A crochet hook (or crochet needle) is an implement used to make loops in thread or yarn and to interlock them into crochet stitches. It is basically a round shaft pointed on one end, with a lateral groove behind it. The point eases the insertion of the hook through the material being crocheted and the groove makes it possible to pull a loop back through the material.
In India, hook sizes are noted in millimeters. While in most countries abroad, they are noted in alphabets. More specifically, aluminum hooks (the ones of larger sizes, used with yarn) are noted in alphabets; and steel hooks (the ones of smaller sizes, used with thread) are noted in numbers in the US. A simple look at a conversion chart like the one below shows us just this, that the sizes are exactly the same. Also you can find out which size corresponds to which alphabet and make a purchase.
Here are the Crochet hook sizes conversion charts.
Yarn Weights and Required Hook Sizes
|Lace||1 Ply||2 Ply||1.50-2.25 mm|
|Fingering||2 Ply||3 Ply||2.25-3.50 mm|
|Sport||4 Ply||5 Ply||3.50-4.50 mm|
|DK||DK||8 Ply||4.50-5.50 mm|
|Worsted||Aran||10 Ply||5.50-6.50 mm|
|Chunky||Bulky||12 Ply||6.50-9.00 mm|
|Super Chunky||Super Bulky||14 Ply||9.00 mm (and above)|