Gutter Size Calculator

5 in or 6 in Gutter, Downspout Count, Hangers & Slope

Gutters are not sized by roof area alone — they are sized by an adjusted drainage load that accounts for how fast water arrives. The Gutter Size Calculator multiplies the roof area feeding each run by a pitch factor, because a steep roof throws water harder and can overshoot a small gutter, then by your local design rainfall intensity in inches per hour, and compares the result to the published capacity of 5 inch and 6 inch K-style gutter. It returns the recommended gutter size, the number of downspouts needed for either 2x3, 3x4 or 4 inch round outlets, the roof area draining to each outlet, the hanger count at 24 inch centres, and the slope drop to set at roughly one sixteenth of an inch per foot. As a rule, adding a downspout is cheaper and more effective than upsizing the whole system, and 6 inch gutter with 3x4 outlets is the right call under heavy tree cover.