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Gutter Size Calculator

Size gutters and downspouts from roof area and rainfall intensity. Calculate required gutter capacity, downspout count, and K-style or half-round sizing.

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Inputs that matter
Roof Drainage Area, Rainfall Intensity, Gutter Run Length
Output to expect
Gutter Size Calculator
  • Check the units and required inputs before comparing results.
  • Keep the assumptions with a copied result so you can reproduce the calculation later.
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Reference & details

How it works

Rainfall Runoff Rate

Runoff equals rainfall intensity (in/hr) times drainage area (sq ft) times runoff coefficient. Roofs use C near 0.95.

Q = C × i × A (C ≈ 0.95 for roof)

Gutter Capacity

Manufacturer capacity tables relate gutter profile to max drainage area at standard rainfall rates.

5″ K-style ≈ 5,520 sq ft @ 1″/hr; 6″ ≈ 7,960 sq ft

Downspout Sizing

Each 2×3″ downspout drains roughly 600 sq ft at moderate rainfall. Undersized downspouts overflow gutters even if gutter profile is adequate.

1 downspout per 600–800 sq ft (2×3″); 2×3″ = 600 sq ft max

Updated: July 2026

Example Scenarios

At 1″/hr design storm, 5″ K-style gutter handles up to ~5,500 sq ft — adequate with 2 downspouts at 600 sq ft each.

Requires 6″ gutter or dual 5″ runs with 4 downspouts minimum to prevent overflow during peak storms.

Small tributary area ties into existing main gutter — verify combined capacity or add dedicated downspout.

FAQ

5″ K-style aluminum is standard for residential roofs under ~5,500 sq ft per slope at moderate rainfall. Larger roofs or heavy rain regions need 6″ gutters.

Rule of thumb: one 2×3″ downspout per 600 sq ft of roof area, maximum 40 ft gutter run between downspouts. Increase count in high-rainfall regions.

Use local code or NOAA 100-year 1-hour rainfall intensity for your area. Many US locations use 1–2 inches per hour for residential sizing.

K-style gutters carry more water per inch of width due to profile shape. A 5″ K-style exceeds 5″ half-round capacity.

Steep roofs deliver water faster to gutters, increasing peak flow. Size gutters for tributary area and intensity — steep pitch may require extra downspouts.

About Gutter Size Calculator

Size residential gutters and downspouts using roof catchment area and local rainfall intensity. Enter tributary roof area and design storm rate to determine minimum gutter profile and number of downspouts needed.