How is a tank measured?

[dmbnpj]

From outside edge to outside edge or from inside edge to inside edge? Including the black trim?

 

[jdieck]

Most manufacturers give external dimensions but not to accuracy (say +/- 1/4") same for the gallonage so it is not that precise.

If you want to know the actual volume you need to take inside measurements.
You can use the volume calculator:
http://jdieck1.home.comcast.net/volcalc.html