Gutter Size Calculator
Size each gutter run for the roof area that drains into it, the roof pitch and how hard it rains where you live.
Gutter and downspout sizing
How the sizing works
| Roof pitch | Pitch factor |
|---|---|
| Flat to 3/12 | 1.00 |
| 4/12 to 5/12 | 1.05 |
| 6/12 to 8/12 | 1.10 |
| 9/12 to 11/12 | 1.20 |
| 12/12 and up | 1.30 |
Steeper roofs catch more wind-driven rain, so they get a larger factor. Widely published capacities for level K-style gutters are about 5,520 sq ft of design area at 1 in/hr for a 5-inch gutter and 7,960 sq ft for a 6-inch gutter; the calculator divides those by your rainfall intensity. Capacities assume one outlet per run; a downspout at each end, or in the middle, splits the area and roughly doubles what a run can handle.
Where to find rainfall intensity
Use NOAA's Precipitation Frequency Data Server: click your location and read the 5-minute duration value (shown as inches; multiply a 5-minute depth by 12 for in/hr). Many designers use the 10-year return period for residential gutters. Values in the US range from about 3 in/hr in the dry West to 8 in/hr or more along the Gulf Coast.
Worked example
Downspouts
A common rule of thumb is 1 square inch of downspout opening per 100 sq ft of roof: a 2×3 inch downspout (≈6 sq in) per 600 sq ft, or a 3×4 inch (≈12 sq in) per 1,200 sq ft. Space downspouts no more than about 30 to 40 ft apart, and slope gutters toward them about 1/4 inch per 10 ft.
Frequently asked questions
Should I get 5 inch or 6 inch gutters?
5-inch K-style gutters handle most houses. Go to 6-inch for large or steep roof areas draining to one run, in heavy-rain regions, or where valleys concentrate water.
How many downspouts do I need?
At least one per gutter run, and roughly one 2×3 downspout per 600 sq ft of roof or one 3×4 per 1,200 sq ft, spaced no more than about 30 to 40 feet apart.
Do I measure roof area along the slope for gutters?
No. Use the plan (flat) area that drains to the gutter, then apply the pitch factor.
Last reviewed 2026-10-10. Results are estimates for planning; confirm quantities with your supplier and requirements with your local building code and the manufacturer's instructions.