4Calculators exists because most calculators on the internet answer the wrong question. They give you a number and nothing else — no formula, no units, no sense of whether the number is reasonable, and no explanation of the assumptions baked into it. If you are sizing a circuit, pricing a loan, charging a refrigerant system, or checking a lab dilution, a bare number is not enough. You need to know why.
So every calculator here does four things: it computes the answer, it shows the formula in real mathematical notation with every variable defined, it walks a worked example you can reproduce by hand, and it explains what a good result looks like and where the method breaks down.
What is on the site
The library currently holds 1292 calculators in 41 subject areas — personal and corporate finance, mathematics from arithmetic through calculus, physics, chemistry and the life sciences, the licensed trades (electrical, HVAC/R, plumbing, construction, machining), transport, agriculture, health and fitness, computing, and the everyday arithmetic of ordinary life. Browse the complete A–Z index or start from a subject area.
Our principles
- Show the work. A result without a method is a guess you cannot check.
- Name the source. Where a calculation is governed by a code or standard — the National Electrical Code, ASHRAE, ACI, Manning's equation, an IRS publication — we say which, and which edition assumption we made.
- Test against known answers. Every calculator ships with test cases whose expected values come from published worked examples, code tables, or textbook problems. See how we calculate.
- Say what we do not model. Every page states its assumptions and limits explicitly.
- Free, no account, no paywall. The site is funded by advertising. We never sell or share what you type into a calculator — it never leaves your browser.
Who makes this
4Calculators is built and published by Michael A. Levy, working on his own. There is no editorial staff, no newsroom and no committee — when this site says “we”, it means one person and the tools he works with. He has worked across the fields the library covers — finance and accounting, engineering and the trades, science, and computing — which is why those are the subjects here.
The calculators are produced with AI. Specifically, they are written using Claude Opus 5 (Anthropic’s model, driven through Claude Code), directed page by page: which formula governs, which standard or text it comes from, what the inputs and units are, what the test cases must prove, and what the article has to explain. The AI drafts against that brief; the build then verifies it mechanically. How we calculate sets out the whole pipeline, and the editorial policy states exactly what is and is not checked by a human.
Being straight about the part that matters: he has personally reviewed many of these calculators, but not all 1,292 of them. Nobody could read that many line by line and keep the library current. Where a human has not gone through a page in detail, it has still been through a separate AI review pass and the automated verification — structural validation plus every one of that calculator's test vectors executed in a real browser — which either passes or the page does not ship. That is a real standard, and it is not the same thing as a licensed professional signing off on your particular calculation — which is why every page tells you to have consequential numbers verified by one.
Corrections are genuinely welcome, and on a library this size they are how the last errors get found. If you find a result that disagrees with your code book, a formula that is wrong, or an assumption that should have been stated, please say so — corrections are made promptly and the page's updated date changes with them.