What rainfall intensity should I use?

Use the 5-minute peak rainfall rate for your area (also called design rainfall intensity). In the continental US this ranges from about 2 in/hr in the arid Southwest to over 8 in/hr in South Florida. NOAA's Atlas 14 or your local building code provides the value. If unsure, 4 in/hr is a safe default for most of the US.

What is the roof pitch factor?

Steep roofs concentrate runoff faster than flat ones, so engineers apply a pitch multiplier to the plan area to get the effective drainage area. For pitches up to 3:12 use 1.0; 4:12 to 6:12 use 1.05; 7:12 to 11:12 use 1.1; 12:12 and above use 1.2. The calculator lets you enter it directly.

What gutter sizes are standard?

Residential gutters come in 4-inch (K-style or half-round), 5-inch, and 6-inch widths. A 5-inch K-style gutter is the most common choice; 6-inch is used on large roofs or in high-rainfall areas. Downspouts are typically 2×3 in or 3×4 in (rectangular) or 3-inch or 4-inch round.

How many downspouts do I need?

Place a downspout at least every 20–40 feet (6–12 m) of gutter run and at every inside or outside corner. More downspouts reduce the peak load on each section. The calculator shows the minimum gutter capacity — adding downspouts lets smaller gutters handle the same roof.

Can I use this for metal or flat roofs?

Yes. Enter the horizontal plan area of the drainage zone and use a pitch factor of 1.0 for flat or near-flat roofs. For standing-seam metal roofs with no valleys you can treat the entire roof as a single drainage zone.

Gutter Size Calculator

Name: Gutter Size Calculator
Availability: InStock
Author: Nham Vu

Enter your roof area and local rainfall intensity to get the recommended gutter width and downspout size.

Roof & Rainfall Inputs

Units:

Roof drainage area (sq ft)

Horizontal plan area of the roof draining to this gutter.

Peak rainfall intensity (in/hr)

US default ~4 in/hr. Check NOAA Atlas 14 or local code.

Roof pitch factor

Steeper pitches shed water faster; multiplier accounts for this.

Gutter run length (ft)

Length of the gutter from end to end (used for downspout spacing).

Enter your roof and rainfall data, then click Calculate.

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Summary

Enter your roof area and local rainfall intensity to get the recommended gutter width and downspout size.

How it works

Measure your roof drainage area in square feet or square meters.
Enter the peak rainfall intensity for your region (inches per hour or mm/hour).
Optionally adjust the roof pitch factor — steeper roofs shed water faster.
The calculator determines the effective drainage area and required gutter capacity.
Results show the minimum gutter width and downspout diameter to handle the load.
Add one downspout per 20–40 ft (6–12 m) of gutter run for best performance.

Use cases

Size gutters when building or reroofing a home.
Verify existing gutters are adequate for your climate.
Plan downspout count and placement for a long gutter run.
Compare 4-inch vs 5-inch vs 6-inch gutters for a specific roof.
Size gutters for a garage, shed, or outbuilding.
Check whether a steep roof pitch requires upsized gutters.
Estimate gutter capacity before a storm season.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu