3/8 inlet and 1/4 return. I found an article that talks about pumps and how to judge what your motor would need. It said the general rule of thumb is to take your GPH and times it by 10 and that would be how many horsepower the pump would support. The current pump is rated at 125 gph so, theoretically it would support 1250 hp...seems like a stretch to me but, at 125 gph I would think it would keep up with the demand I'm putting on it.