Gutter Sizing Calculator

Undersized gutters overflow during heavy rain, causing fascia rot, foundation erosion, and landscape damage. This calculator uses the Effective Drainage Area (EDA) method from the International Plumbing Code (IPC) to determine the right gutter profile and downspout count for your roof.

Roof and Rainfall Data

Gutter Run Length ft

Rafter Length (eave to ridge) ft

Roof Pitch

Max Rainfall Intensity

Outlet Position

Recommended Gutter Size

Based on IPC Table 1106.2 gutter capacity guidelines. Actual installation should account for leaf guards, ice dams, and gutter slope (minimum ¼ inch per 10 ft). Consult local code for exact requirements.

Effective Drainage Area (EDA)

The Effective Drainage Area accounts for the fact that wind-driven rain hits pitched roofs at an angle, increasing the effective collection area:

EDA = Gutter Run (ft) × Rafter Length (ft) × Pitch Correction Factor

The pitch correction factor increases with steepness because a steeper roof presents more surface area to wind-driven rainfall:

Pitch	Correction Factor
Flat (0/12)	1.0
3/12	1.05
4/12	1.12
6/12	1.2
8/12	1.3
10/12	1.4
12/12	1.5

Gutter Capacity by Size (IPC Table 1106.2)

Gutter Size	Max Drainage Area (4 in/hr rainfall)
5-inch K-Style	Up to 600 ft²
6-inch K-Style	Up to 1,440 ft²
5-inch Half-Round	Up to 480 ft²
6-inch Half-Round	Up to 960 ft²
7-inch Commercial	Up to 2,760 ft²

Frequently Asked Questions

How many downspouts do I need?

One standard 2×3 inch downspout handles approximately 600 square feet of drainage area at 4 in/hr rainfall. A 3×4 inch downspout handles up to 1,200 square feet. Divide your total EDA by the downspout capacity and round up.

What size gutters for a 2,000 sq ft roof?

A 2,000 sq ft roof (after pitch correction) in a 4 in/hr rainfall zone needs 6-inch K-style gutters with at least 2–3 downspouts. In heavy rainfall areas (6+ in/hr), you may need 7-inch gutters or additional downspouts.