Rainwater Tank Sizing Calculator

Work out rainwater tank sizing instantly with clear inputs, formula shown and shareable results.

About Rainwater Tank Sizing Calculator

Annual yield is roof area times rainfall times a runoff coefficient of 0.8-0.9 for pitched hard roofs. Tank size is a compromise between yield and demand: sizing much beyond about 18 days of average yield or 30 days of demand gives diminishing returns because the tank rarely fills or empties fully.

Formulas

Annual yield

Yield (L) = roof area (m2) x rainfall (mm) x runoff coefficient

Tank size

Size is the lesser of about 18 days of average yield and 30 days of demand

Worked example

With the default values below, Rainwater Tank Sizing Calculator returns Recommended Tank Size of 6,706.8. Change any input above and every figure here is recalculated in your browser.

Inputs used

Effective Roof Areaminimum 0 m2: 200 m2
Annual Rainfallminimum 0 mm: 800 mm
Runoff Coefficientaccepts 0–100 %: 85 %
Daily Demandminimum 0 L/day: 250 L/day

Results

Recommended Tank Size: 6,706.8
Annual Rainwater Yield: 136,000.0
Annual Demand: 91,250.0
Demand Met From Rainwater: 100.000%

Step by step

1Values used
2Annual yield
3Tank size
4Recommended Tank Size= 6,706.8
5Annual Rainwater Yield= 136,000.0
6Annual Demand= 91,250.0
7Demand Met From Rainwater= 100.000

Assumptions and limits

The result is derived only from the 4 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 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
Rainwater harvesting for potable use requires treatment, backflow prevention and compliance with drinking water regulations. Non-potable systems must be clearly labelled to prevent cross-connection.

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

Rainwater harvesting for potable use requires treatment, backflow prevention and compliance with drinking water regulations. Non-potable systems must be clearly labelled to prevent cross-connection.

Frequently Asked Questions

Why not size the tank for the whole dry season?

Because cost rises linearly with volume while the additional demand met falls away sharply. A daily rainfall time-series simulation gives the true yield-reliability-storage relationship.

What treatment is needed?

First-flush diversion and screening as a minimum. Potable use requires filtration and disinfection with verification, and cross-connection to mains supply must be prevented by design.