How do I calculate the size of the downspout I need for a gutter?

Question

From what I've read, a 2x3 rectangular downspout is meant to drain 600 square feet of roof. But surely that depends on the rainfall intensity, doesn't it?

600 square feet of roof in Idaho is going to be a hell of a lot different than 600 square feet of roof in Hawaii.

I have 385 square feet of roof which will be handled by 25 linear feet of 5" K-style gutter, in an area that receives a maximum rainfall intensity of 6 inches / hour over a 5 minute period [chart]. Will a single 2x3 downspout be sufficient? If not, will I have to upgrade to 6" gutters in order to accommodate a larger 3x4 downspout, or will a 3x4 downspout still fit on a 5" K-style gutter?

Answer 1

Size the gutters for the rainfall, but go with the largest downspout that will fit your gutters.

The issue for downspouts isn't water capacity in a torrential rain, it's debris that gets washed into the downspout. The larger the downspout, the better debris will wash through. Large downspouts are harder to clog, pass more water when they do clog, and are easier to clean out.