About This Calculator
This calculator helps you size gutters and downpipes based on your roof area, local rainfall intensity, and gutter run length. It checks whether your chosen gutter profile has sufficient flow capacity per BS EN 12056-3.
UK Gutter Profiles
| Profile | Flow Capacity |
|---|---|
| 75 mm Half Round | 0.38 l/s |
| 100 mm Half Round | 0.78 l/s |
| 125 mm Half Round | 1.37 l/s |
| 150 mm Half Round | 2.16 l/s |
| 100 mm Ogee | 0.90 l/s |
Tips
- • Use at least one downpipe per 12 m of gutter run
- • Gutter fall should be 3 mm per metre towards the downpipe
- • Add 5% wastage for cuts and joints
- • Consider larger profiles for high-rainfall areas
Ready to Calculate?
Enter your roof area and gutter details to get sizing recommendations.
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CACalifornia Building Code (Title 24)
References & Sources
Standards Cited
- IPC / UPC — International/Uniform Plumbing Code
Gutter and downpipe sizing for roof drainage
Formulas Used
- Required flow rate:
Q = roofArea × rainfallIntensity / 3600
Source: IPC / UPC — Q in litres/second, area in m², rainfall in mm/hr - Number of brackets:
brackets = ceil(gutterRun / bracketSpacing) + 1
Source: IPC / UPC
Key Assumptions
- Design rainfall: varies by location (default 100 mm/hr / ~4″/hr) — IPC / UPC
- K-Style gutters: 5″ and 6″ standard profiles — Industry standard
- Bracket spacing: 30″ (762 mm) maximum — Industry standard