Methodology and editorial standards
How the calculators and their explanations are made, how they are checked, and what that checking does and does not cover.
Who does the work
Smart Financial Calc is operated by Smart Financial Calc. Calculator code and explanations are drafted with AI assistance under the operator’s direction, and the operator decides what is published. Every formula is then checked against its cited sources and by automated tests, as described below.
Pages are not reviewed by licensed financial, tax, legal or medical professionals, and no page claims to be. Treat the explanations as careful educational material, not professional advice.
Sources
- Formulas and factual statements come from primary or authoritative sources: statutes and regulators (for example the CFPB, SEC, FDIC, IRS and Federal Reserve), standards bodies (NIST, BIPM, IANA), official health agencies (CDC, NIH, WHO), peer-reviewed papers for named equations, and standard textbooks.
- Each calculator lists its sources and links to the specific page used, not just a home page.
- Figures that change over time, such as interest rates, tax brackets, contribution limits and prices, are never built in as current facts. They are inputs you enter, and examples label them as examples.
How calculations are checked
- Independent expected values. Every calculator has automated unit tests. Their expected results are established separately from the calculator’s code: by hand derivation, from published worked examples, or with an independent computation such as a Python script. How each value was obtained is recorded next to the test. A test never gets its expected value by running the code it checks.
- Ordinary cases, boundaries and invalid input. Tests cover typical inputs, boundary values, invalid entries and the edge cases specific to each calculator, such as a payment too small to ever repay a loan, a leap-day birthday or an impossible triangle.
- Properties that must always hold. Where it adds assurance, randomized property tests check identities over many generated inputs, for example that an amortization schedule always ends at a zero balance.
- Worked examples match the tests. The worked example on each calculator page uses the same inputs and outputs, with the same rounding, as a unit test.
- Browser tests. Each calculator page is loaded in a real browser: the form is filled in, the results are checked, an invalid entry must remove the results and show a clear message, and the page must load without errors. Automated accessibility checks run on every type of page.
- A separate verification pass. Before a calculator is marked complete, its reference values and edge cases are re-checked in a separate review of the finished calculator, and the outcome is recorded.
What you see when an input doesn’t work
Results update as you type. The moment an input is invalid, the results are removed and the problem is explained next to the field, so an out-of-date result is never left on screen. When the numbers are valid but have no meaningful answer, such as a payment that never repays the balance, the calculator says so in words instead of showing an error code or an endless table.
Rounding and precision
- Money calculations use standard double-precision arithmetic and round only for display, to the cent. Because each figure is rounded separately, a displayed total can differ from the sum of displayed parts by a cent.
- Amortization schedules are the exception: each period’s interest is rounded to the cent and the final payment is adjusted, so the balance ends at exactly zero and the principal payments add up to the amount borrowed.
- Where you expect exact answers, arithmetic is exact: fractions and systems of linear equations use exact rational numbers, and the basic calculator uses decimal arithmetic, so 0.1 + 0.2 gives exactly 0.3. Permutation and combination counts are exact up to a digit limit; beyond it the calculator switches to an approximation and labels it as one.
- Rates that have to be solved for, such as internal rate of return, yield to maturity and APR, use bracketing methods with a stated tolerance and an iteration limit. If no answer is found, the calculator says so instead of guessing.
- Long calculations are bounded, for example to at most 1,200 monthly periods (100 years).
Dates on pages
“Last updated” is the date of the last substantive change to a page: a formula, a worked example, an explanation or a correction. Layout changes and typo fixes don’t change it, and dates are never moved forward without a real change.
Corrections
Reports of errors are checked by reproducing the problem and comparing the result with the cited sources. When an error is confirmed, the calculation or text is fixed, a test that would have caught it is added, the page’s “Last updated” date changes, and the correction is listed on the corrections page with what was wrong and what changed.
What the calculators are not
The calculators give educational estimates. They are not financial, investment, tax, legal or medical advice; not quotes, offers or lending decisions; not a way to prepare or file a tax return; and not a diagnosis. Health and fitness results are population-level estimates for adults. The disclaimer covers each area in more detail.