A calculator is only useful if you can trust the number. This page explains how TavraxTools tools are built, how we check them, and how we handle mistakes. It is written for readers who want to know what stands behind a result.
Where the formulas come from
We do not invent methods. Loan and mortgage payments use the standard annuity formula. BMI categories follow the World Health Organization. Heart rate zones use the Tanaka formula and the Karvonen method. Due dates use Naegele's rule. Web tools follow the documentation of the standards and search engines they refer to, such as RFC 3986 for URL encoding, RFC 4648 for Base64 and Google Search Central for titles, snippets and robots.txt. Each tool page lists its sources under "Sources and further reading".
How we check the results
Every tool is tested with automated checks before it is released. More than a thousand checks run on each release. The important ones compare our results with values computed separately, by independent code that does not share our formulas or libraries:
- Money tools: loan, mortgage, savings and interest results are compared with reference values computed separately in Python, including edge cases such as a 0% interest rate.
- Scientific calculator: 3,000 random expressions are evaluated by the calculator and by Python, and the answers must agree. Order of operations, powers, factorials and error messages have their own checks.
- Numbers and text: Roman numerals are checked for all 3,999 valid numbers, number-to-words for more than 900 numbers in both the international and the lakh and crore systems, statistics against Python's statistics module on 2,000 values, and URL encoding against Python's URL library on hundreds of random strings.
- QR codes: the encoder is compared module by module with two independent open-source QR libraries across all 40 versions and 4 error-correction levels, and the downloaded PNG and SVG files are scanned back with a decoder to confirm they contain the original text.
- Image tools: compress, resize, convert, rotate and color-pick features are tested in a real browser engine with generated pictures. The tests open the downloaded files and check their format, size and pixels.
- Layout and access: pages are checked at phone, tablet and desktop widths for content spilling off the screen, for consistent spacing, and for form fields that have labels.
We also test the tests. Before trusting a test suite, we add a deliberate bug (for example, a rotation that turns the wrong way) and confirm that the tests catch it.
What our tools cannot do
Results are estimates based on the numbers you enter and the assumptions stated on the page. A lender's quote, a doctor's measurement or a tax authority's figure can differ, and those are the ones to rely on for decisions. Where several accepted methods exist (for example, for quartiles or ideal body weight), the page says which method we use. See the disclaimer.
Keeping pages up to date
Each page shows the month it was last reviewed. We review pages when a formula, a standard or a search-engine rule changes, and after reader reports. Health, money and legal information is reviewed most carefully.
Corrections log
Everyone makes mistakes. When we find one, we fix it and record it here.
- September 20, 2026: Paint calculator. The page said 350 square feet per gallon is about 10 m² per liter. It is about 8.6 m² per liter (10 m² per liter is about 407 square feet per gallon). Corrected.
- September 20, 2026: URL encoder and decoder. The parameter table decoded a plus sign that came from %2B as a space, and split values containing %26. It now reads the address as typed, so each value is shown correctly.
- September 20, 2026: Statistics calculator. A decimal comma such as 2,5 was read as two numbers. The page now explains that commas separate numbers and asks for a full stop for decimals.
- September 20, 2026: Meta tag and robots.txt generators. Sample addresses on example.com were pre-filled and could be copied by mistake. The fields are now empty.
To report a mistake, use the contact page with the tool name and the numbers you entered.
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