Frost line · footings · concrete math
Dig below the frost line.
Buy the right number of bags.
The ground under your project freezes to a depth your building code has already decided. Everything below grade starts there.
The tool rack
Instant, no page reloads. Look up your frost line first and the depth fields fill themselves.
Concrete bags
How many bags per post hole — 50, 60, 80 lb, spare included.
Deck footing size
Tributary area and soil bearing → the pier diameter you need.
Fence post depth
The ⅓ rule vs. your frost line — whichever is deeper wins.
Sonotube volume
Tube diameter × height → concrete volume and bags.
Slab volume
Length × width × thickness → yards, bags, rebar, mesh.
The reference shelf
The published figures the tools run on — depths, thresholds, and sizes, each one cited where it came from.
Frost line depth by state
The code-required depth for all 50 states, with the citation — or enter a ZIP code.
Frost depth map
Every state drawn to scale — and where this winter's reading lives.
Water line depth
How deep water service lines are buried, and which code says so.
Gas line depth
Two pipes, two rulebooks — and neither one measures from frost.
Gas pipe sizing chart
BTU capacity by pipe size and run length, with every condition printed.
Cold-weather pours
Can you pour in winter? Yes, with conditions. Here are the conditions.
Form tube sizes
Every stocked diameter, to scale, with concrete per foot of height.
What this site is
One layer of ground, taken seriously. Frost depth is a legal building requirement: deck footings, fence posts, and foundations must reach below it, or the freeze lifts them and the thaw drops them somewhere new. FootingDepth publishes the code-required frost line depth for every state, explains the winter behind the number, and then does the arithmetic every hole demands — volume, bags, footing size, post depth.
The numbers come from state-adopted building codes and state agency publications, cited on every page. Where counties amend the state figure — and they do — the page says so and points you to your local building department.
Straight answers
How deep is the frost line where I live?
Anywhere from nothing (Florida) to five feet in northern Minnesota — and deeper still in Alaska, where permafrost country builds on engineered foundations. Every state has its own page here with the code figure, the range across its counties, and the reason behind it — start at the 50-state frost line chart.
How many bags of concrete do I need per fence post?
For a typical 4×4 post in a 12″-wide hole 30″ deep: about 1.75 cubic feet — four 80 lb bags with a spare. Your hole is not typical; the bags calculator takes thirty seconds.
Why do footings have to reach below the frost line?
Water expands about 9% when it freezes. When the ground under a footing freezes, it lifts — frost heave — and a heaved footing cracks foundations, racks fences, and pops deck piers. Below the frost line, the ground never freezes and nothing moves.
Are these numbers a substitute for a permit?
No. State figures are the floor; counties amend them and inspectors enforce the local number. Call your building department — and always call 811 before digging.