Since it has been asked on the forum several times, here are the Eheim metric tubing conversions:
Eheim Metric Tubing Conversion
9/12mm = 3/8"
12/16mm = 1/2"
16/22mm = 5/8"
25/34mm = 1"
The coverted number is SAE (Standard American English) and is what you might see on a wrench or socket as a unit of measurement
3/8 - 1/2 - 5/8 - 1" are all the inside measurements of the tubing 12mm = 1/2" roughly.
The metric number that Eheim uses are ID first (ie 9/12) and OD is second. In English or SAE this would be 3/8 ID 1/2 OD
ID = inside diameter
OD = outside diameter
Sorry to be so thorough but I wanted no confusion.