Calculator Roof ROI Calculator How Much Rainwater Could You Collect? Answer two questions to get and estimate of how much rainwater you could potentially harvest each month. Please enable JavaScript in your browser to complete this form.Roof Area (m2)CitySelect a CityAucklandHamiltonTaurangaCoromandelNapierPalmerston NorthWellingtonNelsonChristchurchQueenstownWanaka Average litres of rainwater captured per month: 0 litres Calculate