Hey mand working in retail as a kid helped me out if you want to estimate gallons , the average "trashcan" for a computer (or bathroom) is about 3-4 gallons. I've only see a few on average that are bigger so you can eyeball it with that if ya want . Gee wonder what I use for pots he he he.

Mand makes the best point though , deeper is better! (and not just in growing! , sorry bad joke....)