Rainwater Harvesting: Match water tank size to rainfall

jojo water tanks

Various types and sizes of water tanks are available, such as these JoJo tanks in South Africa

As a general rule you should install the largest water tank that you have room for or can afford. This will allow you to use rainwater for as long as possible through droughts and prolonged dry periods.

Tank Size

Rule of thumb
A good rule of thumb is to purchase a water tank that holds a minimum of four weeks supply. For instance, if you use 1,000L of tank water each week in the garden, toilet and laundry combined, you should consider purchasing a 4,000L water tank.

Other factors to consider…

  • How big is your roof?
  • How much rainfall do you get in your area?
  • How much space do you have for a tank?
  • What type and size is your garden?
  • How many people live with you?

If your water tank is going to be large, consult a builder or engineer as it may require structural support or raised water tank stand. You may also need a building permit from your local council.

How much rainfall will I get?

The pattern of rainfall is important. That is, if you have very regular rainfall all year round, you can use a smaller water tank.  Check out the average annual rainfall for South Africa below:
average annual rainfall map south africa

How much rainfall will I collect?

Each square metre of roof area collects 1 litre of water for every 1 millimetre of rainfall received. This is determined by the area of roof connected to the tank via the stormwater down pipe.

As an example:

  • An average house has a roof area of approximately 160 square metres.
  • Assume that 50% of this roof area can be easily connected to a rainwater tank i.e. 80 square metres.
  • Each square metre of roof area collects 1 litre of water for every 1mm of rainfall received.
  • If you have 10mm of rain, the water collected by the tank is 800 litres (80 square metres x 10mm = 800 litres).

<b>Rule of thumb guide:</b> roof collection versus tank capacity for an area receiving 600mm of rain per year courtesy of <br />EME group.

Rule of thumb guide: roof collection versus water tank capacity for an area receiving 600mm of rain per year courtesy of
EME group.

Source: Save Water Adapted

One should also decide whether the rain water tanks should be above ground or underground.  Underground water tank sizes are more limited; the largest JoJo underground tank being 6000 litres in capacity.  Underground water tanks also involve much more installation work than their above-ground counterparts.  If space is not an issue, a 10000 litre tank or even a 15000 or 20000 litre tank will pay for itself when there is a heavy rainfall event.  Water tanks are not only used for rainwater harvesting; storing borehole water or even municipal water is a good idea in these times of water supply cuts around South Africa.  JoJo Tanks manufacture a wide range of water tank sizes and types and are sold with a 5 year guarantee.  Click on the links below to see the various JoJo water tanks, JoJo rainwater tanks, JoJo tank stands, JoJo chemical tanks and other JoJo Tanks products available in South Africa.

JoJo Tanks South Africa offer a wide range of plastic water tanks and chemical tanks to suit every household, business or agricultural need, including silo tanks and silo tank stands.  See JoJo Tanks VERTICAL TANKS, HORIZONTAL TANKS (transporter tanks), STEEL TANK STANDS and OTHER JOJO PRODUCTS. Also see JoJo Tanks’ NEW 6000 LITRE UNDERGROUND TANKS. Also see our FAQ and WATER TANK PRICES.

JoJo’s superior polyethylene plastic water tanks and steel water tank stands make JoJo Tanks the leaders in plastic water tank technology and the quality, affordability and guarantee on these water tanks make JoJo products the best choice in South Africa.  JoJo’s water tanks and chemical tanks come standard with a number of features that are often lacking on cheaper/inferior plastic tanks made by other companies in South Africa.

We are authorised JoJo Tanks dealers in South Africa and dispatch orders directly from JoJo Tank depots to save on transport costs. Full range of JoJo Tanks products and JoJo water tanks for sale.  CONTACT US for a quote on the right JoJo water tank or other JoJo product for you.  Special discounts are available on multiple orders of chemical tanks and water tanks and to our commercial and government customers.


(15000 litre and 20000 litre JoJo vertical water tanks also available on order)

Live in the USA and need rainwater harvesting equipment?  Click HERE!


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