calculator.nu is a library of single-purpose calculators — several hundred of them, covering finance, business, health, tax, science, work and a handful of smaller topics — built on one idea: a calculation that used to need a spreadsheet, a textbook formula and a working memory of which figure goes where should take one page, one set of clearly labelled inputs, and an answer you can trust enough to act on. No page on this site tries to be a full financial adviser, a personal trainer or an accountant. Each one does a single job, does it with a documented formula, and gets out of the way.
What a calculator here actually is
Every calculator is built the same way: a small number of inputs with sensible defaults, an output computed from a formula stated in plain view rather than hidden inside a black box, and a short explanation of what the result means and where it commonly goes wrong. The FIRE number calculator uses the same 25x rule that comes out of the 1998 Trinity Study; the business ratio calculators use the standard accounting definitions taught in any finance course, not a proprietary variant invented for this site. If a formula is disputed or has more than one common convention — EBIT versus EBITDA for interest cover, for instance, or whether debt-to-equity should include all liabilities or only interest-bearing ones — the page says so rather than picking one silently and leaving you to find out later that a different convention gave someone else a different answer.
Why the numbers can be trusted, and where they can't
A calculator can only be as reliable as the arithmetic and the formula behind it, and both of those are checked: every formula is drawn from a published, citable source — a textbook definition, a regulator's own methodology, a well-established convention like the Trinity Study or the CAGR formula — rather than approximated. What no calculator here can do is know your specific circumstances. Tax pages use published rates and bands for the year shown; they do not know about a reliefs claim that changes your particular bill. Insurance and FIRE calculators use standard formulas; they do not know your health, your risk tolerance or your family's actual spending. Treat every result as a correctly computed answer to the question you asked, not as a substitute for advice from someone who knows your full situation.
No accounts, no email gate, no walled-off results
Every calculator on this site works without creating an account, entering an email address or waiting for a result to unlock. Inputs are processed in your browser and are not stored or sent anywhere to generate the answer — there is nothing to log in to see a number you could have worked out yourself with enough time and the right formula. That is a deliberate choice rather than an accident of how the site happens to be built: a calculator that requires a signup before it will do arithmetic has added a barrier the arithmetic never needed.
How the site is organised
Calculators are grouped into sections — finance, business, health, tax, science and so on — and each section has its own landing page like this one, explaining what belongs there and how the tools inside it relate to each other rather than just listing links. New calculators are added where a genuine calculation is missing, not to pad out a section for its own sake, which is why some sections are large and thorough and others are small and will grow as more calculators are built and checked. If a specific calculation you need is not here yet, that is a gap to fill rather than a signal it was judged unimportant.