Downspout Tank Sizer

See how big your rain tank really needs to be. Downspout Tank Sizer runs your roofs, your monthly rainfall and your daily water use through a full year, day by day, and marks the dry months so you can see exactly where a tank runs short.

HOW IT WORKS
- Add catchment roofs with their footprint area and surface. Each surface has an editable runoff coefficient.
- A first-flush diverter volume per area is subtracted from every rain day.
- Pick a generic climate profile (wet, temperate, dry or monsoon) or type your own 12 monthly totals in mm or inches.
- List your water uses - garden, toilets, laundry, livestock - each with a daily rate and a seasonal pattern.
- A daily water balance over a year gives tank level, overflow days, short days and reliability.

FIND THE RIGHT SIZE
A sweep of 21 tank sizes draws the reliability curve and marks the knee as the suggested size - the point where a bigger tank stops buying you much extra reliability.

GUTTERS AND DOWNPIPES
A gutter and downpipe hint comes from your roof area and a design storm intensity.

FEATURES
- Daily water-balance simulation over a full year: tank level, overflow days and short days
- Reliability curve across 21 tank sizes with the suggested knee size marked
- Multiple roofs with editable runoff coefficients and a first-flush diverter
- Generic climate profiles or your own 12 monthly rainfall figures, metric or imperial
- Gutter and downpipe sizing hint from roof area and storm intensity

Switch between metric and imperial at any time. Every field is validated, and only valid values are applied.

PRIVATE AND SIMPLE
Downspout Tank Sizer works fully offline. No account, no ads, no sound and no internet connection needed. Your plan stays on your device.

Five tabs keep it tidy: Balance, Roofs, Rainfall, Demand and Sizes.