How Much Water Will You Collect?
Rainwater Storage Calculator
Use this calculator to estimate how your plot size, rainwater catchment, storage capacity and watering demand work together. The aim is to make harvested rainwater the primary source of irrigation, with mains water used as a supplement during dry periods.
2.5 poles
10 poles
5 poles
2 m²
15 m²
5.0 m²
200 L
5,000 L
1,000 litres
100 L/pole
250 L/pole
150
L per pole/month
Estimated annual performance
Potential rainwater collected
—
Rainfall available from the selected catchment
after collection losses.
Estimated irrigation demand
—
Estimated annual crop-watering requirement
using seasonal demand.
Rainwater actually used
—
Rainwater that can actually be captured,
stored and used when required.
Estimated mains water needed
—
Supplementary water required after available
stored rainwater has been used.
Rainwater contribution
—
Percentage of estimated irrigation demand
supplied from harvested rainwater.
Rainwater lost through overflow
—
A high figure suggests that increasing storage
may capture more of the available rainfall.
Suggested storage capacity
Minimum useful storage
—Captures most of the easy benefit available from adding storage to this catchment.
Recommended storage
—Captures about 95% of the practical mains-water reduction achievable by increasing storage.
High-resilience storage
—Provides additional summer reserve, aiming for approximately 1½ months of peak irrigation demand.
Month-by-month water balance
| Month | Rain collected | Water demand | Tank start | Tank end | Mains needed | Overflow |
|---|
About this calculator:The calculator uses an Oxford monthly rainfall pattern based on
Met Office 1991–2020 averages, proportionally adjusted to the
650 mm annual rainfall assumption used in the TTACIC Fair Use
Water Policy.An 80% rainwater collection efficiency is assumed.The selected litres-per-pole figure represents
peak summer irrigation demand, not demand in every
month. Winter irrigation demand is assumed to be negligible.Actual rainfall, soil type, crop choice, mulching, growing under cover,
watering technique and weather conditions will affect actual water use.The storage recommendations are planning estimates rather than a
guarantee that a particular tank will provide sufficient water in every
year.