How the numbers on this site are derived, where they come from, how they are verified, and what happens when one turns out to be wrong.
Length and angle calculators are built from the underlying geometry — run, rise and the Pythagorean theorem, with adjustments (ridge deduction, overhang tails, birdsmouth placement) modelled the way the cuts are actually made. No formula comes from another calculator site; if a relationship cannot be derived from first principles, it does not ship.
Span and size data follow a stricter rule: they are transcribed from the primary source — the published text of IRC 2021 Tables R802.4.1(3) and (4) — and are never interpolated, extrapolated or silently corrected. Where the code offers more conditions than the site covers (other snow loads, other grades), the pages say so and point to the full tables rather than guessing.
Sources are ranked, and the ranking is enforced. Primary and official sources outrank trade publications; trade publications outrank other calculator sites, which are never used as a source for a number.
Before a calculator ships, its outputs are checked against worked examples computed independently — by hand or by an independent implementation — including the examples printed on the page itself, so a reader repeating them gets the same figures. Fraction handling is tested at the awkward values (the x.5 sixteenths, the carry into the next inch and foot) where rounding bugs live. Span and size tables are proofread against the source document in full, cell by cell, not sampled.
Verification is repeated when a page changes materially, and the 'last verified' date on the page moves only when that check has actually been rerun.
Errors are reported through the report-an-error form or by email, and every report is worked by hand. Confirmed errors are fixed promptly, and where an error was material — a wrong length, a wrong span, a wrong angle — the fix and the date are noted. We do not silently rewrite published worked examples; if an example's figures change, the page shows the corrected working.
If a report turns out to be a misunderstanding rather than an error — most often a span entered as a run — the reporter gets the working that explains the result, and where the same misunderstanding keeps recurring, the page is rewritten to head it off.
No manufacturer, retailer or advertiser influences any result on this site. Calculators return the number the formula or the table gives, whether or not it is the number that sells lumber. If advertising is carried in future, it will be labelled as advertising and kept separate from the tools and tables.