Frost line · footings · concrete math
The ground under your project freezes to a depth your building code has already decided. Everything below grade starts there.
Client-side, instant, nothing tracked. Look up your frost line first and the depth fields fill themselves.
How many bags per post hole — 50, 60, 80 lb, spare included.
Tributary area and soil bearing → the pier diameter you need.
The ⅓ rule vs. your frost line — whichever is deeper wins.
Tube diameter × height → concrete volume and bags.
Length × width × thickness → yards, bags, rebar, mesh.
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 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.
Anywhere from nothing (Florida) to five feet (northern Minnesota). 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 chart.
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.
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.
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.