Tank measurement Clarification

[kit]

Out of curiosity, when measuring or reading specs on a tanks dimensions, say 48"x18" - is it correct to say that would be measured from the outside frame?

I'm trying to measure a tank's capacity in cubic feet but wanted to know if the correct method is to measure from the inside of the tank before any frame or glass is added back into the total length/width.

Most packaging I see for boxes or listed dimensions for tank size from others seem to be the exterior and I'm not sure if it is a correct method for my own use.

And also am I to measure to the top height of the glass and go beyond any plastic lip for the canopy/lid portion?