There are several way to signify the wall thickness of the pipes and pipe fittings.

1st. The wall thickness can be give as mm directly.

2nd. Maybe give as inched.

3rd. Maybe give as SCH No.  Eg, SCH10, SCH20,SCH30, SCH STD， SCH40， SCH80， XS,XXS etc. Then you will look up the wall thickness for what milimeter is stand for. For carbon steel pipes and fiitngs American standard, you can get it in ASME B36.10, For Stainless steel pipes and fittings you should get it in ASME B36.19.