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Rainwater Harvesting Tank Size Calculator

Size a rainwater collection tank from a roof or paved catchment area and the rainfall you're sizing for — a single storm, a typical month, or a year, whichever matters for your setup.

Harvestable volume1125 L
In cubic metres1.13 m³

How to measure your inputs

  • Catchment area: the roof or paved footprint draining to one collection point, measured as length × width.
  • Rainfall: use a single storm total if sizing for one event, or a typical monthly/annual figure if sizing for ongoing supply — check Swales' Precipitation map for a specific site's numbers.

Methodology

The standard rainwater-harvesting yield formula: volume (litres) = catchment area (m²) × rainfall (mm) × runoff coefficient. This works out in litres with no extra unit conversion, since 1mm of rain over 1m² is exactly 1 litre. The runoff coefficient accounts for water lost to absorption, evaporation, and splash before it reaches the collection point — smoother, less porous surfaces (like metal roofing) capture a higher share than rougher ones (like gravel).

Sizing a rainwater tank that actually matches your roof

Supply-based sizing vs. demand-based sizing

This calculator answers a supply question — how much water a given rainfall event or period will actually produce from your catchment. That's different from demand-based sizing (how much storage you need to cover a dry spell based on usage), which needs your consumption rate as well. For a first pass, size the tank close to what a typical significant rainfall period produces; refine from there once you know your actual usage.

Why runoff coefficient isn’t just a rounding error

The gap between a metal roof (0.9) and gravel (0.6) is a real 30% difference in collected volume for the same rainfall — not a minor adjustment. Smooth, non-porous, well-sloped surfaces shed water efficiently; rough or porous ones absorb or lose more to splash and evaporation before it reaches a downpipe.

First-flush diversion and filtration

The first few litres off any roof carry accumulated dust, pollen, and bird droppings; a first-flush diverter (a simple pipe that fills and seals before water reaches the tank) keeps that initial flush out of storage. This calculator gives you the raw harvestable volume — real usable storage is typically a little less once first-flush loss and basic filtration are accounted for.

Frequently asked questions

Is roof-collected rainwater safe to drink?

Not without treatment in most cases — roof runoff picks up dust, bird droppings, and roofing-material residue. It’s commonly used untreated for irrigation, livestock, or non-potable household use; drinking water needs filtration and disinfection appropriate to local health guidelines.

How much rain falls in a typical month?

It varies hugely by region and season — anywhere from near zero in a dry-season month to 200mm+ in a wet-tropical month. Check Swales’ Precipitation map for a specific site’s real figures rather than assuming a global average.

What size tank do I actually need?

Start from what a typical significant rainfall period yields for your catchment (this calculator), then check that against your intended use — a tank meant to bridge dry spells needs to hold closer to your total demand between refills, not just one storm’s yield.

Does tank material affect how much water I collect?

No — the tank only stores what the catchment and runoff coefficient produce; tank material affects water quality over time (some materials leach less than others) and durability, not the collected volume itself.