How we estimate frost depth
1. The freezing index
The air freezing index (AFI) adds up how far, and for how long, daily average air temperature stays below 32°F over a winter. A winter that averages 10°F below freezing for 30 days scores 300 °F-days. NOAA's National Climatic Data Center computed AFI return periods for 3,110 US weather stations from 1951–80 records. We use the 100-year return period: a winter that cold happens about once a century. The same dataset is behind the air-freezing-index map in the International Residential Code, which is used to design frost-protected shallow foundations.
2. From station to ZIP code
For each ZIP code we take the three nearest stations and average their AFI, weighted by the inverse square of distance. County figures average the county's ZIP codes. Stations sit at specific elevations, so mountain ZIP codes far from a station can be colder or milder than shown.
3. From freezing index to inches
We use the modified Stefan equation from frost engineering:
depth = λ · √(48 · k · n · AFI ÷ L)
- k: thermal conductivity of frozen soil (BTU/ft·hr·°F)
- L: latent heat of the soil water, 144 × moisture content × dry density (BTU/ft³)
- n: surface factor, 0.7 for lawn with light snow cover
- λ: correction for the heat held in the soil (Berggren coefficient)
| Soil | k | Moisture | Dry density | λ |
|---|---|---|---|---|
| Sand / gravel | 1.3 | 10% | 120 pcf | 0.65 |
| Silt / loam | 0.9 | 20% | 100 pcf | 0.80 |
| Clay | 0.8 | 30% | 90 pcf | 0.85 |
4. Typical code footing depth
Building codes do not use our estimate. Each jurisdiction adopts a fixed frost-line depth, usually rounded up for safety. To show what codes in a similar climate commonly require, we fit a curve (depth ≈ 1.35 × AFI0.465) to the adopted frost depths of about 20 US cities and show a range of ±20%, rounded to 6 inches, with a 12-inch floor. Local requirements can fall outside this range.
Limitations
- Snow cover, pavement, shade, slope and soil moisture change real frost depth.
- Climate data is from 1951–80; recent winters have been milder in many areas, so these estimates lean conservative.
- These numbers are for planning. Your building department sets the legal requirement.
Sources
- NOAA National Climatic Data Center, Air Freezing Index – USA Method, Return Periods and Associated Probabilities (1951–80).
- US Army Corps of Engineers, TM 5-852-6, Calculation Methods for Determination of Depths of Freeze and Thaw in Soils.
- International Code Council, International Residential Code, sections R301.2, R403.1.4 and R403.3.
- ZIP code locations: zipcodes open dataset (MIT license).