redearslider.com

Here is a website where you can use the dimensions of a tank to learn how many gallons it is. That way you don't pay money for a tank that is smaller than advertised.

http://alysta.com/books/fishtank.htm

Always a good idea. I don't know how many times I've seen a 55 gallon Craigslist and people say "it's at least 100 gallons, it's huge... 48"x13"x20"..."

I almost wasted an hour’s drive to see a 55 advertised as a 75... at the last minute I caught a bit of a moron vibe from the person and asked for the measurements - it was a 55.

Here's another link to THIS website's tank size calculator:
http://www.redearslider.com/calculator.html

I find it quite useful for determining how much water I have in a partially filled tank.

It's also handy to know that approx. gallons can be determined by doing
(length x width x height)/231

All you need to bring with you is a tape measure and calculator.