About factonisam
factonisam is a hundred free calculators, run by a small team who got tired of typing the same arithmetic into a phone. Here is who builds it, how it is paid for, and what we will not do with your numbers.
Why this site exists
This site started with a spreadsheet and an argument about a mortgage. One of us was trying to work out whether an extra £150 a month was worth more than a shorter term, and every calculator we opened wanted an email address before it would show a number. The ones that did not want an email wanted a phone call from a broker. The one that asked for neither showed a payment but not the amortisation behind it, so there was no way to check whether it was right.
That is a small annoyance, and small annoyances are exactly what free tools exist to remove. So we built the mortgage calculator we wanted: one that shows the payment, the escrow, the full schedule and the formula, without asking who we were. Then we built the affordability one, because that is the question that comes first. Then ninety-eight more, because once you start noticing missing calculators you cannot stop.
How a calculator gets built
Each calculator goes through the same four stages before it is published.
- Source the formula. We start from the published definition rather than from another calculator. BMI comes from the WHO's own cut-offs, Mifflin-St Jeor from the 1990 paper, amortisation from the standard annuity formula. Where two credible versions exist, we pick one, say so on the page, and explain the difference.
- Implement and test it. The formula is written as a small JavaScript function and tested against a normal case, a boundary case and a deliberately invalid one. No result may ever reach the screen as
NaN,Infinityor a blank. - Check it by hand. Someone works at least one example on paper and the arithmetic goes onto the page, step by step. If the hand calculation and the tool disagree, we do not publish until we know which one is wrong and why.
- Write the page around it. What the tool is for, how to read the result, where the method breaks down, and what to use instead when it does. The limitations section is the part we spend longest on, because a number without its caveats is worse than no number.
Corrections are treated as urgent. If a formula is wrong, the fix ships the same day we can confirm it and the worked example is recalculated in the same pass so the page never contradicts itself. Our editorial policy sets out how that process runs, and how we calculate covers the rounding and assumption rules every tool follows.
How the site is paid for
factonisam is free and always will be. It is paid for by advertising, which is why you will see clearly labelled ad slots on the pages — never inside a calculator, never above a headline, and never disguised as a result. No advertiser has any influence on what a calculator returns, and there is no affiliate arrangement behind any of the formulas.
We are not a lead-generation business. There is no account system, no email capture gate, and no broker on the other end of a form. If a calculator suggests you talk to a professional, that is because the maths has run out of usefulness, not because someone paid for the referral.
What we do with your numbers: nothing
Everything you type into a calculator on this site is processed by JavaScript running in your own browser. Your salary, your weight, your loan balance and your cost of goods never travel across the network, because there is no server-side application here to receive them. The site is a set of static files; the only thing our host ever sees is a request for a page.
Three small preferences are stored on your device, by your browser: light or dark mode, the last few tools you opened, and the ones you have starred. They live in localStorage, they never leave your machine, and clearing your browser data removes them. The privacy policy spells out the full picture, including what Google AdSense does and does not do, and the cookie policy lists every stored item by name.
Talk to us
We read everything sent to info@factonisam.com and reply to most of it within two business days. The most useful messages we get are corrections — a formula that disagrees with a textbook, a fee schedule that has changed, a worked example with a typo in the third decimal place. The second most useful are requests for calculators that do not exist yet; several of the hundred started as one-line emails. The contact page has the details.