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Rainwater Harvesting Calculator
Estimate theoretical and usable roof rainwater, month-by-month tank storage, overflow and how much of your demand it could cover.
Smart result
Your estimate will appear here
Change any editable assumption, calculate, then review the steps, assumptions and limitations before deciding.
How this estimate works
Theoretical yield is roof area × rainfall (1 mm on 1 m² = 1 litre). Useful inflow applies your runoff coefficient and system efficiency. A month-by-month tank balance then supplies your demand and overflows above the tank size.
Formula
- Theoretical litres = area m² × rainfall mm
- Useful inflow = theoretical × runoff × efficiency
- Each month: storage = min(tank, storage + inflow − used), used = min(demand, storage + inflow)
Limitations
- You enter monthly rainfall; no location data is assumed.
- Monthly modelling ignores intense single-day storms, so real capture is usually lower.
- Water quality and treatment are not assessed.
Data used
No location rainfall dataset is deployed. Enter monthly rainfall from IMD or local records.
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Frequently asked questions
Why is usable water less than the theoretical yield?+
Runoff losses, first-flush and filter losses, and a full tank during heavy months all reduce what you can actually use.
How big should my tank be?+
Compare coverage with your tank against the unlimited-tank figure. If they differ a lot, a larger tank or a recharge pit may capture more.
How accurate is this rainwater harvesting estimate?+
It will provide an indicative estimate based on your inputs and the stated assumptions. Actual results can vary by location, equipment, tariff, and usage.
Can I use the result to make a purchase?+
Use it as a starting point, then confirm current prices, specifications, eligibility, and site requirements with a qualified provider.