What does "yarn weight" mean?
A yarn’s weight is not what it reads on the scales: it is how thick it is. A thin yarn gives a light, detailed piece; a thick yarn works up fast and gives a dense fabric. The Craft Yarn Council, the American body that standardises labels and patterns, sorts yarns into eight categories numbered from 0 (lace) to 7 (jumbo). Each category gathers several everyday names, and that is where the confusion starts: the same yarn is "worsted" in New York and "aran" in London.
The chart above gives, for each category, the American names the source cites, the British equivalent where it names one, the range of crochet hooks and the range of knitting needles, in millimetres. These are starting ranges, not rules: they assume an average hand and an ordinary project.
Reading a ball band
A ball band almost always carries three useful facts. First a symbol, a ball of yarn with a number from 0 to 7: that is the category in the chart, and the quickest thing to spot. Then the length for a given weight, for example 200 metres per 100 grams: it is the most dependable figure for comparing two yarns, because it depends on no name. For the same weight, the more metres, the finer the yarn. Finally the gauge and the suggested tool, a number of stitches and rows over ten centimetres with a given hook or needle size.
Treat country names with care. A yarn labelled "worsted" by an American brand and an "aran" yarn from a British brand both fall into category 4, but they can differ slightly. The symbol and the length settle it.
Substituting one yarn for another
You do not have the pattern’s yarn, or it is no longer made: no harm done. Look for a yarn in the same category with a similar length. If the pattern calls for 400 metres of worsted and your ball offers 200 per 100 grams, you need two 100-gram balls, and a little to spare. Think about the fibre too: cotton does not drape like wool, and a pattern designed for a soft fibre will come out stiff in a rigid one.
Do not redo the stitch arithmetic by hand. Knit or crochet a swatch with the new yarn, measure it, and compare it with the pattern’s: if the numbers match, follow the pattern as written. If they do not, change the hook or needle size, half a size at a time, until you get the right figures. The gauge calculator makes that comparison for you.
The hook and needles in the chart
The chart’s ranges tell you which size to try first. Hooks and needles do not always match millimetre for millimetre, because the yarn is not handled the same way. To go from an American notation ("H-8", "US 8") to a diameter in millimetres, see the crochet hook size chart and the knitting needle size chart. Your swatch remains the only judge, whatever the range.
Frequently asked questions
Worsted or aran?
They are two names for the same category, number 4. Worsted is the American word, aran the British one. Either suits most patterns that call for the other, as long as you check the gauge.
Can I replace one yarn with another?
Yes, if you pick the same category, a similar length and a fibre with a comparable drape. Then knit or crochet a swatch before you start, and adjust the tool size if the numbers differ.
Is DK thinner than worsted?
Yes. DK is category 3 and worsted category 4: DK is slightly finer and takes a slightly smaller hook or needles.