Frost depth in Idaho
Idaho has two code depths: 30 inches in 48 inches — and which one applies depends on the county you build in. Add 6 inches of bearing and that becomes a dig depth of 36" / 54". The numbers below are the code values an inspector measures, not climate averages — the local building department always has the final word.
| Area / zone | Frost depth | Dig depth | Set by |
|---|---|---|---|
| Boise, Treasure Valley & south | 30" | 36" | Idaho Residential Code Table R301.2(1) |
| Northern & mountain counties | 48" | 54" | Local Table R301.2(1) |
Two rows, two numbers: the county you build in decides which one governs your permit. Never take the shallower value to a counter in a deeper zone.
Why Idaho's number looks the way it does
The Rockies split Idaho in two: the Boise and Treasure Valley side of the state digs 30 inches, while the northern and mountain counties — farther north and higher — dig 48.
From frost depth to dig depth in Idaho
Footings bear on undisturbed soil below the frost line, so the hole goes 6 inches deeper than the code number. In Idaho that means:
| Code frost depth | Dig depth | Shortest Sonotube |
|---|---|---|
| 30" | 36" | 4 ft tube |
| 48" | 54" | 6 ft tube |
A 4-inch gravel base at the bottom of the hole is common practice for drainage, and if you use one, add it to the tube length rather than the dig depth — the footing still has to reach the full 30–48 inches below grade.
How much concrete a footing takes in Idaho
Bags of 80-lb mix per footing, for a solid pier with no post in the hole, at Idaho's code depth:
| Frost depth | 10" hole | 12" hole | 16" hole | 20" hole |
|---|---|---|---|---|
| 30" (36" dig) | 3 bags | 4 bags | 7 bags | 11 bags |
| 48" (54" dig) | 5 bags | 6 bags | 11 bags | 17 bags |
The bags calculator converts any total into 80, 60, 50 or 40-pound bags, and the concrete calculator handles slab and footing volumes.
Idaho frost depth FAQ
How deep is the frost line in Idaho?
Idaho has two code depths, and the county you build in decides which one an inspector measures: 30 inches in Boise, Treasure Valley & south, and 48 inches in Northern & mountain counties. These are code values, not the average depth the ground actually froze last winter.
How deep do footings have to go in Idaho?
Bearing sits below the frost depth, so dig to the code depth plus 6 inches: 30 inches of frost means a 36-inch dig, and 48 inches of frost means a 54-inch dig in Idaho. The extra 6 inches is the bearing layer that keeps the footing from sitting on frost-susceptible soil, and it is what Idaho Residential Code Table R301.2(1) expects to see on a permit drawing.
How do I know which Idaho zone my county is in?
The citation in the table above names the authority that drew the lines — Idaho Residential Code Table R301.2(1); Local Table R301.2(1). Your county or city publishes its own amendment to IRC Table R301.2(1) (or its own zone map), and that local document, not a state table, is what a permit reviewer checks. One call to the building department settles it before you rent an auger.
Plan a pour in Idaho
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Last updated: 2026-11-02