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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UKUK Building Regulations
References & Sources
Standards Cited
- Approved Document H — Drainage and waste disposal — H3 rainwater drainage
H3 Table 1 effective-area pitch factors and Table 2 gutter/outlet sizing - BS EN 12056-3 — Gravity drainage systems inside buildings — Roof drainage, layout and calculation
Gutter sizing, flow capacity, and downpipe design
Formulas Used
- UK effective roof area:
A_effective = A_plan × (1 + 0.5 × tan(roofPitch))
Source: Approved Document H H3, Table 1 pitch-factor anchors — Reproduces the published factors 1.29 at 30°, 1.50 at 45°, and 1.87 at 60°. Over 70°, Table 1 requires an elevational-area method, so this plan-area calculator fails closed. - Required flow rate:
Q = effectiveRoofArea × rainfallIntensity / 3600
Source: Approved Document H / BS EN 12056-3 — Q in litres/second, area in m², rainfall in mm/hr - Number of brackets:
brackets = ceil(gutterRun / bracketSpacing) + 1
Source: BS EN 12056-3
Key Assumptions
- Design rainfall: 75 mm/hr for UK (BS EN 12056-3) — BS EN 12056-3
- Half-round and ogee profiles available in 75-150 mm widths — Industry standard
- Bracket spacing: 900 mm maximum — BS EN 12056-3