Online Tool Store
Gutter Sizing Calculator
Calculate required roof gutter dimensions and downspout capacity based on roof catchment area, pitch multiplier, and local design rainfall intensity.
🔒 This tool runs entirely in your browser. Your files are never uploaded to a server.
Effective Catchment Area
Recommended Gutter Size
Downspout Requirement
How it works
- Enter your roof eave length and horizontal run width in meters or feet.
- Select the roof pitch slope multiplier (flat, standard, medium, or steep).
- Choose the design rainfall intensity rating for your local regional climate.
- Select the number of downspouts and preferred gutter profile to view recommended gutter width and downspout diameters.
The formula
Effective Catchment Area (m²) = Length (m) × Width (m) × Pitch Factor
Peak Runoff Flow Rate Q (L/s) = (Effective Area in m² × Rainfall in mm/hr) / 3,600
FAQ
How is effective roof catchment area calculated?
Effective catchment area equals the plan length multiplied by the roof run width, multiplied by a pitch slope factor (1.0 to 1.25). Steep roofs catch driving windblown rain, effectively increasing the volume of runoff entering the eaves trough.
When should I choose 5-inch vs 6-inch gutters?
Standard 5-inch K-style or 112mm half-round gutters handle roof catchment areas up to 100 m² (approx 1,100 sq ft) with standard rainfall. For steep roofs, large roof runs, metal roofs, or heavy downpour zones, 6-inch (150mm) gutters prevent overflow and water damage.
How many downspouts are required for a gutter run?
As a rule of thumb, one downspout is recommended for every 10 to 12 meters (30 to 40 feet) of continuous gutter run, or for every 50 to 80 m² of drained roof area.
What is design rainfall intensity?
Design rainfall intensity is the peak precipitation rate (measured in mm/hr or inches/hr) expected during a 5-year or 10-year recurring storm event in your geographic area.
How we compare
| Feature | Online Tool Store | Gutter contractor charts | Generic area tables |
|---|---|---|---|
| Peak runoff flow rate (Liters/second) | ✓ | ✗ | ✗ |
| Pitch multiplier & windblown rain adjustment | ✓ | ✓ | ✗ |
| Multi-downspout distribution balancing | ✓ | Partial | ✗ |