Sources · method · limits
One subject, taken seriously: the depth where your project meets the winter.
Frost depths are compiled from state-adopted building codes and state agency publications — 16 states publish a statewide figure; the rest set it by county or city amendment, and each state page says which. Every entry names its code and section. Code text itself is ICC copyright and is never reproduced here; the requirement is stated in plain words with the citation alongside. Last checked against current state adoptions: August 2026.
The climate story — why Minnesota digs five feet while Georgia digs one — follows NOAA's Air Freezing Index, the public-domain dataset that tallies how hard and how long each winter freezes. Codes turn that record into the depths on these pages.
Calculator constants — bag yields, presumptive soil bearing values, the ⅓ rule, sonotube sizes — come from manufacturer data sheets and the residential code's foundations chapter, kept in one data file with their sources noted.
Arithmetic, in your browser, instantly. Nothing is sent anywhere, nothing is tracked in the tools, and the formulas are the same ones on the back of the bag — stated plainly on each page so you can check the work.
A permit, a soils report, or an engineer. State figures are the floor; counties amend them and your inspector enforces the local number. Two calls beat any website: your building department before you plan, and 811 before you dig.