Frost depth in Illinois
Illinois has two code depths: 30 inches in 42 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" / 48". 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 |
|---|---|---|---|
| Chicago & northeast | 42" | 48" | Chicago Building Code |
| Central & southern Illinois | 30" | 36" | 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 Illinois's number looks the way it does
Illinois runs on two depths: 42 inches in Chicago and the northeast, where the city writes its own building code, and 30 inches across central and southern Illinois where winters moderate.
From frost depth to dig depth in Illinois
Footings bear on undisturbed soil below the frost line, so the hole goes 6 inches deeper than the code number. In Illinois that means:
| Code frost depth | Dig depth | Shortest Sonotube |
|---|---|---|
| 30" | 36" | 4 ft tube |
| 42" | 48" | 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 30–42 inches below grade.
How much concrete a footing takes in Illinois
Bags of 80-lb mix per footing, for a solid pier with no post in the hole, at Illinois's code depth:
| Frost depth | 10" hole | 12" hole | 16" hole | 20" hole |
|---|---|---|---|---|
| 30" (36" dig) | 3 bags | 4 bags | 7 bags | 11 bags |
| 42" (48" dig) | 4 bags | 6 bags | 10 bags | 15 bags |
The bags calculator converts any total into 80, 60, 50 or 40-pound bags, and the concrete calculator handles slab and footing volumes.
Illinois frost depth FAQ
How deep is the frost line in Illinois?
Illinois has two code depths, and the county you build in decides which one an inspector measures: 42 inches in Chicago & northeast, and 30 inches in Central & southern Illinois. These are code values, not the average depth the ground actually froze last winter.
How deep do footings have to go in Illinois?
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 42 inches of frost means a 48-inch dig in Illinois. The extra 6 inches is the bearing layer that keeps the footing from sitting on frost-susceptible soil, and it is what Chicago Building Code expects to see on a permit drawing.
How do I know which Illinois zone my county is in?
The citation in the table above names the authority that drew the lines — Chicago Building Code; 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 Illinois
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Last updated: 2026-11-02