Yarn comes in various thicknesses, and these thicknesses are organized into categories that are called weights. You can use the standard yarn weight chart below to find the recommended hook sizes for each yarn weight. This way, your next crochet project will come out the size you intended.
To help crocheters select the correct yarn for their projects, the Craft Yarn Council has set up a system of guidelines manufacturers use to categorize their yarn.
The Craft Yarn Council has assigned numbers and names to each yarn weight category. The lightest yarn weight is called lace yarn and is given the number "0." The largest classification of yarn is called jumbo yarn and is given the number "7."
The symbol weight yarn chart is helpful because you can use it to determine which weight of yarn you will need for either your hook size or the type of project you choose to crochet.
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1.4 mm to 2.25 mm
2.25 mm to 3.5 mm
3.5 mm to 4.5 mm
Steel: 6, 7, 8 Regular: B-1
B-1 to E-4
Steel: 5 to 1 1/2 Regular: 13 to 14
Steel: 1 1/2 Regular: 10 to 13
Yarn manufacturers have come up with a system to measure yarn. They have determined that a yarn's weight should be measured by its thickness or diameter. To do this they measure a yarn strand using WPI or wraps per inch.