Methodology & sources

Last updated: August 28, 2026.

This page exists so you can doubt the site methodically: know where each number comes from, what was verified, and what remains an approximation. No calculator here is a black box.

Who is responsible

The project is maintained by Rafael Rossi, who writes, checks and tests every calculator. Corrections and questions: rafaelrossi.apps@gmail.com or the contact page.

The four criteria every page meets

1. An explicit formula. The maths is written on the page, not just implemented in code. You can redo it on paper.
2. A worked example with real numbers. Every example was calculated and checked before publishing — no table is written from memory.
3. Sources where the subject demands them. Health, finance, tax and construction pages list the references used, with the year they apply to.
4. A visible revision date. Each page shows when it was last reviewed. When a rule changes, the date changes with it.

Sources and formulas by area

Finance

• Compound interest: M = P(1+i)ⁿ + PMT·[((1+i)ⁿ−1)/i]. Simple interest: J = P·i·t.
• Loans and mortgages: the standard amortising payment, with a month-by-month amortisation table separating interest from principal.
• Indicators: CAGR, ROI, APY and margin follow standard finance definitions, and the difference between nominal and effective rates is always spelled out.
• Rates on the English pages are quoted per year, matching how they are published in the US market — not per month, as is the Brazilian convention.

Health

• BMI: World Health Organization (WHO) categories.
• BMR and calories: the Mifflin-St Jeor equation, with conventional activity factors.
• Body fat: the US Navy circumference method. Pregnancy: Naegele's rule. Blood alcohol: the Widmark formula.
• ⚠️ Health formulas are population averages. Individual variation is wide, and it is stated on the pages where it changes how the result should be read — the blood-alcohol page, for instance, says outright that it must not be used to decide whether you can drive.

Construction and home

• Material take-offs (paint, tile, roofing, brick, concrete) use average manufacturer coverage rates and the roof pitch factor. They are for budgeting and do not replace a licensed professional's design.

Maths, physics, chemistry and conversions

• Standard formulas (quadratic, LCM/GCD, areas, proportion, progressions, kinematics, molarity).
• Conversions use official International System (SI) factors. Where a unit is ambiguous across cultures (US vs imperial gallon, short vs metric ton), the page states which one it is using.

Reference years

Tax tables and official figures change, usually at the turn of the year. Every affected calculator displays the reference year it is using, so that an out-of-date page is visibly out of date rather than silently wrong. If you see a year that has passed, treat the result as a rough order of magnitude and check the current figure at the source.

How the site is verified

Beyond checking the worked examples by hand, an automated battery runs before each release: every interactive lesson is executed in a simulated browser (which has already caught errors that only appeared at runtime), internal links are swept against the pages actually generated, the sitemap is compared item by item with the site, and colour contrast is measured in both themes.

When we find an error

Errors happen — the project's history has them: a pace table that said 1:46 where 1:45 was correct; a text that rounded differently from its own calculator. The procedure is always the same: fix it, update the page's revision date and, when the error exposes a flaw in the method, change the method so it cannot recur. If you find one, the contact page says what to include.

Privacy of your calculations

All calculations run in your browser — the values you type are never sent to or stored on a server. Details in the Privacy Policy.