utilduck is a flock of everyday tools. The name joins utility and duck — like the rubber duck on a programmer's desk, it sits quietly beside you while the small calculations of daily life get done: percentages, dates, units, passwords, conversions.
Calculator inputs are processed in your browser and are not submitted to utilduck servers. Visit and device analytics, advertising requests, and browser-provided features are described separately in the Privacy and Cookie policies. There is no account or paywall.
When a tool relies on an official standard, we cite the primary source at the bottom of the page — for example the WHO Child Growth Standards, the US FDA's unit-conversion guidance, USDA and NOAA reference data, national dietary intake standards, and Korea's Ministry of Government Legislation (법제처) statutes. The full list of sources by tool is at sources.
Some of our calculation engines are open source under the MIT license at github.com/Kyunghwan-Sohn/utilduck-engines. The blackjack EV engine matches 339 of 340 cells in the published basic-strategy chart (the one mismatch is a 0.0009 EV difference, an effective tie), and the lunar calendar engine matches all 31 recorded Vietnamese Tết dates from 1996 to 2026 and round-trips 100 of 100 dates between 1900 and 2100. Both are covered by automated tests — 24 for blackjack, 36 for the lunar calendar. Details are at methodology. The published validation set—31 Vietnamese Tết dates from 1996–2026, 100 round trips and the lunar engine tests—covers only the UTC+7 Vietnamese mode. We do not claim that validation for this UTC+8 output.
Before each release, a real-browser regression suite checks representative inputs, boundary cases and error handling across the supported tools and guides.
If you spot an error, we check it and fix it once confirmed — we do not publish a number we cannot verify.
Results are reference estimates, not medical, legal, tax, or financial advice. The flock keeps growing: 133 tools across calculators, converters, health, dates, office, developer and study categories, in English, Korean, Japanese, Chinese, German, French, Spanish, Vietnamese, Indonesian, Portuguese and Italian.
The public catalog contains 133 tools: 120 are available in all 11 languages, while 13 Korea-specific tools are published only in Korean so that country-specific rules are not mass-translated into irrelevant pages.