A footing must extend below your local frost line — the depth to which the ground freezes in winter. That depth ranges from roughly 12 inches in warm climates to 48 inches or more in cold ones. The exact number is set by your local building code, and that's the figure to build to, not a rule of thumb from the internet.
This one detail separates structures that stand for decades from ones that crack, tilt, and pull apart after a single hard winter. Get the depth right and your deck, porch, fence, or foundation stays put. Get it wrong and frost will do the demolition for you.
What frost heave actually does
When water in the soil freezes, it expands. If a footing sits above the frost line, that expanding, freezing soil pushes up against the bottom of the footing and lifts it — that's frost heave. The next thaw lets it settle, often unevenly. Repeat that cycle every winter and your posts rise, your slab cracks, and doors stop closing.
The fix is simple in concept: put the bottom of the footing below the deepest frozen layer. Down there the soil never freezes, so there's nothing to heave the footing upward. The footing bears on stable ground that stays the same temperature year-round.
Depth by region — and why you still must check code
Frost line depth tracks your climate. As a rough orientation:
- Warm / mild climates (much of the South and coastal areas): around 12 inches, sometimes less where the ground rarely freezes.
- Temperate climates (Mid-Atlantic, lower Midwest): commonly 24 to 36 inches.
- Cold climates (Upper Midwest, Northeast, Mountain West): 42 to 48 inches or more.
Treat these as a sanity check, not a spec. Two houses a few miles apart can have different code-required depths because of soil type, elevation, and local frost history. Before you dig, call or look up your local building department and use their published frost depth. Many footing jobs also require a permit and an inspection of the open hole before you pour — skipping that can mean tearing out finished work.
The frost line isn't a national number. It's a local one, written into your code for a reason.
Footing basics beyond depth
Depth gets the most attention, but a sound footing needs a few more things right:
- Width. The footing must be wider than the post or wall it carries so the load spreads onto enough soil. A common minimum is twice the width of the supported element, but your code or plan specifies it.
- Bearing on undisturbed soil. Pour onto firm, undisturbed ground — not loose backfill, which will settle.
- Below-grade protection. In freezing climates, keep the footing draining well so water doesn't collect and freeze around it.
For a slab-on-grade structure rather than a post-and-footing build, the slab itself follows different rules — see how thick should a concrete slab be for thickness and base requirements. Footings and slabs solve different problems: footings beat frost, slabs spread load.
Estimating the concrete for footings
Footings are usually small individual pours, but they add up across a deck or fence line. To size them, calculate the volume of each footing (a cylinder for a tube form, a box for a continuous footing) and multiply by the number of holes. The concrete calculator handles this — feed it your diameter or dimensions and depth.
Because each footing is modest in volume, bags almost always make sense here rather than ordering a truck. As a reference, an 80-lb bag yields 0.60 cubic feet of mixed concrete, so a 12-inch-diameter footing poured 48 inches deep — about 3.1 cubic feet — needs roughly 5 to 6 bags each. Mix small, consistent batches in a concrete mixing tub and pour each hole in one go.
Once poured, give footings time before you load them with framing — the same curing rules apply, which you'll find in concrete curing time.
The practical wrap-up
The whole job comes down to one number: dig the bottom of every footing below your local frost line, confirmed with your building department, not guessed. Pour onto undisturbed soil, make the footing wide enough for the load, and respect the curing window before you build on top. Estimate your bags in the concrete calculator and you can usually pour each footing in one go. Measure twice. Buy once.