Frost depth and footing depth guide
Frost Line Depth Calculator
Answer first: frost line depth (frost depth) is the minimum depth a footing must be placed below grade to avoid frost heave. It ranges from 0 inches in Florida to 72 inches in Minnesota. Use this calculator to find your state's frost depth and calculate your required footing depth before pouring.
Fence posts below frost line (TikTok most-watched)
Post depth = frost line + 6 in minimum
The most common DIY mistake: setting fence posts too shallow. In Minnesota (72 in frost), posts need to be at 78 in = 6.5 ft deep. Trick: mark your post-hole digger with paint at the target depth before you start.
Deep frost states (MN, ND, WI)
48–72 inches below grade
The northern tier has the deepest frost lines. MN: 72 in, ND: 72 in, WI: 60 in, MI: 42 in. A 72-in frost depth means footing bottom must be at 6 ft below finished grade.
Moderate frost states (IL, PA, CO, OH)
36–48 inches below grade
The most common range in the US. Illinois: 48 in, Pennsylvania: 48 in, Colorado: 36–48 in. 42 inches is the most-used depth for Midwest and Mid-Atlantic.
Mild and no-frost states (VA, TN, FL)
0–24 inches below grade
Virginia: 24 in. Tennessee: 12 in. Florida: 0 in (no frost). Even with zero frost depth, local codes usually require 12–18 in for bearing capacity. Always check your AHJ.
Interactive workflow
Run the numbers, then save or export the project
Related concrete planning tools
Move from a single calculation into a purchase, proposal, or pour-day decision with connected calculators and checklists.
What is the frost line depth in my state?
Frost line depths by state: Minnesota/North Dakota: 72 in. Wisconsin/Michigan: 60 in. Illinois/Iowa/Pennsylvania/Massachusetts/Connecticut: 48 in. Colorado/New York/Ohio: 36–48 in. Virginia: 24 in. Tennessee/Arkansas: 12–18 in. Texas/Georgia: 0–12 in. Florida/Hawaii: 0 in. For exact local requirements, check your county or municipality — local codes may exceed the state minimum.
How deep should fence posts be below the frost line?
Fence posts should extend at least 6 inches below the local frost line. If your frost line is 42 inches, posts should go to 48 inches (4 ft) deep. Contractor trick: mark your post-hole digger with spray paint at your target depth so you don't have to stop and measure every hole. Posts above the frost line will heave in winter and settle in spring, loosening over time.
How far below the frost line should concrete footings be?
The bottom of your footing must be at or below the frost depth — not just 1–2 inches below. The IRC requires footing bottoms to be below the frost line. Additional depth (6–12 in beyond the frost line) is common for a safety margin. Frost depth is measured from the LOWEST exposed grade, not from the top of a raised area. A retaining wall must have footings below frost from the LOW side.
What is frost heave and why does it damage foundations?
Frost heave is the upward movement of soil caused by freezing water expanding. When water in soil freezes, it expands 9% by volume and can generate thousands of pounds of upward force per square foot. Footings above the frost line get pushed up in winter and may not fully settle back in spring — the soil under the footing can fill in, permanently lifting the structure. Over years this causes cracking, tilting, and structural damage.
How deep should deck footings be?
Deck footings must extend below the local frost line. For a 48-inch frost line (common in PA, IL), deck post footings should be 50–54 inches deep. The IRC requires deck footings to extend to undisturbed soil below the frost line. Tube form footings (Sonotubes) are most common: 10–12 in diameter for a 6×6 post, 12–16 in for heavier loads.
Does soil type affect frost line depth?
Yes — clay soils retain more water and are more prone to frost heave than well-draining sandy or gravelly soils. Frost penetrates deeper in wet clay than dry sand. For this reason, some engineers specify excavating to the frost line depth and replacing susceptible soil with gravel that drains well and is not frost-heave-prone. This "over-excavate and replace" approach is common on sloped sites and in cold climates.
What happens if I pour a footing above the frost line?
Footings above the frost line are vulnerable to frost heave. The soil freezes, expands, and lifts the footing upward. When it thaws, soil can fill in underneath — so the footing never fully settles back. Over multiple freeze-thaw cycles, structures shift, walls crack, and structures can become unsafe. Building departments will not pass inspections for footings above the frost line.
How do I find the frost line depth for my specific location?
Three ways: (1) Check your local building department or county permit office — they have adopted frost depth in their code. (2) Use ASHRAE 99% design freezing temperatures — most frost line maps are based on this. (3) Ask your ready-mix concrete supplier — they deal with frost all the time and know local requirements. The frost line in our calculator is from IRC Table R301.2(1) and state/local amendments.
Do I need to go below the frost line for a shed foundation?
It depends on the shed type and local permits. A permanent shed on concrete footings: yes, go below frost line. A floating slab or gravel pad: some codes allow this for small sheds (under 200 sq ft) since the whole structure can move with frost without damage. A shed on deck-style pier blocks: pier blocks need to go below frost line or the floor will rack and doors won't open. Always check local permit requirements.
What is the minimum footing depth required by code?
The IRC sets two requirements that both must be met: (1) footing bottom must be below the local frost line, AND (2) footing must extend at least 12 inches into undisturbed soil. So in a frost-free area, minimum footing depth is still 12 inches. In a 42-inch frost area, your footing bottom is at 42+ inches, plus the footing itself is typically 8–12 inches thick.
How does the frost line depth vary within a state?
Frost line depth varies within states due to elevation, climate, and local code amendments. Colorado ranges from 36 inches in Denver to 48+ inches in mountain towns. Texas ranges from 0 in South Texas to 12–18 in in the Panhandle. Local jurisdictions can adopt frost depths higher than the state minimum. Always confirm with your local building department — the county or city code, not just the state code.
How much concrete do I need for footings below the frost line?
For tube form footings: Volume = π × r² × depth. A 12-inch diameter tube at 48 inches deep = 3.14 × (0.5 ft)² × 4 ft = 3.14 ft³ per hole. That is about 5 bags of 60 lb concrete or 4 bags of 80 lb. For 8 holes on a deck: 25 ft³ = 0.9 yd³. Use our concrete post hole calculator to get exact bag counts for your footing dimensions.
Planning disclaimer
Frost line depths are general guidelines based on ASHRAE 99% design temperatures. Always verify with your local building department before construction. Actual required footing depth may exceed frost line due to soil conditions, load requirements, and local code amendments.