What we do before a calculator is allowed to go live, and what we refuse to do.
Every calculator states the equation it uses, defines each variable, and names the assumptions baked into it. If a tool models a fixed rate, it says so and does not silently extend to adjustable-rate products. If it models monthly interest accrual, the method section says monthly.
The purpose of publishing the formula is falsifiability. A reader who disagrees with a result can find the step where the disagreement lives, instead of having to trust a black box.
Each calculator carries at least three worked examples with a specific expected output, and the tests are run before publication. The expected values are generated by an independent implementation of the same mathematics, written separately from the code that runs in your browser. Two independent implementations agreeing on the same input is a real check; a single implementation agreeing with itself is not.
When a test fails, the page does not publish. That is the whole point of having it.
Where a page depends on a published rate or limit, the figure is attributed to its publisher — Freddie Mac, the Federal Reserve, the FDIC, the IRS — with the date the figure was retrieved. Sources are restricted to the issuing institution's own publication. A number reported secondhand is not a source.
Where a page is pure arithmetic on figures you enter, there is no external number to source, and the page says that instead of padding the sources block.
Every calculator has a section describing what it does not include and where its output diverges from a real-world document. A mortgage payment excluding escrow is the clearest example: the number is correct for principal and interest and understates your actual monthly outflow. The page says this beside the result.
If a page is wrong, the fix is dated and described on the corrections page. Silent edits are not made to substantive figures.