The question comes up quite a bit on the forums, “Can my drain line handle the flow from my return pump?” Then come the usual questions about what type of drain setup is being used, how many tees and elbows, how far the drop is, etc. Really what most people want to know is if they’re in the right ballpark or if they’re way out of bounds. I’ve frequently used this excellent calculator over at Melev’s Reef. Just a few simple questions and it tells you roughly what the flow rate you can expect is for your plumbing.
Melev also talks a bit about the theory behind how the calculations are made, if you’re a big ole nerd like me you’ll enjoy that part too.