Drainage Calculator

Recommended Channel Drain:

Disclaimer: This calculator provides an indicative sizing guide only. It uses simplified assumptions based on typical rainfall intensities and runoff coefficients for major New Zealand centres. Actual drainage design must comply with NZ Building Code (E1) and AS/NZS 3500.3:2021 requirements.

Design rainfall intensities are generally based on 10-minute duration storms for roof and surface drainage. Typical figures for a 10-year ARI, 10-min duration are:

• Auckland: ~95–110 mm/hr
• Wellington: ~105–120 mm/hr
• Christchurch: ~75–85 mm/hr

These values are drawn from NIWA’s HIRDS database and reflected in NZBC E1 and AS/NZS tables. BRANZ guidance suggests ~100 mm/hr for most regions as a practical design figure, with significantly higher values for alpine/high-rainfall zones (e.g., Arthur’s Pass, Fiordland). This calculator does not account for alpine or extreme rainfall zones and uses the upper end of typical figures for major centres.

Important: Rainfall intensity drives peak flow in the Rational Method. Underestimating it can lead to undersized drainage systems. If you are working to NZ compliance or detailed engineering design, you should:

• Reference NIWA HIRDS v4 or AS/NZS 3500.3 Table E.1 for accurate intensities.
• Consider return periods (e.g., 10-year vs 50-year) and storm durations.
• Consult a qualified specifier or civil engineer for site-specific design.