Frost depth in Missouri
Missouri has two code depths: 30 inches in 36 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" / 42". 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 |
|---|---|---|---|
| St. Louis & east | 30" | 36" | St. Louis amendments to IRC Table R301.2(1) |
| Kansas City & north | 36" | 42" | Kansas City Building and Rehabilitation Code |
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 Missouri's number looks the way it does
Missouri follows the Ozark line: 30 inches in St. Louis and the east, 36 in Kansas City and the north, where the state starts to feel like the plains.
From frost depth to dig depth in Missouri
Footings bear on undisturbed soil below the frost line, so the hole goes 6 inches deeper than the code number. In Missouri that means:
| Code frost depth | Dig depth | Shortest Sonotube |
|---|---|---|
| 30" | 36" | 4 ft tube |
| 36" | 42" | 4 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 30–36 inches below grade.
How much concrete a footing takes in Missouri
Bags of 80-lb mix per footing, for a solid pier with no post in the hole, at Missouri's code depth:
| Frost depth | 10" hole | 12" hole | 16" hole | 20" hole |
|---|---|---|---|---|
| 30" (36" dig) | 3 bags | 4 bags | 7 bags | 11 bags |
| 36" (42" dig) | 4 bags | 5 bags | 9 bags | 13 bags |
The bags calculator converts any total into 80, 60, 50 or 40-pound bags, and the concrete calculator handles slab and footing volumes.
Missouri frost depth FAQ
How deep is the frost line in Missouri?
Missouri has two code depths, and the county you build in decides which one an inspector measures: 30 inches in St. Louis & east, and 36 inches in Kansas City & north. These are code values, not the average depth the ground actually froze last winter.
How deep do footings have to go in Missouri?
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 36 inches of frost means a 42-inch dig in Missouri. The extra 6 inches is the bearing layer that keeps the footing from sitting on frost-susceptible soil, and it is what St. Louis amendments to IRC Table R301.2(1) expects to see on a permit drawing.
How do I know which Missouri zone my county is in?
The citation in the table above names the authority that drew the lines — St. Louis amendments to IRC Table R301.2(1); Kansas City Building and Rehabilitation Code. 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 Missouri
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Last updated: 2026-11-02