Use the horizontal footprint feeding the downspout, rather than the total sloped area of the roof. If two gutters drain different roof sections, calculate the section that feeds the outlet you are planning around.
What the result means
One inch of rain over one square foot equals 144 cubic inches of water. A US gallon contains 231 cubic inches, so one inch over 1,000 square feet is about 623 gallons before losses. Some water may be retained or absorbed.
The same volume arriving in ten minutes or over several hours can place different demands on an inlet. Choosing pipe size requires rainfall intensity, route levels, obstructions and outlet conditions. Use the drainage guide to consider the complete path.