Gutter Size Calculator

Hydraulic Sizing & SMACNA Gutter Drainage Metrology

Gutter sizing requires matching catchment runoff rate to gutter carrying capacity:

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• SMACNA Adjusted Drainage Area Formula**:

A_adj = Roof Footprint Area (sq ft) × Pitch Factor

• Flat to 3/12: Pitch Factor = 1.00

• 4/12 to 5/12: Pitch Factor = 1.05

• 6/12 to 8/12: Pitch Factor = 1.10

• 9/12 to 11/12: Pitch Factor = 1.20

• 12/12+: Pitch Factor = 1.30

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• Maximum Gutter Capacity Matrix (per Downspout)**:

Max Area (5" K-Style) = (5,500) / (Rainfall Intensity) sq ft

Max Area (6" K-Style) = (7,900) / (Rainfall Intensity) sq ft

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• Downspout Sizing Rules**:

• 2" × 3" Rectangular: Drains up to 600 sq ft at 4 in/hr

• 3" × 4" Rectangular: Drains up to 1,200 sq ft at 4 in/hr

• 4" Round Corrugated: Drains up to 1,250 sq ft at 4 in/hr

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• Standard Sizing Benchmarks**:

• < 1,375 sq ft per spout: 5" K-Style Gutter

• 1,375 - 1,975 sq ft per spout: 6" K-Style Gutter

• > 1,975 sq ft per spout: 7"/8" Commercial Gutter (or add more downspouts)

How to Use This Calculator

Enter your Roof Drainage Catchment Area (e.g. 1200 in sq ft), Roof Pitch Slope — choose from Flat to 3/12 Pitch (Pitch Factor = 1.00), 4/12 to 5/12 Pitch (Pitch Factor = 1.05), 6/12 to 8/12 Pitch (Standard - Pitch Factor = 1.10), Maximum Design Rainfall Intensity (SMACNA) — choose from 3.0 in/hr (Pacific Northwest / Moderate Climate), 4.0 in/hr (US Midwest & Northeast Standard 100-Year Storm), 5.0 in/hr (US Southeast & Mid-Atlantic Heavy Thunderstorm), Planned Downspouts along Gutter Run — choose from 1 Downspout (Short run ≤ 35 ft), 2 Downspouts (Dual ends / Standard run 35-70 ft), 3 Downspouts (Long run 70-100 ft), and Hydraulic Drainage Benchmark — choose from Custom Hydraulic Design, Suburban Roof Face (1,200 sq ft, 6/12, 4 in/hr, 2 Drops) → 5" K-Style Gutter, Steep Large Roof (2,500 sq ft, 10/12, 5 in/hr, 2 Drops) → 6" K-Style Gutter. Measure twice before entering dimensions — small measurement errors scale up significantly in material estimates.

The Formula

Peak Discharge in GPM: Q = (Area_sqft Rain_in_hr 0.0104) / 60 ... 1 in/hr on 1 sqft = 0.0104 gpm

Maximum drainage capacity at rainIntensity for 5" K-Style = 5,500 / rainIntensity sq ft

Maximum drainage capacity for 6" K-Style = 7,900 / rainIntensity sq ft

Worked Example

A homeowner with a 2,500 sq ft steep roof in an area with 5.0 in/hr storms calculates that each of their 2 downspouts must drain 1,500 sq ft, confirming that 6-inch K-style gutters and 3x4 downspouts are required to avoid overflow.

Understanding Your Result

Construction estimates are starting points — site conditions, material specifications, and local code requirements will affect final quantities and costs. Always obtain at least three contractor bids for major projects. Local building departments can confirm permit requirements and inspection milestones. Add waste factors appropriate to the material type and complexity: 10% for simple rectangular shapes, 15–20% for complex geometries or custom cuts.