About the Gutter Size Calculator

Gutters must move the water that falls on the roof they serve. The load calculation starts with the roof area draining into each run, adjusts upward for steeper pitch (faster runoff), and applies the local rainfall intensity — a 1 in/hr storm deposits roughly 0.0104 gallons per minute per square foot of roof. Sum the flow and compare it to gutter capacity: about 7.4 gpm for a level 5-inch K-style and 12 gpm for a 6-inch.

Beyond gutter width

Size alone isn't the whole story. Downspout count and placement matter just as much — each downspout resets a run's capacity, and runs over 30–35 feet overwhelm a single outlet even in modest rain. Sloping the gutter a quarter-inch per 10 feet toward the outlet measurably boosts capacity, and keeping gutters clean is non-negotiable: a clogged 6-inch gutter behaves like a 2-inch one. Sizing correctly at install prevents the most common homeowner complaint — water pouring over the fascia in heavy rain.