Water tank sizing: calculate your usable water supply
The right tank size starts with actual water use. Calculate the supply you need and understand why nominal capacity is different from the water your system can use.
Measure actual water withdrawal
Record how much water leaves your supply over several representative days. Note every refill and changes to your irrigation settings. Plant count alone is a poor sizing method: container volume, growth stage, climate and watering technique all affect consumption.
With drip irrigation, measure delivery to the plants separately. Our guide to calculating and checking irrigation volume helps you compare timer settings with actual output. In a recirculating system, pumped volume is not water consumption. Measure the net loss from the complete system instead.
Multiply consumption by the refill interval
The starting calculation is usable water supply = daily withdrawal × days between refills. At 10 litres per day over five days, you need 50 litres of usable water. This is a worked example, not a standard allowance for every growing setup.
Use a realistic higher daily figure if consumption may increase during that interval. Keep daily level and equipment checks in your routine. A calculated supply does not guarantee that the installation can run unattended for five days.
Separate nominal volume from usable capacity
A 65-litre tank does not automatically deliver 65 litres to the plants. Some water remains below the pump's minimum operating level. Leave room at the top for filling and, where relevant, returning water. Establish both limits with the pump and pipework you will actually use.
For a recirculating setup, check the highest level after the pump stops and water drains back from lines or other components. The reservoir must hold that return volume. Do not fill to the rim before checking how much returns.
Check the tank against the available space
A 65-litre rectangular tank and a 100-litre collapsible Aquaking tank differ in shape and access. The collapsible 100-litre version measures 40 cm in diameter and 80 cm high. The 160-litre collapsible tank measures Ø 50 × 90 cm. Allow working space beyond those outside dimensions.
Check the route through the doorway, the support beneath the tank and whether the pump remains removable. A litre of water weighs approximately one kilogram. Add the weight of the container and equipment when assessing the complete installation.
Make the final selection
Measure daily net water withdrawal.
Choose a refill interval that fits your inspection routine.
Establish the lowest and highest usable water levels.
Allow room for returning water, the pump and maintenance.
Compare suitable water tanks.
Read our collapsible versus rigid tank comparison if you are still choosing the construction. Include cleaning and refilling the reservoir in your plan.