In the US you can specify the length of a short haircut by giving a number. So a "Number 2 Haircut" means using the #2 guard on a clipper to cut the hair 1/4 inch long. A "Number 4 Haircut" will cut it 1/2 inch long; and so on.
I know that clippers are used the same way in China but the numbering system seems to be different--and of course in China they use metric. I and a short-haired friend have both gone in sort of blind about exactly how say what we want in Chinese. (Well, travel is an opportunity to try new things!)
But now I would like to know the right way to specify it. Suppose I want 1/2 inch, or more or less 1.5cm. What is the barber's terminology for this?
The site linked by @user3306356 has this list, which is true in the US also, but is humorously vague:
I will say I'd been getting haircuts in the US for a long time before I knew this system existed here. A grumpy woman barber who wants men to pay more attention to their hair told me to remember it.