Tank size calculator

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.