How to specify the length of a short haircut in China? 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:

1号是光头

2号是1号长一点点的

3号也是

0号是可以发光的

NS要1号

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.
nothing.

Actually there is not a term to describe the length of the haircut in China.

So, how to decide the length of the haircut? keep communicate with barber.

eg.

I: 'please cut my hair with 毛寸(is an adjective for style, not length)'

B: after a while 'Is it fine?'

I: 'shorter'

B: 'Okay, let me see'