Frost depth in Montana
Montana has two code depths: 42 inches in 60 inches — and which one applies depends on the county you build in. Add 6 inches of bearing and that becomes a dig depth of 48" / 66". 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 |
|---|---|---|---|
| Eastern plains (Billings) | 42" | 48" | Local Table R301.2(1) |
| Mountain valleys (Bozeman, Missoula) | 60" | 66" | 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 Montana's number looks the way it does
Montana's mountains set the split: 42 inches on the eastern plains around Billings, 60 in the mountain valleys — Bozeman and Missoula dig as deep as Minnesota's north.
From frost depth to dig depth in Montana
Footings bear on undisturbed soil below the frost line, so the hole goes 6 inches deeper than the code number. In Montana that means:
| Code frost depth | Dig depth | Shortest Sonotube |
|---|---|---|
| 42" | 48" | 4 ft tube |
| 60" | 66" | 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 42–60 inches below grade.
How much concrete a footing takes in Montana
Bags of 80-lb mix per footing, for a solid pier with no post in the hole, at Montana's code depth:
| Frost depth | 10" hole | 12" hole | 16" hole | 20" hole |
|---|---|---|---|---|
| 42" (48" dig) | 4 bags | 6 bags | 10 bags | 15 bags |
| 60" (66" dig) | 5 bags | 8 bags | 13 bags | 20 bags |
The bags calculator converts any total into 80, 60, 50 or 40-pound bags, and the concrete calculator handles slab and footing volumes.
Montana frost depth FAQ
How deep is the frost line in Montana?
Montana has two code depths, and the county you build in decides which one an inspector measures: 42 inches in Eastern plains (Billings), and 60 inches in Mountain valleys (Bozeman, Missoula). These are code values, not the average depth the ground actually froze last winter.
How deep do footings have to go in Montana?
Bearing sits below the frost depth, so dig to the code depth plus 6 inches: 42 inches of frost means a 48-inch dig, and 60 inches of frost means a 66-inch dig in Montana. The extra 6 inches is the bearing layer that keeps the footing from sitting on frost-susceptible soil, and it is what Local Table R301.2(1) expects to see on a permit drawing.
How do I know which Montana zone my county is in?
The citation in the table above names the authority that drew the lines — 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 Montana
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Last updated: 2026-11-02