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Rainwater Tank Calculator

Enter the roof area that drains to the tank, your average yearly rainfall, a runoff coefficient, your daily use from the tank and the tank size. The calculator shows the water collected per year, how long a full tank lasts and what share of your demand rain could cover. A 100 m² roof with 600 mm of rain collects up to 60,000 litres a year.

Measurements

Units

Plan area of the roof that drains to the tank.

From your national weather service for your location.

1 means every drop is collected. Real roofs lose some to evaporation, splash and first-flush diverters; lower it to suit.

For example toilet flushing, laundry or garden.

Results

Water collected per year

14,961 gal

Days a full tank lasts

50 days

Average collected per day
41 gal
Share of your demand covered
158 %

Rain doesn't fall evenly through the year, so a tank may run dry in a dry season even when the annual total is enough. Check local rules for rainwater use.

Show the working
  1. Roof × rainfall × coefficient1,000 ft² × 24 in × 1 = 56634 L (14961 gal)
  2. Tank ÷ daily use1300 ÷ 26 = 50
Paper and orientation
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How to use the rainwater tank calculator

  1. Choose Metric (litres) or Imperial (US gallons).
  2. Enter the roof catchment area that drains to the tank, measured in plan.
  3. Enter the average yearly rainfall for your location.
  4. Set the runoff coefficient.
  5. Enter your daily use from the tank and the tank size.

How the calculation works

Water collected per year = roof area × rainfall × coefficient (1 mm on 1 m² = 1 litre). Average per day = yearly total ÷ 365. Days a full tank lasts = tank size ÷ daily use. Share of demand covered = yearly total ÷ (daily use × 365).

Worked examples

Suburban house. 100 m² roof, 600 mm rain, coefficient 1: 60,000 L a year, about 164 L a day on average. A 5,000 L tank at 100 L a day lasts 50 days.

US home. 1,000 ft² roof, 24 in of rain: about 14,960 gallons a year at a coefficient of 1.

How to interpret the results

The yearly total shows the potential; the tank size decides how much you can store for dry periods. Because rain is seasonal, a tank can run dry even when the yearly total exceeds your demand.

Tips and common mistakes

  • Use plan area, not the sloped roof area.
  • Fit a first-flush diverter and leaf screens to keep water cleaner.
  • Use an overflow that drains safely away from foundations.
  • Check local rules on rainwater use and tank installation.

Sizing for dry periods

A simple way to size a tank is to decide how many days of supply you want to store and multiply by your daily use. If the longest dry spell where you live is a month and you use 100 litres a day for the garden and toilets, a tank of about 3,000 litres gives you that buffer. A month-by-month balance using local rainfall records gives a more accurate answer.

The square footage calculator measures the roof plan area, and the roof pitch calculator converts between plan and sloped area if your drawings show the sloped roof. Add the tank, gutters and pipes to the project materials list when you plan the installation.

Assumptions and limitations

The calculation uses yearly averages and doesn’t simulate dry spells. For important supplies, a month-by-month analysis with local data is better.

Frequently asked questions

How much rainwater can I collect?

Roof area × rainfall × runoff coefficient. One millimetre of rain on one square metre of roof is one litre, so 100 m² with 600 mm of rain gives up to 60,000 litres a year.

What runoff coefficient should I use?

The calculator starts at 1, which means every drop is collected. Real systems lose some water to evaporation, splash, gutters and first-flush diverters, so reduce it to reflect your roof and system.

How big should my tank be?

Think about the longest dry spell you want to cover. Tank size ÷ daily use gives the number of days a full tank lasts.

Where do I find rainfall data?

Your national weather service publishes average annual rainfall for your area.

Can I drink harvested rainwater?

Rules vary by country and region, and drinking water needs treatment. Check local health guidance and regulations.

Does roof material matter?

Yes. Smooth metal roofs shed water efficiently, while rough or porous surfaces hold back more. Reflect this in the runoff coefficient.

Sources