How to know "your number" for a machine haircut in a foreign country?

In Spanish Wikipedia, Hair clipper article has an explanation about that. I'm Spanish and I have always seen this system:

(roughly translated)

Each number means 3 mm or 1/8 inches (= 3,175 mm).

Number 1: 3 mm - 1/8".
Number 2: 6 mm - 2/8" (1/4").
Number 3: 9 mm - 3/8".
Number 4: 12 mm - 4/8" (1/2").
Number 5: 15 mm - 5/8".
Number 6: 18 mm - 6/8" (3/4").
Number 7: 21 mm - 7/8".

Now I'm living in Italy and I bought here an electric clipper (Philips) that uses this same system.

If you went to Morocco and they used this system and nº2 is equivalent to a UK&US 5, it's pretty probable that you measure it in millimeters (maybe you already know it, anyway it is only my guess), but I have no idea what German number (12) can mean.

Maybe people from these countries can tell us how many millimeters it is "a number".

EDIT: Unfortunately I can't speak German but I have googled for understanding UK & US notation and I have found this web. It seems to be American and, although it depends of the clipper brand, as an example this is the representation:

Blade - Length:
00000 - 1/125"
0000 - 1/100"
000 - 1/50"
0A - 3/64"
1 - 3/32"
1A - 1/8"
1.5 - 5/32"
2 - 1/4"
3.5 - 3/8"
3.75 - 1/2"

So my supposition above was wrong.

This other page, maybe American too, or perhaps international, shows a system like the Spanish one.

What I usually do is use pictures instead. The next time you get a good haircut, ask a friend to take a few pictures of your head or take some selfies. Then every time you go to a barber shop in any country show them these pictures and they will know what to do. A lot more reliable than trying to guess the local number system :)

This is a very old question but the truth is there is no real answer

All counties have different systems, having said that all (usually) follow a simple rule - which do they use day to day? imperial or metric? If its metric its 3mm per grade and 1/8 if imperial. This is the only answer to this kind of travel problem without listing every single country

Overall the best way of dealing with with this problem is to show them a photo

If you like this please also like @machlas answer as its absolutely correct (as far as it can be) but offers no solution to the problem - so while it doesn't answer the question you should take a photo if you are unsure and think you will stay long enough to need a haircut

In the US we use the 8th inch system:

1 = 1/8 inch 2 = 2/8 inch 3 = 3/8 inch 4 = 4/8 inch 5 = 5/8 inch 6 = 6/8 inch 7 = 7/8 inch 8 = 1 inch

However, I live in Rome, Italy and when I go to a barber sometimes they use this system and sometimes they use another system, which may just be millimeters. I had a really bad surprise this way last time I went and lost all of my hair. In both the US and Rome most barbers I have seen use Wahl brand clippers.