How much rainwater can you harvest from your rooftop?

Before deciding on size and number of water tanks for rainwater harvesting, you should have an approximate idea of how much rainwater your rooftop can potentially collect.

how to calculate amount of rain from rooftop

You need to know your rooftop surface area and local annual rainfall to calculate how much rainwater you can harvest

Firstly, ensure that your rooftop is suitable for rainwater harvesting. Corrugated iron or metal rooftops are best for harvesting rainwater, followed by tiled and concrete rooftops. Unfortunately, thatched roofs are not suitable for practical rainwater harvesting. Gutters are also a requirement for the effective channelling of rainwater to downspouts leading to water tanks or rain barrels.

There are various methods of determining how much rainwater your rooftop can collect. The basic variables which need to be known (at least approximately) are:
1. Your roof surface area
2. Mean annual rainfall for your area

Here is a simple online rainwater harvesting calculator to get you started:

Once you know how much rainwater you can potentially collect annually, you need to get an approximate annual water usage for your household (by looking at your water bills or perhaps you have water meter). An ‘average’ household of 4 people with a small garden could use around 300000 litres of water per year. Let’s use this as an example and assume a rooftop surface area of 150m2 and an annual average rainfall of 1000mm. If you plug these figures into the above calculator, you’ll see that you can harvest  135000 litres of rain per year, i.e. half your annual requirement. Of course, this is a simplified calculation and one should also consider the efficiency of your roof at collecting the rain (iron roofs are best, tiled roofs have a lower collection efficiency) as well as evaporation and leaks (the online calculator above takes into account some losses). But even if you can harvest 100000 litres of rainwater every year, that is a substantial quantity of water!

Now to calculate what size tank you should buy:

Most days without rain X Average daily water use = Storage volume neededd on day one of a dry period
If for example, the most days without rain was 50 days, and the average daily usage was 400 litres, the volume on hand on day one of 50 days would have to be 20000 litres.
It’s quite difficult to predict the required volume, because a dry period may begin when the water tank is not yet full. However, this method provides a guideline, and you can add a safety factor on top of this requirement.
When calculating how big the storage capacity of your rainwater tanks should be, plan the tank placement area so that extra storage tanks can be added later if needed. So the above example would require a 20000 litre water tank or perhaps two 10000 litre tanks or four 5000 litre tanks. This is assuming that you have space for installing this number of tanks. If space is limited, underground water tanks could be installed, for example three 6000 litre underground water tanks and a 2000 litre above-ground water tank.

In South Africa, JoJo Tanks manufacture a wide array of water tank types and capacities so that there will be the right water tank or water tanks available for your specific rainwater harvesting requirements.

JoJo Tanks South Africa offer a wide range of products such as plastic water tanks and chemical tanks to suit every household, business or agricultural need, including silo tanks and silo tank stands as well as other high quality JoJo Tanks products.  See JoJo Tanks’ VERTICAL TANKS, HORIZONTAL TANKS (transporter tanks), STEEL TANK STANDS and OTHER JOJO PRODUCTS and 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 5-year guarantee on these water tanks make JoJo products one of the best choices 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. We can usually delivery tanks and tank stands free of charge to business/commercial addresses in South Africa.


(15000 litre and 20000 litre JoJo vertical water tanks & chemical tanks are available on order directly from JoJo Tanks’ main factory)

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

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