Gutter Downspout Sizing Calculator

First find the peak runoff flow: the drained roof area times the rainfall intensity gives the volume per hour, which converts to gallons per minute. Then divide that flow by the capacity of one downspout to get the number of downspouts needed. Larger storms and bigger roofs need more or larger downspouts.

Use the design rainfall intensity for your location and a chosen storm return period and duration, which you can look up from the NOAA National Weather Service precipitation frequency data. It is location specific, not a universal number, so it is an input here. Designers commonly use a 5-minute intensity for a 100-year storm for roof drainage.

One square foot of roof under one inch per hour of rain sheds about 0.0104 gallons per minute, because a square-foot inch is 0.623 gallons spread over an hour. Multiply the drained area by the intensity in inches per hour by 0.0104 to get the peak flow in gallons per minute reaching the gutters.

Downspout capacity depends on its size and shape (for example a 2-by-3-inch or 3-by-4-inch rectangular spout, or a round leader) and on the gutter slope and outlet. Capacities are published in plumbing and roof-drainage standards, so enter the rated gallons-per-minute capacity for your downspout rather than assuming a single value.

Gutters only collect the roof area that drains to them. A long ridge may split runoff between front and back gutters, so each gutter handles roughly half. Enter the area that actually drains to the run of gutter you are sizing, which may be a slope or a section of the roof rather than the whole footprint.