Gutter Calculation Formulas
Capacity Check: Gutter Capacity ≥ Required Flow Rate
Downpipes: ceil(Gutter Length ÷ 12) — one per 12m run
Note: Gutter capacities are nominal values. Actual capacity depends on fall, outlet position, and installation. Check local rainfall intensity for your area.
Gutter Calculator — Gutter Sizing & Downpipe Calculator
AS/NZS 3500.3 requires all roof drainage to be designed for 100-year ARI rainfall. With intensities ranging from 150mm/hr in southern Victoria to 300mm/hr in tropical Queensland, correct gutter and downpipe sizing prevents overflow and foundation damage.
Gutter Profile Capacities
| Profile | Width | Capacity (L/s) |
|---|---|---|
| Quad 115mm | 115mm | 1.8 |
| Quad 125mm | 125mm | 2.4 |
| Quad 150mm | 150mm | 3.6 |
| Half Round 125mm | 125mm | 2.2 |
| Square Line 125mm | 125mm | 3.0 |
| Box Gutter | Various | 4.0+ |
Downpipe Capacities
| Size | Capacity (L/s) | Max Catchment (m²) |
|---|---|---|
| 65mm Round | 1.0 | 18 |
| 75mm Round | 1.5 | 27 |
| 90mm Round | 2.2 | 40 |
| 100mm Round | 3.0 | 54 |
Flow Rate Formula
Flow Rate (L/s) = Catchment Area (m²) × Rainfall Intensity (mm/hr) / 3600.
Frequently Asked Questions
What size gutter do I need?
Calculate catchment area × rainfall intensity / 3600 for flow rate. Match to gutter capacity. A 60m² Sydney catchment: 60×200/3600 = 3.3 L/s needs Quad 150mm.
How many downpipes do I need?
Divide total flow by downpipe capacity. 3.3 L/s with 90mm pipes (2.2 L/s): 1.5 → 2 downpipes.
What is ARI 100?
Average Recurrence Interval of 100 years — the design rainfall intensity standard for roof drainage under AS/NZS 3500.3.
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