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Rainwater Tank Size

Free Rainwater Tank Size calculator with clear step-by-step results.

About Rainwater Tank Size

Two numbers size a rainwater system: annual yield, which is roof plan area times rainfall times a runoff coefficient, and storage, which is set by the longest dry period you need to ride out. A tank larger than the dry-period requirement adds cost without adding usable water.

Formulas

Annual yield

Litres = roof plan area m2 x rainfall mm x runoff coefficient, less first-flush losses

Storage requirement

Litres = daily demand x longest dry period in days

Worked example

With the default values below, Rainwater Tank Size returns Recommended tank capacity of 5,400. Change any input above and every figure here is recalculated in your browser.

Inputs used

Roof catchment area (plan)minimum 1 m2
120 m2
Annual rainfallminimum 10 mm
750 mm
Runoff coefficientaccepts 0.2–1
0.85
Daily demandminimum 0 L
180 L
Longest expected dry periodaccepts 1–365
30
First flush divertedaccepts 0–5 mm
0.5 mm

Results

Recommended tank capacity
5,400 L
Water harvestable per year
72,900 L
Annual demand
65,700 L
Share of demand rainwater can meet
100.0% %

Step by step

  1. 1Values used
  2. 2Annual yield
  3. 3Storage requirement
  4. 4Recommended tank capacity= 5,400 L
  5. 5Water harvestable per year= 72,900 L
  6. 6Annual demand= 65,700 L
  7. 7Share of demand rainwater can meet= 100.0 %

Assumptions and limits

  • The result is derived only from the 6 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax or fees or charges into account.
  • Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why use plan area rather than sloped area?

Rain falls vertically, so the catchment is the horizontal footprint. A steep roof catches no more rain than its plan area regardless of how much surface it has.

What runoff coefficient should I use?

Around 0.9 for metal roofing, 0.8-0.85 for tile and 0.7 or less for porous surfaces. Losses come from wetting, evaporation and gutter overshoot.

Related Calculators

Rainwater Tank Size – Calculate Recommended tank capacity