Yarn crafts have no international bureau of weights and measures. What they have is the Craft Yarn Council, a body of yarn companies, tool makers and publishers that agreed on a common chart so that a pattern printed in one magazine could be worked with tools bought from another. That chart is the source these tools quote, and its limits are theirs too: it maps millimetres to U.S. sizes and stops there, because the letters and numbers stamped on a needle vary between manufacturers in a way no standard has ever settled. The millimetre measurement is the only figure on the packaging that means the same thing everywhere, which is why every tool here treats it as the real size and the U.S. number as a label for it.
Convert knitting needle sizes between millimetres and U.S. numbers using the Craft Yarn Council chart, and see which yarn weights each size is meant for.
Convert crochet hook sizes between millimetres and U.S. letters and numbers using the Craft Yarn Council chart, and see which yarn weights each hook is meant for.
Turn the stitch and row count of your swatch into the number of stitches to cast on and rows to work for a piece of the size you want.
Your swatch does not match the pattern and you would rather not reknit it. Convert the pattern's stitch counts to your own gauge and keep the finished measurements.
Weigh your swatch and this works out how much yarn the finished piece will take, in grams, metres and whole balls — from your own knitting rather than from a table of averages.
Turn a head measurement into the number of stitches to cast on, with the negative ease a hat needs to stay on and the Craft Yarn Council size band the head falls into.
Turn a foot measurement into a sock cast-on, with the shoe size band and the sock height published by the Craft Yarn Council for that foot.
Spread increases or decreases evenly across a round or row and get the instruction to work from, including the two run lengths that never divide out neatly.
Tell us what is missing and where its numbers should come from.
What should it convert, calculate or generate, and where would the authoritative numbers come from? A link to the standard or publication that defines them is the most useful thing you can send.
Copies this page address so you can paste it wherever you report it.
Two kinds of thing live here. The size charts are transcribed from the Craft Yarn Council standards row by row, keeping the repeated numbers and the gaps exactly as the standard prints them. The calculators are arithmetic over figures you supply — your swatch, your ball band — because the alternative is an average of somebody else's knitting, and averages are what make a sweater come out the wrong size. Every tool carries worked examples that run as tests, so a change that breaks one of them never ships.