Roof Gutter and Downspout Sizing Calculator

Explanation

Gutter sizing is based on how much water a roof can shed during a peak rainfall event. The key inputs are the projected roof area, the local rainfall intensity, and a pitch factor that scales the area for slope. The runoff rate is expressed in gallons per minute (GPM). Once the runoff rate is known, you can compare the flow to typical gutter capacities and choose an appropriate gutter size and number of downspouts.

This calculator uses a common conversion: 1 inch of rain over 1 square foot equals 0.623 gallons per hour. Dividing by 60 converts to gallons per minute, which yields the 0.0104 multiplier. The pitch factor slightly increases the effective roof area for steeper slopes. It does not replace a full roof drainage design, but it provides a strong estimate for residential and light commercial planning. Downspout count is estimated by dividing total runoff by a typical downspout capacity, and rounding up to ensure adequate drainage.

Because roof geometry and local code requirements vary, always verify with manufacturer sizing charts and plumbing or building codes. Valleys, long gutter runs, and debris-prone areas may require additional capacity or larger downspouts. Use this calculator as a planning tool to compare options and avoid undersized gutters that can lead to overflow, fascia damage, and foundation moisture issues.

If your roof drains into multiple gutter runs, divide the roof area by the number of drainage zones and run the calculation for each. This yields a more accurate downspout count for each run. Also consider climate factors such as snowmelt or ice dams, which can alter runoff behavior and may justify larger gutters or extra downspouts even if the rainfall intensity is moderate. For long eaves, add outlets to prevent mid-run overflow and standing water. Oversized gutters are cheaper than water damage repairs.

Formula

Runoff rate (imperial): GPM = area x intensity x 0.0104 x pitchFactor

Gutter size: 5 in if GPM <= 45, 6 in if GPM > 45

Downspouts: count = ceil(GPM / 30)

Example Calculation

A 1,500 sq ft roof with 2 in/hr rainfall and a 1.05 pitch factor yields about 32.8 GPM of runoff. The tool recommends 5 inch gutters and 2 downspouts at 30 GPM capacity each.

FAQs

What rainfall intensity should I use?

Use local design rainfall or a common value like 2 to 5 inches per hour for planning.

Are 6 inch gutters always better?

Larger gutters handle more flow but can be more expensive and visually larger on the fascia.

Do longer runs need more downspouts?

Yes. Long runs increase flow concentration and should be broken up with additional downspouts.

How accurate is the downspout estimate?

It uses a typical capacity. Adjust for downspout size and local guidelines.

Does this include overflow guards?

No. Guards and leaf screens can reduce capacity and should be considered separately.

How it works

This tool converts rainfall intensity and roof area into GPM, then recommends gutter size and downspout count based on flow thresholds. All calculations run client-side.

Disclaimer

This tool provides a simplified estimate. Confirm gutter sizing with local code and manufacturer capacity charts.