I want to install pvc below grade at my downspouts. There will be several downspouts that run into the pvc trunk line via t connectors. Trying to find out how much water 4" and 6" pvc will carry to see if I can start with 4" then down the line as more downspouts are connected to the system step up to a 6" to carry the water away from my home.

Thanks.
Mike

Chances are, you will never come up with all the data you need for actually calculating what you need. I can say the 6'' pipe will carry about 5 times as much water as the 4'' one at the same pressure drop at moderate flow rates. So you plan should work with a little guess work on where to go up to the 6''. For the average house, the 6'' may be over kill.

Thanks the quick response. It's a fairly large house so we'll just go for the 6".

Thanks again!