Maybe not the best place, but it's really a plumbing-type question. I'm designing an underground drainage to take downspout water away from the house and out towards the street (which is legal, in case you're wondering). The downspouts are 2x3. If my calculations are correct, for each downspout I would need a 3" diameter piece of pipe to carry away the max. downspout capacity. Alternatively, a 4" diameter pipe for two 2x3 downspouts. What I'm calculating is the cross-sectional area of each.

But, should I oversize the underground drains because they won't flow as fast? Do I need to expect that the full cross section of a 2x3 downspout will actually be filled? Don't want overkill, but also don't want too little.

Lazypup? Anyone?