Five-inch K-style gutters or 6-inch half-rounds, the most common residential sizes, are able to handle the rainfall on most houses in most parts of the country. But houses with big, steep roofs or those located in climates prone to heavy downpours may need wider gutters and extra downspouts to keep rainwater from overflowing.
To figure out what size gutters you need, first you’ll need to calculate the square footage of the gutter’s drainage area. For a simple gable-end roof, you would only need to make two calculations, one for each slope. Hip roofs and intersecting roofs have multiple facets, and for those you’ll need to add up the area (length x width) of each surface within a drainage area to get the total square footage.