For fifty years or more (I'm withholding that info) I never really learned what exactly was meant by pot sizes, especially when expressed in gallons. Gallons are a liquid measure to me. Soil would be a volume measurement. The database has available spots for referencing a plant's suitability for being grown in a container. Sometimes there you'll see something like "Suitable in 3 gallon or larger". What, then, is the equivalent to that in a container's dimensions? I've been researching. It's somewhat lacking, but here's a simple conversion that approximates the size container by cross-referencing the diameter. I didn't know where to post in ATP, but since containers are used broadly across the plant spectrum this seemed the best Forum. I hope someone else may find it useful. The meat and potatoes information:
4" pot= pint (0.5 quart)
5-6" pot = quart (0.25 gallon)
7-8" pot= 1 gallon
8.5" pot = 2 gallon
10" pot = 3 gallon
12" pot = 5 gallon
14" pot= 7 gallon
16" pot= 10 gallon
18" pot = 15 gallon
24" pot = 25 gal
30" pot= 30 gal

This doesn't address the depth of a pot, but is still a good idea of the pot sizes. It's all about what I can lift and what I need a forklift to move:smily: ! I like thick clay pots, but age may dictate that I need to make peace with plastic and resin which weigh less (but are also easier for the ever present wind here to topple over).

And then, to complicate it further, there is a "nursery" gallon and a "true" gallon size pot. The nursery gallon is really about 0.9 gallons I think.

True. Using gallons as a measurement isn't the most accurate method. For me the translation is useful. My preferred container for potting up the daylilies is a standard clay pot with 16" diameter. Now I know that container is more or less equivalent to a 10 gallon container. If I can just get that one size stuck in my head, there will be more hair due to a lot less scratching. Much as I would like the next size up, then they get really awkward to move around.