Tank size calculator

I always find this helpful, especially when bringing home a used tank that may or may not be what it was advertized as.

Generally once you have fish you can tell sizes, but there are always oddballs that one comes across!

TankCalc

I always knew my "40 gallon Seaclear" isn't 40 gallon lol. It's 37 gallon even if 100% filled confirmed by the tool you posted. Of course I will not fill it to the very top, so there is probably around 32 gallon water in there at most, consider the space above waterline as well as the a substrate. In fact I regret that I did not go for the next step up, which would be 4 inch taller. I always consider the water in there is too shallow.

Pretty useful tool.