches and rod or vibrate each layer to eliminate voids. Set the anchor bolt or post base hardware immediately after the pour, before the concrete begins to set (typically 30–60 minutes in warm weather).