How to Use the Gutter Capacity Calculator
An undersized gutter overflows during a downpour, sending water down your siding or back under the roofline. Oversizing wastes money, so it helps to calculate the drainage flow your roof actually produces before picking a gutter size.
One inch of rain over one square foot of roof produces about 0.623 gallons of water. So roof area (ft²) × rainfall (in/hr) × 0.623, divided by 60, gives the flow in gallons per minute your gutters need to handle. We compare that against typical capacities for 4, 5, and 6-inch K-style gutters.
Keep in mind that leaves and debris reduce real-world capacity over time, and downspout placement matters too — a single downspout at one end of a long run can leave the far end backing up even if the gutter itself is sized correctly.
Frequently Asked Questions
It's safest to use your area's peak hourly rainfall from the last 10 years if you can find it. If not, a typical heavy-storm design value of around 4 in/hr works as a reasonable default in most of the US.
Yes, sizing up is generally recommended. Leaves and debris reduce effective capacity over time, and choosing a larger size gives you a safety margin for unusually heavy storms.