Beer line diameter

[cunnol]

Looking into finally buying a chest freezer to house my unused kegs. One thing I'm confused by is whether people are referring to inside or outside diameter when discussing beer lines. I assumed inside but looking at the the homebrew company I see they list their lines by OD. If I'm using a line length calculator I need the ID right? I assume the OD is irrelevant.

http://www.thehomebrewcompany.ie/beergas-line-38-od-per-metre-hard-p-71.html