This page describes exactly how the numbers on this site are produced, so you can judge whether to trust them. If you are checking our work, this is the page to read first.
Start with the part most sites leave out: 4Calculators is built by one person, Michael A. Levy, and the calculators are written with AI (Claude Opus 5) against a specification he writes for each page. Many pages have been personally reviewed; with 1,292 of them, not all have. What backs every page regardless is the verification below — the formula is stated, the source is named, and the arithmetic is tested against independently derived answers before it ships. The editorial policy gives the full production and review pipeline.
Where the formulas come from
Every calculator starts from a published source: a governing code or standard, a peer-reviewed method, a government publication, or a canonical engineering or finance textbook. The source is listed under References at the foot of each calculator page. Where a field has a governing document — NFPA 70 (the National Electrical Code) for electrical load calculations, ASHRAE handbooks and standards for HVAC, ACI 318 for concrete, the IRC for residential framing spans, Manning's equation for open-channel flow — we follow it and name it.
Where a quantity genuinely varies by jurisdiction, manufacturer, or year, we do not invent a number. We tell you which value the calculator assumes, and where to find the one that applies to you.
How each calculator is tested
Every calculator carries a set of machine-readable test cases. Each test fixes a set of inputs and an expected output, and the expected value is derived independently of our code — from a worked example in a standard, a table in a code book, a textbook problem with a published answer, or hand calculation. The whole suite is executed in a real browser before a build is published; a calculator whose tests fail does not ship.
Tests always include at least one edge case: a zero rate, a boundary condition, a degenerate geometry, or the point at which a formula stops being valid.
Rounding, precision and units
Calculations run in IEEE 754 double precision and are rounded only for display. That means intermediate steps are never truncated, so results do not drift the way they can when you round by hand at each stage. Displayed precision is chosen to suit the quantity: currency to the cent, engineering results to a sensible number of significant figures, and very large or very small numbers in scientific notation.
Where a calculator accepts both imperial and metric input, the unit selector converts your entry to the calculator's base unit before any arithmetic happens; conversion factors are exact where an exact definition exists (for example 1 in = 25.4 mm exactly).
Rounding up to a standard size — the next larger conductor, the next joist size, the next whole bag of concrete — is applied where practice requires it, and the page says so.
What we do not do
- We do not give personalised financial, legal, medical, or engineering advice. Our calculators produce estimates from the inputs you supply.
- We do not replace a licensed professional's stamp, a permit-office review, or a physician's judgement.
- We do not silently change a formula. When a method is revised, the page's updated date changes and the reason is noted.
- We do not track what you type. Calculations run entirely in your browser and inputs are never transmitted to us.
Reporting an error
If a result disagrees with your code book, your instrument, or your own hand calculation, we want to know. Send us the calculator name, your inputs, the result you expected and the source you are comparing against. Substantive corrections are made promptly and the page's updated date is changed.