What size gutters do I need for my house?

For most residential homes, 5-inch K-style gutters are standard. However, if your roof area exceeds 600 sq ft per downspout, or you live in a high-rainfall area, 6-inch gutters are recommended. Homes under 400 sq ft per section can use 4-inch gutters.

How many downspouts do I need?

A standard rule is one downspout for every 30-40 feet of gutter run, or every 600-800 square feet of roof area. In heavy rainfall areas (>6 inches/hour), reduce this to every 25 feet or 400 sq ft.

What is the difference between K-style and half-round gutters?

K-style gutters have a flat back and decorative front that resembles crown molding. They hold more water than half-round gutters of the same width. Half-round gutters are traditional, often copper, and used on historic homes. A 5-inch K-style holds about the same as a 6-inch half-round.

How do I calculate roof drainage area?

Measure the footprint of the roof section that drains to one gutter. For a gabled roof, this is typically (Length × Width) / 2 per side. For complex roofs, measure each plane separately. Include overhangs in your measurement.

What is a 100-year storm?

A 100-year storm refers to rainfall intensity that has a 1% chance of occurring in any given year. In the US, this ranges from 3-4 inches/hour (arid regions) to 10+ inches/hour (Gulf Coast, Hawaii). Most residential gutters are sized for 5-7 inches/hour.

💧 Gutter Size Calculator

Calculate Gutter Capacity for Rainfall

Roof Area (sq ft)

Total drainage area per gutter section

Rainfall Intensity (in/hour)

Roof Pitch

Affects runoff coefficient

Gutter Length (feet)

Length of one gutter run

💧 Gutter Sizing Guide

Properly sized gutters prevent overflow, ice dams, and foundation damage. Undersized gutters are a common cause of water intrusion and basement flooding.

Gutter Capacity Formula

The primary factors are:

Roof Area: More surface = more water collection
Rainfall Intensity: Measured in inches per hour during peak storm
Roof Pitch: Steeper roofs concentrate water faster (higher coefficient)

GPM = (Roof Area × Rainfall × Pitch Factor) / 96.3

Standard Gutter Sizes

Size	Max Roof Area (5" rain)	Max Flow (GPM)	Typical Use
4" K-Style	~400 sq ft	~20 GPM	Small homes, sheds
5" K-Style	~600 sq ft	~30 GPM	Standard residential
6" K-Style	~1,200 sq ft	~50 GPM	Large homes, high rainfall
7" K-Style	~2,000 sq ft	~80 GPM	Commercial, extreme conditions

💡 From Dan Williams, Gutter Installation Expert (20 years):

"Don't skimp on downspouts! I see homeowners install 6-inch gutters but only put one downspout on a 50-foot run. That's asking for overflow. My rule: One 3x4 downspout every 30 feet, MAX. In heavy rain areas, every 20 feet. Also, never trust 'one size fits all'—I've seen Pacific Northwest homes with 7-inch gutters because they get 10+ inches of rain in November."

⚠️ Common Mistakes

1. Ignoring Roof Pitch: A steep 10:12 pitch sheds water twice as fast as a 4:12. You need larger gutters or more downspouts even with the same roof area.

2. Undersizing Downspouts: A 6-inch gutter with a 2x3 downspout is like a fire hose with a garden hose nozzle. Use 3x4 or 4x5 downspouts for 6-inch gutters.

3. Using National Averages in Extreme Climates: A 5-inch gutter works in Denver (3 in/hr rainfall). It will FAIL in Miami (8+ in/hr). Always check local rainfall data.

Reviewed by Dan Williams

Professional Gutter Installer & Drainage Specialist, 20 years experience in residential and commercial systems.

💧 Gutter Sizing Guide

Gutter Capacity Formula

Standard Gutter Sizes

⚠️ Common Mistakes

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