In the US, knitting needle sizes are in numbers; crochet hooks use both numbers and letters.
that would be a size P crochet hook
You should use a size J/10 crochet hook for a project that requires a 6 mm crochet hook.
The recommended size of crochet hook for working with yarn that is 5mm thick is typically a 5mm crochet hook.
A 00 crochet hook is typically around 9.0 millimeters in size.
The recommended size of crochet hook in the US for working with a 3mm yarn is typically a size C or D hook.
You should use a crochet hook that is labeled as 5.00 mm for the project.
For this project, you should use a size 6 yarn crochet hook.
A U.S. size 7 yarn hook is 4.5 mm. http://www.crochetmagazine.com/hook_sizes.php
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.
it is best to use an H or I size hook
In UK sizes, a 2mm crochet hook is a 14, it doesn't not have a US size equivalent. Here is a chart with crochet hook conversions from metric to US to UK.
A 5.0mm would be an H-8 hook.