See what one rain event can put in the tank.

Turn roof catchment area and rainfall depth into a practical water-volume estimate, then compare it with tank capacity and your garden's weekly irrigation target.

Roof catchment area (m²)Rainfall (mm)Collection efficiency (%)Tank capacity (L)Tank currently full (%)Garden area (m²)Weekly water target (mm)

RAINWATER COLLECTED800 L50.0 m² × 20.0 mm × 0.80

TANK AFTER RAIN1000 L100% full

ESTIMATED OVERFLOW50 Lstorage capacity exceeded

GARDEN COVERAGE2.5 weeks400 L weekly target

CORE RULE1 mm rain on 1 m² = 1 litre

Collection losses are represented by the efficiency setting for gutters, first-flush diversion, splash and other system losses.

Compare with rain-adjusted irrigation →Open My Garden →

This is a volume-planning estimate, not a plumbing or structural design. Real collection depends on roof material, gutter capacity, filters, first-flush diversion, leaks and how evenly rainfall reaches the catchment. Check local rules before connecting tanks to buildings or household systems.

HOW IT WORKS

Rainfall depth converts cleanly into volume.

One millimetre of rain over one square metre is one litre of water. The calculator multiplies roof catchment area by rainfall depth, then applies a collection-efficiency factor to account for real-world losses.

Tank fill is calculated from the current stored volume plus the new collection estimate. Anything beyond tank capacity is reported as potential overflow. The garden-coverage figure compares stored water with a simple weekly irrigation target; actual demand still depends on crops, soil, shade, wind and useful rainfall.

See what one rain event can put in the tank.

Rainfall depth converts cleanly into volume.

Use the stored water intelligently

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