This short guide will help you choose the right capacity for your underground rainwater storage tank - without difficult formulas. We will show you how to estimate your needs, count the amount of water from your roof, select the capacity and plan a simple garden installation.

Step 1: Determine what you will use the rainwater for.

List the uses: watering the garden, flushing toilets, laundry, washing the car, housework. This will allow you to estimate the real need for water in a typical week.

• Garden watering - take a guideline of 20-25 l/m² per month per season.
• Toilet flushing - approximately 35-50 l/person/day.
• Laundry - approx. 15-20 l/person/day.

Step 2: How much water you collect from the roof (no complicated formulas)

In simple terms, assume: annual rainfall × roof area × runoff coefficient. For most roofs, you can assume a coefficient of around 0.9 (depending on the roofing and the layout of the gutters).

Brief example: calculation of possible rainwater yield

Roof 120 m², annual rainfall ~600mm (approx. 600 l/m²). Estimate: 120 × 600 × 0.8 ≈ 57,600 l/year. This figure helps to select the capacity for a period without rain. And a period without rain can last for 3 weeks. Therefore, you must divide the result by 21 days. That is: (57600 l /365 days)*21 days = 3314 l, the result according to the estimation should be taken as a minimum. Therefore then we choose 4000l , 5000l, 6000l tanks - because of how we will use the rainwater.

Step 3: Selecting the tank capacity

It is most practical to select a tank that will store a supply of water for 2-3 weeks of your needs.


How to calculate capacity: annual rainfall × roof area × runoff coefficient ≈ annual amount of rainwater to be collected.

• 2000-3000 l - small garden, 1-2 people.
• 4000-6000 l - family of 3-4 people + garden 200-300 m².
• 8000-10 000 l - larger garden, intensive watering. 300 m² - more , and with automatic watering.

If you prefer, open the calculator in a new tab: Tank selection calculator. If you have any problems, please contact our customer service.

Step 4: Tank selection - material

In detached houses, underground rainwater tanks are the best choice - they do not take up space in the garden, provide stable temperatures, no water freezing and allow larger capacities without affecting aesthetics.

The best material is PE, HDPE, MDPE plastic - it is lightweight, corrosion resistant, easier to install and above all airtight for many years .
It is worth paying attention to water and ground conditions. If the ground water level is high or the soil is clayey, flat tanks such as the F line, Bolt, Eco line Slim, etc., are the best choice.
On the other hand, if the conditions are normal, any solution can be used. If you have any problems, please contact our customer service.

Step 5: Simple installation in the garden

The simplest arrangement: gully filter in front of the tank or basket filter, underground tank, floating intake, automatic pump and point of use (tap). This makes watering convenient and economical.

FAQ: How to choose a rainwater storage tank

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