About this site

Who runs it

I publish this site under the name Stator. I write every calculator here, write the explanatory article that sits under it, and answer the email myself. There is no editorial team and no agency involved. If something on this site is wrong, it is mine to fix — and I would much rather hear about it than not.

I am not a financial adviser, a doctor or an accountant, and nothing here is presented as advice from one. What I can offer is the opposite of the usual calculator site: every formula is stated openly, every result is printed alongside the inputs that produced it, and every page tells you where the method came from and where it stops being reliable. If a number here matters to you, the page gives you enough to check it yourself or take it to a professional.

Why this site exists

Most online calculators hand back a single figure and no way to audit it. If the answer is wrong — a percentage read as a decimal, pounds silently treated as kilograms — nothing on the page tells you. Unit mistakes are by far the most common source of a wrong result, and a bare number hides them perfectly.

So every tool here prints a receipt: the answer, the figures that produced it, and a note naming the formula. If the receipt says something you did not intend, you have caught the mistake before it costs you anything.

How the calculators are tested

This is the part I take most seriously, and it is the main reason to trust a number from this site over one from a page that looks similar.

Every page runs against an automated test suite before anything is published. The suite loads all 98 calculators, confirms each one executes cleanly and produces a result, then checks roughly 160 specific answers against examples worked by hand — one inch is 2.54 centimetres exactly; 31 January plus one month is 28 February, not 3 March; a /24 subnet has 254 usable hosts; 100 square feet of 12-inch tile needs about 108 tiles once you allow for waste. A formula change that breaks any of those fails the build and does not ship.

Deliberate constraints follow from the same concern:

• Unit conversions use the exact 1959 international definitions — an inch is exactly 25.4 mm, a pound exactly 0.45359237 kg — so no rounding error enters at the definition itself.
• Imperial input is converted to metric first, so both unit modes return identical results rather than drifting apart.
• Date arithmetic counts whole months, then remaining days, clamping to the end of the month. Dividing by 365 accumulates error and is never used here.
• The random number generator uses crypto.getRandomValues with rejection sampling, not Math.random() % n, which quietly biases the low end of the range.
• The scientific calculator has a hand-written parser rather than calling eval, so no input you type is ever executed as code.

Where the formulas come from

The finance tools use standard amortisation mathematics: M = P·r / (1 − (1+r)−n), with zero-rate loans special-cased so they do not divide by zero. BMI categories follow World Health Organization adult ranges, and the page states plainly that they do not apply under age 20 or during pregnancy. The calorie tool uses the Mifflin-St Jeor equation published in 1990, typically within about 10% of measured values.

Where a method has known limits, the page says so instead of hiding it. The ovulation and due-date pages state explicitly that they are not contraception. Health and money pages carry stricter disclaimers than the rest of the site because they deserve them.

Privacy, in one paragraph

Every calculation runs in your browser. Nothing you type — dates of birth, weight, income, loan balances — is transmitted anywhere, written to a database, or kept after you close the tab. The site sets no localStorage and no sessionStorage; even your language choice travels in the URL rather than being stored on your device. There is no account to create and nothing to install. The privacy policy sets this out in full.

Corrections

If a calculator returns a result you believe is wrong, please tell me. Include the numbers you entered and the answer you expected, and I will check it. Corrections take priority over everything else on the site — ahead of new calculators, ahead of design work. A tool that quietly returns a wrong number is worse than no tool at all.

Contact

Email: contact@hasibai.com

Errors, corrections, accessibility problems, suggestions for calculators that do not exist yet, and general questions are all welcome. Expect a reply within a few working days.

Advertising

The site is funded by display advertising, which is what keeps every tool free and account-free. Adverts are kept clearly separate from the calculators and never influence a result — there is no sponsored placement in any formula, and no advertiser has ever seen a page before it went live. See the privacy policy for how advertising cookies work and how to opt out of personalised ads.