Water & environment · Calculator companion · By K Imports
Rainwater harvesting potential: practical guide
This guide explains how to use the Count.ie rainwater harvesting potential. Estimate annual collection and a chosen dry-period storage buffer. Follow the inputs, method and worked example below, then compare your own scenario.
Guide written .
Open the Rainwater harvesting potential calculatorWhat to prepare
- Record the relevant area, volume, mass, rainfall or usage measurement.
- Identify the source and period of any entered factor.
- State the assessment boundary: what activity is included and what is outside the model.
Understand the inputs
The calculation method
Collection litres = roof area × rainfall × runoff fraction × filter fraction. Buffer = daily demand × chosen dry days.
Worked example
Illustrative inputs and their result.
Example inputs
- Roof catchment area (m²)
- 120
- Annual rainfall (mm)
- 1000
- Roof runoff efficiency (%)
- 85
- Collection/filter efficiency (%)
- 90
- Daily non-potable demand (litres)
- 150
- Tank usable volume (litres)
- 3000
- Chosen dry-period buffer (days)
- 21
Calculated example result
- Annual Collected Litres
- 91,800 L
- Tank Days Of Demand
- 20
How to interpret the result
An entered-factor estimate is not an audited footprint or certification. Results depend on the selected boundary, factor, units and local conditions.
Use the same boundary and factor source for each scenario. Report assumptions alongside savings or changes so the comparison can be reproduced.
Common mistakes to avoid
- Do not present an assumed factor as a measured local result.
- Avoid counting an activity in two categories of the same assessment.
- Distinguish potential collection or diversion from the quantity actually achieved.
Assumptions and sources
Annual yield is an upper-level estimate, not a time-series storage simulation. Seasonal mismatch and overflow reduce usable yield. No drinking-water suitability is assessed.
If your case falls outside this scope, use a more suitable calculator or contact us about an unclear method.