Frost Depth by State

Frost depth by state

These are the depths that state codes and the larger city codes actually require, in inches below finished grade. Where a state leaves the number to local government the table lists the value used by its largest city that publishes one, and the basis column says so.

Where one state has two answers

Treating a state as one climate is the most common mistake in reading a frost depth chart. 22 states publish two different depths, and in most of them the gap is a foot or more of digging:

Minnesota is the clearest case because it is written into the rule rather than left to interpretation. Minnesota Rules 1303.1600 sets five feet in Zone I and three and a half feet in Zone II, and then lists every county by name in each zone. Hennepin, Ramsey and every other Twin Cities county is Zone II at 42 inches. St. Louis County, which is Duluth, is Zone I at 60 inches. That is a difference of 18 inches of digging for a state you could drive across in four hours.

The pattern repeats wherever elevation or a mountain range interrupts the climate. Western Washington is 12 inches because the Pacific keeps the ground above freezing most winters; drive four hours east over Snoqualmie Pass and it doubles. The Cascades are the dividing line, not the state border.

Why the code number is not the measured number

If you look up frost depth somewhere else and get a different answer, both numbers can be right. They answer different questions.

Measured frost depth is what the ground did. It depends on soil type, moisture content, whether there was snow on it in January, whether it was under a driveway or under grass, and how bad that particular winter was. Sandy, well-drained soil can freeze deeper than wet clay, because dry soil has less latent heat to give up. A foot of snow before a cold snap insulates the ground and cuts penetration by half or more. Two sites a mile apart with different cover can differ by a foot.

Code frost depth is what a footing has to be built to. It is a conservative envelope chosen so that a footing does not move in a bad winter, with no credit taken for snow cover or drainage. That is why the code value is usually deeper than the average measured penetration, and why Minnesota's code says 60 inches in the north while a measured average might read closer to 50.

The practical consequence: use the code number for anything that will be inspected, and use the measured number to understand why your neighbor's deck heaved even though he dug to what he thought was the local depth.

From frost depth to dig depth

The frost depth is the depth the ground freezes. A footing needs to sit at or below it, and it also needs soil under it that has not been disturbed and can carry load. The usual rule is to add 6 inches of bearing below the frost line, so:

dig depth = frost depth + 6 inches

In Minneapolis Zone II that is 42 + 6 = 48 inches. In Boston it is 48 + 6 = 54. In Miami there is no frost, so the 12-inch minimum foundation depth governs and bearing is the only real question. Some jurisdictions write the plus-six into the code as the footing depth directly, which is why you sometimes see 48 inches for Minnesota instead of 42. Either way you end up at the same hole.

If you use a gravel base

A 4 to 6 inch gravel layer at the bottom of the hole is good practice in clay and in wet spots. It does not replace the bearing layer, so add its depth on top: 42 inches of frost, 6 inches of bearing, 4 inches of gravel is a 52-inch hole, not a 44-inch one. Digging the frost depth and filling the bottom with gravel is the same mistake as digging too shallow.

What that means in Sonotube lengths

Cardboard forms are sold in 4, 6, 8 and 12 foot lengths. At a 42-inch frost depth you are digging 48 inches and a 4 foot tube is exactly right, with nothing to spare. At 48 inches of frost you are digging 54 and you need the 6 foot tube, with 18 inches left over to cut off above grade. Going up a size is cheaper than discovering the tube is short with the hole already dug.

How much concrete a frost-depth footing takes

This is the conversion most people actually need: frost depth to hole depth to bags. Bags of 80 lb mix, solid footing with no post in the hole, no waste allowance.

Two things that table makes obvious. First, depth is expensive: going from a 12-inch frost depth to 60 inches takes a 12-inch hole from 2 bags to 7. Second, diameter is more expensive still: at 60 inches of frost, a 20-inch hole is 20 bags against 7 for a 12-inch hole. If you need more holding power in deep frost, a bell-shaped bottom costs less than a wider hole.

A 4x4 post standing in the hole displaces about 11% of the concrete in a 12-inch hole, which is roughly one bag on a 60-inch dig. The fence post calculator does that subtraction per post; this page ignores it so the numbers stay a pure depth-to-volume conversion.

Frost depth from the air-freezing index

If your state is not on the list, or you are on a county line, there is a federal method that skips the state table entirely. The air-freezing index is the cumulative number of degree-days the air spends below 32°F over a winter. HUD publishes a direct conversion from that index to a design frost depth for footing design, and it is the same climate measure the IRC uses to size frost-protected shallow foundations.

Read the index off IRC Figure R403.3(2), which maps it by county, then read the depth off the table. Northern Minnesota and northeastern North Dakota sit at 4,000, which is 62 inches. Georgia is listed at 1,500 or less across every county, which is 32 inches on paper — but Georgia's own residential code still only requires 12, because the index-based table is a national envelope and the state amendment is more specific.

Where the two disagree, the state or local amendment wins. Use the index when nothing more specific exists.

Frost-protected shallow foundations

There is a legal way to avoid all of this digging. IRC R403.3 lets a building keep a shallow footing if rigid insulation holds the frost out of the soil under it, instead of putting the footing under the frost. The insulation is vertical against the slab edge plus a horizontal wing that extends out from the building, with the R-values and wing dimensions set by the air-freezing index, straight off the table above.

The conditions are specific. The building has to be maintained at a monthly mean of at least 64°F, so an unheated garage, porch or three-season room does not qualify, and the foundation cannot be attached to a basement or crawl space that falls below that. Insulation below grade has to be rated for wet conditions, and its R-value is derated: extruded polystyrene counts as R-4.5 per inch vertically instead of its dry rating.

In Zone I Minnesota, where the alternative is a 60-inch frost wall, this is not a technicality. It is the reason a lot of new construction there uses a turned-down slab instead of a basement.

Frost depth FAQ

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Last updated: 2026-10-29