Gutter Size Calculator
Free online gutter sizing tool. Enter your roof area and local rainfall intensity to get the right gutter size (5 or 6 inch) and recommended downspout count — essential for construction and remodeling.
🏗️ Gutter Type
📐 Roof Parameters
Horizontal projected area, not sloped surface area
Typical: 0.02-0.03; storm-prone: 0.04-0.06
1.2-1.5 recommended for debris and extreme weather
Length of one continuous gutter section
✅ Recommendation
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Required Flow = Roof Area × Rainfall Intensity × Safety Factor =
-- L/sDownspouts ≈ Gutter Length ÷ 10m =
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How to Choose the Right Gutter Size
Choosing the wrong gutter size leads to overflowing water, foundation damage, and structural issues. The three key factors are roof projected area, local rainfall intensity, and a safety margin. Most residential homes work well with 5-inch K-style gutters; larger roofs or storm-prone areas should upgrade to 6-inch.
K-style gutters are the most popular — they offer high flow capacity and structural strength. Half-round gutters have a classic traditional look but lower capacity. Box gutters are typically used in commercial buildings. This calculator compares all three types.
❓ FAQ
5-inch or 6-inch gutters — which do I need?
5-inch (125mm) works for most residential roofs up to ~150 m². 6-inch (150mm) is recommended for larger roofs, steep slopes, or high-rainfall regions. This calculator makes the recommendation automatically.
How do I find my local rainfall intensity?
Check your local weather bureau's rainfall intensity data. Typical values range from 0.02-0.04 L/s/m² in moderate climates to 0.05-0.07 in storm-prone regions. When in doubt, use a slightly higher value for safety.
How many downspouts do I need?
Plan one downspout per 10-12 meters (33-40 feet) of gutter run. Include downspouts at corners and at both ends of long runs. Downspout size typically matches gutter size — 5-inch gutters pair with 2×3-inch downspouts.