Reference tools for yarn crafts, built from the published standard rather than from memory.
Every size on this site is transcribed from the Craft Yarn Council standards: the hook and needle conversion chart and the Standard Yarn Weight System. The Council represents yarn companies, accessory manufacturers, magazine and book publishers and consultants in the yarn industry, and its charts are what patterns and packaging in this trade are written against.
Transcribed means row by row, in the form the Council prints it. Where two rows share a U.S. number, both are kept. Where a row has no U.S. number, the gap is kept too. Nothing is filled in from another source to make a table look complete.
Each tool carries reference cases taken from the standard — a size, the answer the chart gives, and a citation for it — and those cases run as automated tests. A change that breaks one of them fails the build. The tests also cover the awkward rows on purpose: the U.S. numbers that map to two diameters, the diameters with no U.S. number, and the sizes that sit exactly on the boundary between two yarn weights.
They will not invent a UK or Canadian equivalent, because the chart does not publish one. They will not tell you the size is right for your project either — gauge decides that, and the standard says as much: always follow the gauge stated in your pattern. A size chart is a starting point and a swatch is the answer.
Corrections are welcome, particularly if you own a needle or hook whose markings disagree with the chart. Every tool page has a panel for reporting a problem or asking for a new tool.