How to Calculate Gutter Size for Roof

🏠 Choosing the right gutter size is essential to protect your home from water damage. Undersized gutters can overflow during heavy rain, causing issues like erosion, foundation cracks, and basement flooding.

In this guide, you’ll learn how to calculate the proper gutter size for your roof using rainfall data, roof dimensions, and a free calculator.

📐 What Affects Gutter Size?

To determine the ideal gutter size, you’ll need to know:

• Roof Area (sq ft) – how large the surface is
• Roof Pitch Factor – how steep your roof is
• Maximum Rainfall Intensity – usually in inches per hour, varies by location

Together, these determine the drainage load, or how much water your gutters need to handle.

🧮 Gutter Size Calculation Formula

The most common formula for sizing gutters is:

Effective Roof Area = Roof Area × Roof Pitch Factor

Drainage Requirement (in³/hr) = Effective Roof Area × Rainfall Intensity

Gutter Size is chosen based on this drainage volume

📏 Roof Pitch Factor Table

Roof Pitch	Factor
Flat to 3:12	1.0
4:12 to 5:12	1.05
6:12 to 8:12	1.10
9:12 to 11:12	1.20
12:12 and above	1.30

🔧 Example: Calculate Gutter Size

Let’s say you have:

• Roof Area: 1,000 sq ft
• Roof Pitch: 6:12
• Rainfall Intensity: 2 in/hr

Step 1:
Pitch Factor = 1.10 (from table)

Step 2:
Effective Area = 1,000 × 1.10 = 1,100 sq ft

Step 3:
Drainage = 1,100 × 2 = 2,200 in³/hr

Based on this, you’d choose a gutter size capable of handling this volume, such as a 5″ or 6″ K-style gutter.

🛠️ Gutter Sizing Tips

• If you have valleys or long roof sections, consider oversized gutters.
• Use downspouts every 30–40 feet for optimal flow.
• Install gutter guards to keep debris from blocking water flow.