Where the numbers come from
Every calculator on this site implements a published, named formula rather than something I invented. The methodology page writes each one out in full — the equation, where it comes from, and the assumptions baked into it. If a tool's result surprises you, that page will usually explain why.
It also states what each formula cannot tell you. BMI does not distinguish muscle from fat. A cycle prediction is a calendar estimate, not contraception. A due date is a 40-week convention, and most babies do not arrive on it. An amortisation schedule is arithmetic, not a mortgage offer. Those limits are printed on the tools themselves, not buried here.
Health tools are not medical advice
I am a developer, not a clinician. The BMI, period, due-date and sleep tools implement widely used public formulas so you can see the arithmetic for yourself. They are not a diagnosis, they do not replace a professional, and they are written to be honest about that rather than to sound authoritative.
Corrections
If something here is wrong, I would rather know. Use the contact form and pick "Correction to a formula or figure". Messages reach the person who builds the site — there is no support queue. A genuine error in a formula or a figure gets fixed and noted in the changelog; if I disagree, I will tell you why rather than quietly ignore it.
Advertising does not influence anything
Ads are the only revenue this site has. Nobody pays for placement, for a mention, or for a recommendation, and no advertiser sees anything before it is published. Crucially, advertising has no effect on what a calculator returns: the formulas are fixed, documented, and identical for every visitor. If that ever changes, it will be written here first.
How this is written and checked
Formulas are checked against their published source before a tool ships, and re-checked whenever the tool changes. The guides are worked examples with real arithmetic — where a figure appears, I have run it.
Your data stays in your browser
Every tool runs entirely in your browser. The letters you unscramble, the dates you track, the passwords you generate and the JSON you paste are never sent anywhere — you can confirm that in about ten seconds with your network tab open. What is collected, and by whom, is set out in the privacy policy.