Ecology
Roof rainwater collection
Estimate collected rainfall from a roof’s horizontal catchment area and an assumed efficiency.
Calculators / Ecology / Stored water autonomy
Stored water autonomy calculator
Estimate how long stored water lasts after setting aside an untouched reserve.
On this page
Usable stored water500L
Theoretical days of supply12.5days
Complete days of supply12days
Calculator controls
How to calculate
Estimate how long stored water lasts after setting aside an untouched reserve.
Usable stored water =
stored-reserve LTheoretical days of supply =
(stored-reserve)/daily daysComplete days of supply =
floor((stored-reserve)/daily) daysWorked example
- Stored water (L): 600
- Untouched reserve (L): 100
- Planned daily use (L): 40
- Usable stored water: 500 L
- Theoretical days of supply: 12.5 days
- Complete days of supply: 12 days
Assumptions and limits
Assumes constant daily use with no new inflow, leakage or evaporation. Water quality and permitted uses require separate checks; stored rainwater is not assumed safe to drink.
Results are rounded for display; calculations use unrounded values. Read our calculation methodology.
Understanding the result
Estimate how long stored water lasts after setting aside an untouched reserve. Subtract the reserve before dividing by daily demand. The complete-day result rounds down rather than promising a partly supplied extra day.
When this tool is useful
Subtract the reserve before dividing by daily demand. The complete-day result rounds down rather than promising a partly supplied extra day.
Understanding your inputs
| Input | Example |
|---|---|
Stored water (L) stored | 600 |
Untouched reserve (L) reserve | 100 |
Planned daily use (L) daily | 40 |
Frequently asked questions
What assumptions should I check?
Assumes constant daily use with no new inflow, leakage or evaporation. Water quality and permitted uses require separate checks; stored rainwater is not assumed safe to drink.
How can I check the result?
Start with the labelled example. Keep units consistent, calculate without intermediate rounding and change one input at a time. The diagram explains the model; the result table supplies the numerical answer.