In USA, the size is C. In Europe, 2.75 mm. (mm is millimeters) This is a very small hook, but they get smaller.
A 3mm crochet hook is halfway between a C and D size in US sizes.
A 3mm aluminum hook (in the U.S.) would probably be a C hook. (Hook sizes are not always the same when made by different manufacturers.) Fingering or sock yarn would be best suited to this size hook. That said, manufacturers generally note the suggested hook size on the label of the yarn. That is a suggested size only and you are free to choose a different size hook with which to work that yarn. Also, your pattern will likely tell you what size hook to use and the appropriate yarn for your project. If you are looking at steel hooks, you would have a size between a 1 and 0 hook which would be used for thread, probably size 5.
It is 275mm wide
275mm equals 27.5cm
It is 275 mm wide at the tread.
A size 5 steel hook. http://www.crochetmagazine.com/hook_sizes.php
it is best to use an H or I size hook
that would be a size P crochet hook
27.5cm (divide mm by 10)
It depends on the size of yarn used. For worsted weight size 4 yarn, it reccommends to use a size g or h hook. Check the yarn cover to see which size hook is reccommended for the yarn.
You can use another size that gives you the gauge you want.
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