Ecology & Environment
Rainwater Harvesting Calculator
Estimate annual roof runoff after collection efficiency, equivalent tank fills, annual demand days represented, and annual entered demand. The roof-to-tank diagram distinguishes gross rain, captured water, storage capacity, overflow exposure, and demand coverage.
Annual harvest potential (L)-
Equivalent tank fills-
Annual demand days represented-
Annual entered demand (L)-
Decision view
Roof rainfall to tank and demand
Roof rainfall to tank and demandGross roof rainfall passes through collection losses into storage, while annual demand and tank throughput remain separately visible.
Exact scenario comparisonAnnual rainfall (mm) changes while all other entered assumptions remain constant.
| Annual rainfall (mm) | Annual harvest potential (L) | Equivalent tank fills | Annual demand days represented | Annual entered demand (L) |
|---|
How to use Rainwater Harvesting Calculator
- Measure the effective roof catchment area draining to the tank rather than total building floor area.
- Use representative local rainfall and a collection efficiency that reflects roof losses, first flush, gutter loss, and system condition.
- Compare annual potential with tank capacity and a monthly water-balance model before selecting storage.
Calculator guide
Understanding Rainwater Harvesting Calculator
Annual rainfall volume describes collection potential, not guaranteed usable supply. Tank size, rainfall timing, demand timing, first-flush diversion, overflow, and water-quality rules determine how much of that potential can actually be used.
Calculation method
How the calculation works
Use one liter per square meter per millimeter of rain and apply collection efficiency to estimate harvest potential. One millimeter of rain on one square meter of roof equals one liter of gross water. Multiply roof area by annual rainfall and collection efficiency for annual harvest potential. Divide by storage for equivalent fills and by daily demand for demand-day equivalent.
Storage design
Why annual yield cannot size the tank by itself
Storage bridges the timing gap between rainfall events and water use.
Equivalent tank fills describe throughput potential; they do not prove that a particular tank size meets reliability goals.
Worked situations
Practical examples
- A 180 m2 roof receiving 850 mm annually has 153,000 L of gross roof rainfall.
- At 82% collection efficiency, modeled harvest potential is 125,460 L per year.
- A 12,000 L tank could theoretically cycle more than ten times, while 250 L/day demand equals 91,250 L per year.
Better inputs
Useful tips
- Use monthly or daily rainfall data when drought resilience matters; annual totals can hide long dry periods.
- Reserve unusable dead storage and account for first-flush diversion when sizing usable tank volume.
- Separate potable and non-potable demand and follow local treatment, plumbing, overflow, and backflow rules.
Before relying on the result
Limitations and common mistakes
- The annual model does not simulate storm timing, tank state, overflow, drought, evaporation, leakage, or demand seasonality.
- Collection efficiency is one blended assumption and may change with rainfall intensity and maintenance.
- The page does not assess structural loading, water quality, treatment, mosquito control, permits, or potable-water safety.
Reference
Key terms
- Catchment area
- Effective roof area that drains toward the collection system.
- Collection efficiency
- Share of gross roof rainfall assumed to reach usable storage.
- First flush
- Initial runoff diverted to reduce debris and contaminants entering storage.
- Equivalent tank fills
- Annual harvest potential divided by nominal tank capacity; not a count of actual complete fills.
Important note
Calculated from the entered environmental values using the displayed model. Measurement quality, local conditions, system boundaries, and source data affect interpretation.
Frequently asked questions
Why can annual harvest exceed tank capacity many times?
The same tank can fill, supply demand, and refill repeatedly. The ratio does not mean all annual water is stored at once.
Does one millimeter really equal one liter per square meter?
Yes for gross geometric volume. Collection efficiency then reduces that volume for practical losses.
Can supply days exceed 365?
The displayed value is an annual volume divided by daily demand. More than 365 means potential volume exceeds one year's entered demand, not that water can be carried through drought without adequate storage.
Can harvested rainwater be used for drinking?
Only under applicable local rules and with appropriate catchment, treatment, testing, maintenance, and professional guidance.