About CalcOtter

CalcOtter is 100 everyday calculators across 6 areas — money, health, dates, arithmetic and unit conversion. It exists because the calculator you actually need is usually buried under a sign-up wall, an app download, or three paragraphs of filler above the input box. Here the box is the first thing on the page, the answer appears as you type, and the explanation sits underneath for when you want it.

How a calculator gets built

Every calculator starts as a pure function: a small piece of code that takes numbers in and returns numbers out, knowing nothing about the page around it. That function is tested against a published reference value before it can go live — a bank's own amortization table for the loan math, a worked example from the paper that defined the equation for the health formulas, the exact international definition for a unit conversion. The page is then built on top of that function. The ordering matters: it means a redesign cannot change an answer, and a wrong answer is always a bug in one small, testable place.

The formula each calculator uses is printed on its own page in ordinary notation, with the variables named, so you can reproduce the result by hand or in a spreadsheet. We would rather you checked us than trusted us.

Where the numbers come from

The money calculators use the standard amortization formula banks use for fixed-rate lending, and compound-interest math with the compounding frequency stated on the page rather than assumed. The health calculators use the Mifflin–St Jeor equation for resting metabolic rate, the World Health Organization's classification bands for BMI, and Naegele's rule for pregnancy due dates — each with its published limitations set out on the page, because a screening formula presented as a verdict does harm. Unit conversions use exact international definitions rather than rounded factors: one inch is exactly 25.4 millimetres and one pound is exactly 0.45359237 kilograms, which is why these conversions agree with a metrology table to the last decimal you ask for.

Where two respectable conventions disagree — the several competing formulas for "ideal" body weight, or whether a date range counts both endpoints — we say which one the page follows and why, instead of quietly picking one and presenting it as the answer.

Privacy by architecture, not by promise

The calculations run entirely inside your browser. Loan amounts, salaries, body measurements and due dates are never transmitted anywhere, because there is no server doing the arithmetic — these are static files, and the maths happens on your device after the page has loaded. Put your phone in aeroplane mode and every calculator here still works.

That is a structural guarantee rather than a policy we ask you to believe. A promise not to store your data can be broken quietly; an architecture with nowhere to store it cannot. It also means there is nothing for us to hand over, sell, or lose. The privacy page covers the small amount we do see — aggregate page views, and the ad network's own behaviour.

What these numbers are, and what they are not

These are calculators, not advisors. A financial result is a projection at the rate you typed, ignoring fees, tax, insurance and every rate change over the term — a real lender's quote will differ, usually upward, and that gap is exactly what checking is for. A health result is a population-level screening number: useful as a starting point, meaningless as a diagnosis, and wrong for plenty of individual bodies. A date result follows the calendar and knows nothing about your public holidays or your leave policy.

Used properly, a number from this site makes a professional conversation shorter and better informed. Used as the sole basis for a decision that matters, it is a liability. The terms say the same thing in more formal language.

Corrections

If a calculator here disagrees with a textbook, the textbook wins and we fix it. Reports of wrong numbers are the messages we most want to receive and the ones we answer first — send the page, the values you entered and what you expected to hello@calcotter.com. When we fix an error we also pin the corrected value in the automated tests, so that particular mistake cannot come back.

How the site sustains itself

CalcOtter is free and carries a single clearly-labelled advertisement per page, in a fixed reserved space that never moves the content or covers an answer. There are no accounts, no paywalled features, no newsletter and no data to sell. We do not take payment for placements inside the calculators or for links in the explanatory text: a calculator that quietly favours whoever paid is worthless, and being checkable is the only thing this site has.

Who runs it

CalcOtter is built and operated by Iris Consulting and Engineering (Pty) Ltd, a private company registered in South Africa. Questions, corrections and requests for new calculators all go to the same address, and are read by the person who writes them.