Actual capacity of aquarium

My aquarium was advertised as having a 55 gallon capacity.

Filling it with water – aside from decorations and rocks – this just didn’t seem like it was true.

Here’s some good information on the actual capacity of your aquarium / fish tank.

Good explanation of why your tank is NOT really the size you purchased and simple formula to figure out the actual capacity.
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More complex formula to figure out your tank’s actual capacity. If you hate numbers or thinking don’t go here. But if you really hate numbers and thinking, consider a gold fish bowl, eh?
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So, what about my 55 gallons???

• First off, here are the approximate dimensions of my tank:
  48x21x12.5 outside dimensions (actual edge to edge)
  47.5x17x12 inside dimensions (inside glass measurements and height from rock substrate to the top edge of the water, so just the space that actually CONTAINS water)
• Using the first (simple) link above:
  Multiple out the inside dimensions and divide by 231 (Length x Width x Height / 231).
  My 55 gallons? Actually about 42 gallons in reality! However this does not account for the space taken up (displacement of water) by decorations or anything else in the tank.
• Using the second (more complex) link above:
  The chart at the second link uses the outside measurements from what I understand. For mine, the height was a little off because I think they go by a purely glass tank and mine has a plastic rim on the top and bottom which increases the height a bit. You plug in your numbers and follow down the right column in the chart to find your tank capacity.
  My 55 gallons? Actually about 40 gallons in reality! Now this chart I think is considering the fact that you have other things in the water taking up space and after considering the first number I got (42 gallons) less other things in the tank, the 40 gallon number makes sense – for my tank, at least.

Why does it all matter? Well, first off I just like to know what I really have. 😉 Beyond that, the capacity of your tank makes a difference in trying to figure out how many fish you can have or dosing out medication to the tank if needed. Think about it – if I were to treat my tank for a sickness, 40 gallons vs 55 gallons on dosage would make a pretty big deal! Why might my fish get sick? Well, if I filled the tank with fish for 55 gallons of water vs 40 gallons of water, I effectively became a fish slum lord with all the sickness hazards of overcrowding.

I’m a noob, but it makes sense to me. Anyone else? Love to hear from you in the comments!