For me, I don't care how deep the flowers need or the fish. The way I decided was to stand with both feet planted firmly on a flat surface. Then I bent over extending my hands down to the floor. I turned my head to the side. Then my husband measured up to my nose. That's how deep my pond is mostly. I wanted to be able to reach easily to the bottom to retrieve stray rocks that fell in or pots that might tip over, or even leaves off the bottom. Sounds stupid but when I had the string algae invasion, I'm sure glad I did it that way. Another thing, when your pond is REALLLLL deep, you can't easily see the bottom or scoop the bottom to get up the crud when you want. I know, the professionals won't agree with my reasoning but I'm happy with the depth of my pond. I have healthy Koi and lilies both. Oh, it goes from 24 inches to 30. The 30 is at the opposite end from the waterfall and closest to the skimmer and my front porch.