Like I said before I has more floor space than my 29g aquarium. It is very deep, 18" to be exact. It is 24" wide as well. So the floor area is pretty good. I have a 10g which I plan to use for feeding, so he is much less likely to ingest any substrate when eating since that 10g won't have any substrate, I may put down some artificial grass turf. Anyway my 10g fish tank is 9.5"x12" so I'm sure if a 10g aquarium would last a year then I'm sure this Terrarium would last long since the total floor space is equal to almost four of my 10g tanks. Even my 29g has a surface area of 29.5"x12" so again my terrarium has more floor space, no it isn't as long but it is deeper.
I looked at a 30g rectangle aquarium and it was 30"x12" which is a surface area of 360 square inches. I figured a 24"x18" tank with would be suitable since it has 432 square inches of surface space making it bigger than a rectangular 30g tank. My tank is 18" high and since just about every 30-40g rectanglar tank I've seen are around 16" high I don't really see me have all that much extra height.
Everything I read, and I did a lot of reading, before buying a snake said a terrarium like the one I bought would be suitable for a ball python when compared to a similar sized rectangular aquarium would be fine since ball pythons love to curl up in a ball. The capacity of my 24x18x18 tank is actually 36g so if you only factor in height then 36g and 18" high means that if I were to fill this with water every 1" would be 2g of water. So if I reduced the height to 16" like most aquariums in the 30-40g range I would be able to hold 32g of water. This is a pretty big tank, I think it may just be deceiving by its looks in a picture.
I considered a 40g tank too but just didn't have the room to accomodate the 36" width. The 40g tanks are 36"x15" so they have 540 square inches of surface area on the floor. This is 108 more square inches on the floor than my tank, which is a nice bit of room. However my tank as 72 square inches more space than the average 30g rectangle aquarium so that is a good difference too. I could understand if this was a 24"x12" tank (the size of a 20g rectangle tank) the space being too little to the extra 6" in depth certainly makes a difference. In fact it makes the surface are 50% bigger than a 20g rectangular tank.
This terrarium is actually the 3rd largest Exo-Terra make according to their website. Mine is 24x18x18 but they also make a 34x18x24 and a 36x18x18.
Everything I read online suggested a 20-30g would be fine for most ball pythons for their life, infact more places recommended against going larger than 40g so I though I was right in the sweet spot the a more squarish tank with the surface area larger than a 30g rectangular tank. If I ever feel this tank isn't suitable for him I'm buy a bigger one but I'm certain it should last sometime.
Now I do have one more question, I'm just wondering what dimensions are of all your tanks. The width and lenght should be all that matters since height isn't a big deal for these snakes, although mine can't stop climbing up the back wall of the tank, he seems to really enjoy doing that. He is doing it right now and I'd take a picture if it wasn't so dark. He stretches his full length (about 16-18") and almost makes it right to the top, which is why I want to get him a tree.
Anyway I'd really apprentice the dimensions of your tanks guys so I can compare how mine really stands up. Also like I said if I ever need to I'll just buy a bigger rectangular tank I have a wall that could fit a 60 or 70g tank in my basement den if I moved a few things around.