WI · statewide code figure
How deep do deck footings and fence posts have to go in Wisconsin? Below the frost line. Here's the number, and the winter behind it.
The requirement
48″
Footing bottoms in Wisconsin must reach 48 inches below grade — the frost line. Across Wisconsin it runs roughly 48″–60″ by county.
Reference: Wisconsin Uniform Dwelling Code, Wis. Admin. Code SPS 321.16(1)(a) (2016 edition). Counties amend their codes — confirm the exact figure with your local building department before you pour.
Depth pre-filled to the Wisconsin 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.
Long continental winters keep air below freezing for weeks at a time, letting frost drive four feet or more into the soil. Penetration is deepest in the north, where exposed ground under thin snow cover freezes hardest.
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.
The statewide Uniform Dwelling Code requires footings to reach the frost penetration level or 48 in. below grade, whichever is deeper, so far-northern counties can need more than 48 in. Frost-protected shallow foundations per ASCE 32 and footings on bedrock are recognized exceptions.
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 (statewide code figure). Across Wisconsin it runs roughly 48″–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 Wisconsin 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.