Frost Line Depth by Zip Code

Soil freezes from the surface down, and the frost line is the deepest it gets in a hard winter. The damage comes from water. As the freezing front moves down it pulls moisture toward it, which freezes into ice lenses and swells the soil upward — frost heave. Anything anchored in that layer rides up with it: a concrete post footing, a deck pier, a shed block. When the ground thaws the soil settles, but a footing rarely drops all the way back, so the movement adds up winter after winter.

A footing whose bottom sits below the frost line has nothing to push up against it. That is why building codes set a frost depth and why IRC R403.1.4 requires footings to bear below it and at least 12 in below grade regardless. It is also why the same 6 ft fence needs a 30 in hole in one state and a 60 in hole in another.

Code depths are a legal figure, usually one number for a whole state or city. The climate estimate is a physical one: how deep frost could reach in bare, snow-free ground in a typical winter and in a 1-in-100 winter. Comparing the deep end of each state’s code with its counties’ estimates:

• In 1,645 of 3,108 counties (53%), the adopted depth is within 12 in of, or deeper than, even the 100-year estimate.
• In 1,212 (39%), code covers a typical winter but not the 100-year winter on bare ground.
• In 251 (8%), even a typical winter is estimated to freeze bare ground 12 in or more past the state figure — most of them in North Dakota, Minnesota, Wisconsin, Iowa, South Dakota, Idaho and Colorado.

That is not a finding that codes are wrong. The bare-ground estimate is a deliberately severe case: snow, turf and moist soil all slow freezing, and codes are built on local experience of what actually heaved. A state summary also hides local variation — many colder counties and towns adopt deeper figures than the cities the state entry is drawn from. Where the gap is large, it is a reason to ask your building department, not to ignore it.

Well-known places

Amber marks a 100-year estimate 12 in or more deeper than code. The Upper Midwest, the Intermountain West and the northern Plains carry the biggest gaps; Idaho (44 of 44 counties), Minnesota (87 of 87 counties), North Dakota (53 of 53 counties), Wisconsin (69 of 72 counties), South Dakota (62 of 66 counties) and Iowa (92 of 99 counties) have the highest share of counties whose 100-year estimate runs past the state figure.

The opposite case is rarer and coastal. Only 8 counties, all in Massachusetts and New York, have an adopted depth 12 in or more deeper than the climate estimate — among them Nantucket (27 in), Dukes (28 in), Barnstable (30 in), Kings (28 in) and Plymouth (35 in). Ocean-moderated winters freeze less than inland ones, but a single statewide code figure follows the colder interior. Build to the code number there anyway: it is the one inspected.

The zip lookup gives these for your area. The rules behind them, and where the frost line stops being the right question:

For a hole depth, post length and bag count, the fence post calculator→ takes the depth straight from the lookup. The post depth guide→ covers concrete versus gravel and why posts rot at ground level.

The pipe that brings water to a house has its own frost rule, and it is written in plumbing codes rather than the footing rules above. All three model codes measure it from the same frost line your building department adopts:

• IRC P2603.5 (houses): water service pipe goes “not less than 12 inches (305 mm) deep and not less than 6 inches (152 mm) below the frost line.” 2024 IRC
• IPC 305.4: exterior water supply piping goes at least 6 in below the frost line and at least 12 in below grade. 2024 IPC
• UPC 609.1 (Uniform Plumbing Code, where a state or city adopts it): building supply yard piping goes at least 12 in below the average local frost depth, with at least 12 in of cover. 2024 UPC

So the zip lookup takes the deep end of your state’s adopted frost depth and adds 6 in, never going under 12 in, and shows the UPC figure beside it. A 42 in frost depth gives 48 in (54 in under the UPC); where no frost depth is adopted, 12 in. Where our county climate estimate for a typical winter runs past the code figure, the lookup says so — that is a reason to ask your utility or plumbing inspector what they expect.

Your side and the utility’s side. The IRC defines water service pipe as the outside pipe from the water main or other source of supply to the building, ending at the service valve. That covers the line from a city connection to your house and the line from a private well. The main, and usually the pipe up to the curb stop or meter, belongs to the water utility, which sets its own depth in its service standards — and your line has to meet theirs where they join. If you are on city water, call the utility before you plan the trench.

Local rules vary. Your local plumbing code and water utility decide the depth — this is a planning figure. Call 811 before you dig so buried utilities are marked.

Adopted code depth from our state table, with the range of 100-year climate estimates across each state’s counties and the county with the deepest one. Where the code column shows a range, counties or cities within the state adopt different figures.

Planning figures only. Frost depth is set by your local building department, and it is the legal number — confirm before you dig.

Everything above is about the worst a winter can do. For how deep the ground is frozen this week — useful if you are deciding whether you can dig at all — the National Weather Service’s North Central River Forecast Center publishes a current frost depth map from observer reports across the Upper Midwest and beyond.

Climate source. NOAA NCEI’s 1981–2010 supplemental normals publish the air-freezing index — the season’s accumulated degree-days below 32 °F — at return periods from 1.1 to 100 years for 5,600 stations in the lower 48 states. We use the 2-year value (a typical winter) and the 100-year value (the severity ASCE 32 designs to). All 5,600 stations carry NOAA completeness flags of complete, standard or representative, so none were excluded; 615 are marked “too warm to compute” because fewer than half their winters froze at all, and we count them as zero.

From stations to counties. 2,639 counties take the median of the NOAA stations inside their boundary. The other 469 have no station and take an inverse-distance weighted average of the 5 nearest; they are hatched on the map where the nearest is over 40 km away (12 counties). Alaska and Hawaii are not in NOAA’s dataset, so their 34 counties and boroughs show code depth only. Zip codes are matched to counties with the Census Bureau’s 2020 ZCTA-to-county file: each zip goes to the county covering most of its land, and any other covering at least 20% is shown too (4,236 of 33,791 zips).

From freezing index to depth. We use Brown’s (1964) empirical relation as NOAA’s own analysts applied it to these normals (Bilotta et al. 2015, Journal of Applied Meteorology and Climatology 54:69–76): depth in metres = 0.0174 × AFI^0.67, with the index in °C-days. It describes bare ground with no snow or vegetation, so it is an upper-end estimate for a typical yard. Physically based modelling for the Northeast (DeGaetano, Wilks & McKay 1996) maps bare soil, sod and snow-free soil separately for exactly this reason.

What moves the real number. Snow cover insulates the ground; turf and mulch do too. Wet, fine-grained soils freeze more slowly than dry sand and gravel but heave far more. Hilltops and valleys in the same county differ, and a county median can come from a valley town. And winters have warmed: NOAA found the 1981–2010 index typically 14–18% below 1951–80, and 2011 onward is not in these normals.

Code depths come from the state table this site maintains for the post calculator, with the codes and ordinances each is drawn from. They are planning summaries, not legal determinations: IRC Table R301.2(1) leaves the figure to each jurisdiction, and a state entry built from its largest cities can miss colder counties.

Local rules vary. These are estimates for planning, not code. Frost depth is set by your local building department — confirm it before you dig, and call 811 to have buried utilities marked.

The county table — 3,108 counties with the freezing index, typical and 100-year depth estimates, method, station count, confidence and the state code range — is free to reuse under CC BY 4.0: credit FenceLevel with a link to this page. The NOAA and Census inputs are public domain.

Download the CSV (247 KB)

Linking to one county? Add ?county= and its five-digit FIPS code, or ?zip= and a zip, to this page’s address and the lookup opens on it.