MT · set by county and city amendment
How deep do deck footings and fence posts have to go in Montana? Below the frost line. Here's the number, and the winter behind it.
The requirement
48″
Footing bottoms in Montana must reach 48 inches below grade — the frost line. Across Montana it runs roughly 36″–60″ by county.
Reference: Montana Residential Building Code (2021 IRC adopted under ARM Title 24, ch. 301), Section R403.1.4 with ARM 24.301.154 minimum footing embedment; frost depth set locally in Table R301.2 (2021 edition). Counties amend their codes — confirm the exact figure with your local building department before you pour.
Depth pre-filled to the Montana 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.
Montana winters bring long stretches of subzero air, so frost drives three to five feet into the ground. Eastern plains and valley floors freeze deep, and high mountain areas freeze deeper still.
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.
Frost depth is filled in locally: Billings, Missoula and Bozeman use 36 in, Great Falls and Kalispell 42 in, Helena 48 in, and mountain jurisdictions up to 60 in. A state amendment (ARM 24.301.154) sets an absolute floor of 24 in below finished grade, so always confirm the local number.
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.
48 inches is the working number (set by county and city amendment). Across Montana it runs roughly 36″–60″ 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 Montana that means at least 48″. The post depth calculator checks both rules for you.
Below the frost line — 48″ 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.