This is a calculator to see how much GPH you should have from each style of filter. 
The most popular method of sizing a filter to an aquarium is by GPH.
While there are many other factors to consider, this calculator will give a rough estimate and a starting point to base sizing your filter off of. 



                                                            
Aquarium size in gallons      
         
Canister Filter   TO
HOB filter   TO
Sump   TO