What could your roof collect?
Rainfall depth and connected roof area give you a starting point for estimating how much water could reach your tank. One millimetre of rain over one square metre represents one litre of water before losses.
The collection calculation
Collected litres = rainfall (mm) × connected roof area (m²) × efficiency
With 15 mm of assumed rainfall, 200 m² of connected roof and an illustrative 80% collection efficiency, the estimate is 15 × 200 × 0.80 = 2,400 litres.
Use only the roof that feeds your tank
Count the horizontal catchment area draining to the connected downpipes. Roof sections that drain elsewhere do not contribute. Collection losses can arise from first-flush diversion, gutter leakage, splash, evaporation and other factors. The 80% example is a scenario assumption, not a measurement of your system.
Leave room for the rain
Your tank cannot store more than its capacity. A 22,500-litre tank holding 21,000 litres has 1,500 litres of space. If 2,400 litres arrives before any use, up to 900 litres exceeds that space. Tankwise caps the stored amount and reports the excess as scenario overflow.
Rainfall is uncertain
Tankwise does not fetch a live forecast. Enter rainfall as a scenario to explore, not a guaranteed refill. A forecast for your region may differ from what falls on your roof. Keep a no-rain plan in view when deciding whether to contact a supplier.
For simplicity, the planner adds a rain event at the start of its selected day, adds any delivery, then subtracts that day’s use. Actual timing may change overflow and availability.
Try the rainwater planner →