How we build and verify our tools

We treat every calculator as if someone will use it for a real decision: taking a loan, budgeting a purchase, planning a pregnancy. Every tool goes through five steps before it goes live, and we publish them here so you know where each number comes from.

1. We start from the official source, not other websites

For any tool based on a law, a regulation or an official rate, we go straight to the authority:

Area	Primary sources
Holidays and dates	U.S. Office of Personnel Management (federal holidays); Government of Canada (statutory holidays)
Sales tax	Canada Revenue Agency and provincial revenue ministries; state tax rate tables compiled from state law
Loans	Consumer Financial Protection Bureau; Financial Consumer Agency of Canada
Health	World Health Organization; U.S. CDC and NIH; the American College of Obstetricians and Gynecologists
Faith calendars	Published calendars of the communities that use them, cited on each tool page

We never copy a rate or a formula from another calculator site. When sites disagree, and they often do, we go back to the official text and cite the exact article on the tool page.

2. We show the formula

Under every calculator you'll find the formula it uses and a step-by-step worked example. You can redo the math on paper. That's the point.

3. We test against known answers

Before launch we run each tool against cases whose answers we already know from the official source or by hand, plus edge cases: zero, very large values, leap years, the last day of a month.

4. We date everything

Every financial, legal and health tool shows a last reviewed date. It changes only when we actually re-check the tool against its source, not when we adjust the design or wording. We review these tools at least every six months, and immediately when we learn of a regulatory change.

5. We correct in public

When we find an error, or someone reports one, we fix it and note on the tool page what changed and when.

What our tools don't do

• Results are estimates for guidance. For financial and legal matters, your employer or the relevant authority has the final word. For health, your doctor does.
• We don't store what you type. Calculations run in your browser. Your numbers are not sent to our servers.
• Image and PDF tools run on your device. Your file is never uploaded. To check, open the page, turn off Wi-Fi, and see that the tool keeps working.

How we use AI

We use AI tools to help draft some explanatory text. AI does not write our numbers, rates or formulas: worked examples and tables are computed by the tool itself, and every regulatory fact is checked by hand against its official source before publishing.

Found a mistake?

Email corrections@toolida.com with the tool name, the values you entered, the result you expected, and your source if you have one. We review every report and reply within 7 business days.