Straight from the Craft Yarn Council standards
Size charts read straight from the published standard, and calculators that work from the piece you actually knitted rather than from an average of somebody else's. No accounts, no uploads, no pattern database.
Try
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.
Convert sewing machine needle sizes between the metric NM numbering and the American one, and read the blade diameter the metric number actually states.
Work out how much fabric a set of pattern pieces needs at your bolt width, with seam allowance included and the layout it assumes shown alongside.
Work out how big a cross stitch design comes out on a given fabric count, how much fabric to cut, and roughly how much floss the whole thing will take.
The Craft Yarn Council is blunt about it: letter and number sizing varies from company to company, and it recommends relying on the millimetre marking on the packaging because that is an actual measurement. The U.S. number is a label somebody chose. That is why the chart has U.S. 3 sitting against both 3.125 mm and 3.25 mm, and why a hook stamped G might be 4 mm or 4.25 mm depending on who made it.
The chart maps millimetres to U.S. sizes and stops. It carries no UK or Canadian column, so neither do these tools — a conversion we could not attribute would be a worse answer than none. Where the standard prints a dash, we print a dash: 3 mm and 7 mm needles are perfectly ordinary, they simply have no U.S. number to convert to.
Yarn estimates published as tables are averages of other people's tension, needles and stitch patterns — and a cabled sweater can eat half again as much yarn as a plain one at the same size. Weighing your own swatch removes every one of those variables at once, which is why the yarn calculator here asks for a scale reading instead of offering you a table to look yourself up in.
There is no upload, no account and no server doing the work — the chart ships with the page and the lookup happens in your browser. Which also means the tools keep working on a train with no signal, once the page has loaded.