Gather some basic weather information to calculate the proper size for gutters and downspouts for a house. The total rainfall and how rapidly it falls will affect the runoff from the roof. An inch of rain is not much, but it if falls in half an hour it could flood a gutter. Total roof area also affects runoff. A roof of 1,200 square feet in size will collect more water than one that is 600 square feet. Pitch or steepness also is a factor. Water will run off faster from a very steep roof than from one with a gradual slope.
Things You'll Need
 Tape measure
 Rainfall data
 Calculator

Measure the roof to get the watershed area, the area from which water will run off. Multiply length (peak to eave) by width to determine square footage. Write down those dimensions. Note places where two roofs form a valley, which will affect runoff because water will come from two sides. Note logical places for downspouts, like building corners.

Find the rainfall intensity for your locale. Rainfall intensity is measured in a 5minute interval, translated to inches per hour. The Weather Bureau calculates and keeps these figures.Tables are available that show rainfall intensity for most metropolitan areas. Find out how often major storms occur  intensities that are exceeded only once in 10 years or once in 100 years. These tables are available, too. If your community does not have these, use the figures for the closest community.

Divide the inch per hour watershed of the roof by the 5minute rainfall intensity. That is how much rain a roof can drain during the most intense rainfall. That will show how much water a gutter will have to drain. Ask gutter suppliers what size gutters will be needed to handle that amount. Measure the length of the gutter for downspout locations. The number of downspouts will affect a gutter's ability to handle water. Increasing from one downspout to two doubles the capacity of a gutter to handle runoff without overflowing.

Size downspouts to fit the gutter. A square inch of downspout can drain 1,200 square feet of roof during a rain with an intensity of one inch per hour. Roof area, the locale's rainfall intensity and gutter capacity determine how many square inches of downspout are needed. Doubling the number of downspouts halves the required capacity for each. A round downspout is slightly more efficient in draining water than square or rectangular ones.
Tips & Warnings
 Many online resources provide rainfall figures and tables or formulas for calculating gutter and downspout size. A gutter supplier or building supply store should also have these tables.
 Gutter slope will affect drainage. A gutter should not slope by more than one inch in 17 feet. It will take two downspouts to drain a 34foot gutter.
References
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