'Nother tank question

[Yellow Turtle01]

So awhile ago I asked what size this tank I found in a closet was (30x12x13) and was told it was a 20 long.
(this is NOT for a tort) I was cleaning it out, I decided to fill it up with water, and it held more than 20 gallons?? Confused?

 

[jaizei]

The names are usually nominal and the actual volume may vary.

 

[Yellow Turtle01]

The names are usually nominal and the actual volume may vary.

Oh okay, so it held almost 28 gallons (kind of like 27 1/3) it technically is still a 20 long, but is just bigger?

 

[Dizisdalife]

I believe that the size, or rating, of a tank in Gallons is calculated as 231 cubic inches = 1 US gallon. If I divide the volume (in cubic inches) of your tank by 231 the answer is 20.5xx. Here is a website with some additional information.
http://www.anapsid.org/resources/tanksize.html

 

[Yellow Turtle01]

I believe that the size, or rating, of a tank in Gallons is calculated as 231 cubic inches = 1 US gallon. If I divide the volume (in cubic inches) of your tank by 231 the answer is 20.5xx. Here is a website with some additional information.
http://www.anapsid.org/resources/tanksize.html

Hmm, okay. The website has two different 20 longs, so mine might be the bigger size, because it's 13 not, 12. I'm not an expert at metric, but that makes sense so I have the 'bigger' 20 long tank size.
Thanks