How to size your rainwater collection tank: the 3-step guide
In recent years, we have experienced increasingly frequent and longer periods of heatwave.
So you want to install a rainwater recovery system.
Many questions come to your mind
- How do you know how to choose the right recuperator, the right tank?
- How much can I save on my water consumption?
- What to do in case of overflow
- What type of tank? buried tank? What volume? How many liters?
- How to treat and filter this water to make it drinkable?
- How much will this reduce my water bill?
To answer these questions, our guide helps you calculate the right size for your tank (also called a cistern).
This rainwater is a precious resource. It is therefore important to have the right installation, knowing that water consumption is increasing (multiplication, swimming pool, bathroom, outdoor kitchen, etc.).
It is simultaneously an ecological gesture to avoid using the drinking water network and a way to reduce your bill.
1. Calculate the volume of recoverable water = Precipitation x Loss coefficient x Surface area
What level of precipitation is in your area?
This map shows you, in millimeters per year , the average annual rainfall for your home. Quite simply, it is the amount of rain, expressed in mm. This is the first indication to consider for your rainwater collector.
Remember to use recent data, precipitation is rarer and heavier.
What is the loss coefficient of your roof?
- Corrugated roof coefficient = 0.8
- Tile roof coefficient = 0.9
- Flat roof coefficient = 0.6
2. Calculate your rainwater need = garden need + house need + car need
How much water do you use to water your garden and vegetable patch?
To calculate the capacity of your tank, you need to know what you will be watering. The needs are different between a lawn, a flower bed or even a vegetable garden.
Here is a table that can help you calculate the flightume of your rainwater tank:
| Garden area | 500 m² | 1000 m² |
| Watering flower beds and lawns | 60 L/m² | 45 L/m² |
| Watering the flowerbeds only | 42 L/m² | 32 L/m² |
| Watering a vegetable garden | 100 L/m² | |
How much rainwater do you need to supply your house?
Rainwater can be used to flush your toilet and washing machine.
With this table, you can estimate the volume of drinking water you consume:
| Calculations for | 2 adults | 2 adults and 1 child | 2 adults and 2 children |
| A toilet | 16,512 L | 20,640 L | 24,768 L |
| A washing machine | 8,820 L | 14,700 L | 17,640 L |
| Annual requirement | 25,332 L | 35,340 L | 42,408 L |
How much rainwater should you collect to clean your car?
Considering 3 phases of cleaning (pre-washing, washing with shampoo then rinsing with water), with a flow rate of 15 L/minute = 225 liters of water as soon as you clean your car.
3. Tank volume = ((Recoverable volume + Required volume)/2) x 21 days reserve/365)an style="font-weight: normal;">
In addition, here we are, we have all the elements to calculate the volume of your tank, let's take our example again:
- Recoverable volume: 40,000 L
- Volume of estimated water needs: 84,400 L
- Wish to have a water reserve for 21 days .
Tank volume = ((40,000 + 84,408)/2) x21/365) = 3,578 liters (3.5 m³)
Access the calculator for the volume of your rainwater tank
Calculate the ideal volume of your tank in a few clicks and discover our report on the installation of a tank and the regulations to respect
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