You’re measuring from the substrate or finding the volume of the empty tank? Enquiring minds have a need to know. I first put all of the hardscape I know I’m using into a container I know the volume of. Fill with water. Remove. Then I know how much displacement this causes. Then I measure the tank from the substrate, figure the volume, subtract the displacement of the hardscape. Then I’ve got an approximate idea of my tanks volume. For finding the volume of the empty tank, your equation is most helpful. Thank you.

I just say my tank is the same amount no matter how much decor I put into it. That would really confuse everyone that I showed my tanks. For example, “here’s my five gallon tank, but really it’s only a 3.47 gallon tank if you want to be precise” which would really make my guests confused.

I think when you're stating the size of the tank, the empty tank size is sufficient. For stocking issues, I think you should also take into consideration the volume of water it holds, as that can be vastly different and can effect the number and types of fish you stock.

I just call mine what they are. Like the twenty nine or forty long. But for meds and such, I’m glad to know the actual volume of water.