How we build and review our calculators

A calculator is a claim. When our mortgage tool says your monthly payment is $1,517, we are claiming that a specific formula, applied to your inputs, produces that number. This page explains how those claims get made, tested, and fixed when they are wrong.

Where the formulas come from

We do not invent math. Every calculator implements a standard, documented method: the amortization formula lenders use for level-payment loans, the Mifflin-St Jeor equation for resting energy, the CDC's ranges for body-mass index, the exact 2.54 cm definition of an inch fixed by the 1959 international yard and pound agreement. Each tool page states which formula it runs, in symbols, next to the result. If a method has competing conventions (compounding frequency is the classic case), the page says which convention we chose and why.

How a tool is tested before it ships

Before a calculator goes live we run it against reference values and check the output digit by digit:

• Textbook worked examples. Finance and health formulas appear in course materials with fully worked numbers. Our implementation must reproduce them exactly.
• Institutional figures. Loan and mortgage outputs are compared with lender-published amortization schedules; unit conversions are checked against the exact legal definitions rather than rounded factors.
• An independent spreadsheet. We rebuild the same formula in a spreadsheet from scratch and compare outputs across a range of inputs, including awkward ones: zero interest, one payment period, very large balances, negative dates.

Rounding is handled at display time only. Tools compute at full floating-point precision and round the final figure, so intermediate rounding error cannot compound. Each result card notes this.

What the numbers do and do not mean

A formula is exact; your situation is not. A loan calculator cannot know that your lender rounds payments up to the next cent, and a calorie calculator cannot know your actual metabolism. So every result card carries the caveat that applies to it: gross versus net, estimate versus quote, general information versus advice. We are practitioners and writers, not licensed advisors. Nothing on this site is personalized financial, medical, legal, or tax advice, and the disclaimer spells that out in full.

Review cadence

Tool pages carry a "last reviewed" date next to the byline. A review means someone re-ran the reference checks, re-read the copy against current conventions, and updated anything stale. Pages whose subject matter shifts (tax-adjacent tools, tipping norms, platform character limits) get reviewed more often than pages built on fixed definitions (a centimeter is not going to change).

Corrections

When we find an error, we fix it and note the fix on the affected page. When a reader finds one first, we credit the catch if they want it. If a number here looks wrong to you, take thirty seconds and tell us, and include the inputs you used so we can reproduce it. A real person reads every report, usually within a couple of days.

Privacy as an engineering constraint

Every tool runs entirely in your browser. The salaries, balances, and measurements you type are never transmitted, logged, or stored. This is not a privacy setting, it is the architecture: the pages are static files and there is no server-side code to send your inputs to. It also means the tools keep working on a flaky connection once the page has loaded.

How the site makes money

RabixAI is independent and ad-supported. We show a small number of clearly labelled ads through Google AdSense. Advertisers have no influence over which tools we build, what the formulas say, or what the guides recommend. There are no affiliate links inside calculator results, no sponsored numbers, and no premium tier.

Questions this page didn't answer

The about page covers who we are and why the site exists. For anything else, email hello@rabixai.com.