MO · set by local jurisdictions
How deep do deck footings and fence posts have to go in Missouri? Below the frost line. Here's the number, and the winter behind it.
The requirement
36″
Footing bottoms in Missouri must reach 36 inches below grade — the frost line. Across Missouri it runs roughly 12″–36″ by county.
Reference: State of Missouri Frost Depth Map, Missouri Public Service Commission, Manufactured Housing and Modular Units Program (2016) (2016 edition). Counties amend their codes — confirm the exact figure with your local building department before you pour.
Depth pre-filled to the Missouri frost line. The hole bottom has to reach it — a few inches deeper seats the base cleanly below.
| Bag size | Exact | Buy (with spare) |
|---|
The buy column adds a 10% spare margin and rounds up. Running out mid-pour is the one mistake you can't fix.
Missouri sits in a transition zone: northern counties near the Iowa border get sustained continental cold that pushes frost close to 3 feet down, while the Ozarks and the Bootheel see mostly brief freezes with shallow penetration. Repeated freeze-thaw cycling, rather than one deep hard freeze, drives the footing depths used across most of the state.
The depth tracks how much freezing a winter actually delivers. NOAA's Air Freezing Index keeps a running score of how far and how long air temperatures sit below freezing; deeper scores mean deeper frost, and building codes translate that climate record into the footing depth above. Water expands about 9% when it freezes — when that happens under a footing, the ground lifts it. That's frost heave, and it cracks foundations, tilts fence lines, and pops deck piers. Getting below the frost line puts your concrete where the ground never freezes.
Missouri has no mandatory statewide residential code, so each city or county sets its own frost depth: about 36 inches in Kansas City and northern Missouri, roughly 30 inches around St. Louis, about 24 inches near Springfield, and as little as 12 to 15 inches in the Bootheel. Even where frost is shallow, footings still need the code-minimum 12 inches below grade wherever the IRC is adopted, so always confirm with the local building department.
Treat this page as the starting number, not the permit. County and city amendments are where the real precision lives, and inspectors enforce the local figure — one phone call to the building department settles it.
36 inches is the working number (set by local jurisdictions). Across Missouri it runs roughly 12″–36″ by county. Confirm the exact figure with your local building department — county amendments are common.
Bury one third of the above-grade height, never less than 24″, and never shallower than the frost line — in Missouri that means at least 36″. The post depth calculator checks both rules for you.
Below the frost line — 36″ minimum, bearing on undisturbed soil beneath it. Size the diameter with the deck footing calculator, then figure bags with the depth pre-filled above.
Yes — 811 covers every state, and the locate is free. Call a few business days before any post hole or footing dig and crews mark the buried utilities.