Module 03 · Core engineering
The logic behind tank sizing
There is no single correct tank size — only a sensible range set by four variables. Understanding how they pull against each other is more useful than any rule of thumb.
The four variables
Roof area
More catchment means more litres per millimetre of rain. Capture scales almost linearly with effective roof area.
Rainfall & its variability
Annual total matters, but so does how rain is spread. A peaky climate needs more buffer to ride out dry gaps.
Household demand
What the water is used for — garden, laundry, toilets — sets the draw-down rate against which storage is judged.
Storage limit
A tank can only hold so much. Beyond a point, extra capacity sits empty most of the year and adds little.
The relationship is multiplicative, not additive. Doubling roof area roughly doubles inflow; doubling tank volume does not double useful yield, because a larger tank only helps if inflow can fill it and demand can empty it. The sweet spot is where storage is large enough to carry typical dry spells but small enough to fill regularly.
A conceptual sizing model
A useful first pass treats annual capture as roof area × annual rainfall × a runoff coefficient (around 0.8) × a system efficiency factor. That yields litres per year. Comparing it to annual demand shows whether the constraint is collection (not enough rain hits the roof) or storage (plenty of rain, nowhere to keep it).
Storage is then sized to bridge the longest expected dry run, not the whole year. A tank that could theoretically hold a year of rain would overflow rarely and sit part-empty constantly — expensive and inefficient. The estimator applies exactly this chain so you can watch the numbers move.
This is a conceptual model for learning how the variables interact. It does not size a specific tank for a specific property and is not a substitute for site-specific design.
Overflow probability
Overflow probability rises with roof area and with peaky rainfall, and falls with larger storage and steady demand. In monsoonal climates a modest tank overflows almost every wet season regardless of size; in dry climates the same tank may never fill. Neither is wrong — each reflects matching storage to a different rainfall rhythm. The estimator reports a qualitative overflow likelihood so the trade-off stays visible.