How the footing calculator works
The cylinder math
A hole is a cylinder: π × r² × depth. A 12" hole 4 feet deep holds 3.14 cu ft — about 5.3 bags. The flared-footing option (the bell at the bottom) spreads the load and is required by many codes in poor soils; it adds a shallow pad's worth of concrete per hole, which the calculator prices in.
Frost depth is the law of the land
Frost lines run 12 inches (deep South) to 48+ inches (upper Midwest and mountain West). Footings above the line freeze, expand, and heave — lifting decks, pergolas, and fences a visible amount every few winters. The county building department publishes your line; dig past it and the heave question disappears forever.
Hole logistics
Drill and pour the same day: open holes collect water (which ruins the mix ratio) and crumble walls (which adds surprise volume). A two-person auger rents cheaply; the post-hole digger works for six holes and sells itself short on twelve. Post bases sit on the cured footing — never wood directly in concrete.