Hydroponic Reservoirs Cheat Sheet: Calculate Reservoir Size in 5 Easy Steps!

Hydroponic Reservoirs

Any system that you have that is going to support more than a single plant or two is going to need Hydroponic Reservoirs.  Since you don’t have soil to hold in moisture and nutrients and you probably aren’t planning on hand feeding your babies’ roots, you’re going to need a ‘fridge’ to hold it all.  This is where the Hydroponic Reservoirs come in!

Remember that Hydroponic Reservoirs don’t have to be complex and are actually rather easy to DIY for 90+% of all systems.  Going to Walmart or, if you’d prefer to do it all from your chair, heading to Amazon and checking out the plastic storage bins (with lids!) should be all it takes to get a reservoir started. 

Storage Bin Hydroponic Reservoirs
Totes Storage Bin Hydroponic Reservoirs, Source


Modding a reservoir primarily involves cutting holes in the lid, which can be accomplished with sturdy cutting tools.   More advanced holes, such as reservoir side holes for drainage and feeding into water pumps, will require more skill and finesse as you will have to seal the area around the feeder tubes.




Calculating Reservoir Size for Yours Systems

Plants need water.  Reservoirs hold water.  You should clearly understand that by now… but it isn’t quite so obvious exactly how much water plants need or how that translates into a proper reservoir size.  Let’s work through the tricky math and all of the considerations here!

To quote Greenhouse Mag, “small plants used 1 tablespoon per day, while large plants used slightly less than 2 tablespoons per day.”  While this is based on greenhouse-grown plants that are in the soil, this should give you a good idea of where to start:  Determining how many plants in your system will be supported by this reservoir.

Different Sized Hydroponic Reservoirs
Different Sized Hydroponic Reservoirs, Source


Step 1:  Determining system size:


This step should be pretty straightforward.  How many plants is your system going to hold?

Step 2:  Determining water per plant:


The next factor to consider is your water delivery system.  While we might consider just going with 2 tablespoons of water for a basic system, there are plenty of reasons to bump this up to 3 tablespoons or higher:  If your water is being sprayed, fogged, misted, or dripped onto plants then – simply due to surface area of the droplets of water – you’re going to face a lot more evaporation than if you have more or less stagnant water sitting in a lidded reservoir.  For these systems, you might really consider boosting your estimated water usage at 7 tablespoons or higher.  Hot greenhouses, also, evaporate water faster than indoor systems.

Step 3:  Decide how often you want to interact with the system:


Are you a weekend warrior or a daily doter?  This step should be a bit self-explanatory, but really do keep in mind that enthusiasm for projects does wane a bit with time.  Even industrial size greenhouses need to remember that there may be labor shortages (ahem, Covid) or desires to expand that come in the way of repeated maintenance.  That being said, you will probably find it beneficial to check on your system at least once a week and, when you first set up a system, a bit more than that.  If you’re unsure about this step, go with a good 7.

Step 4:  Determining the size of extras in your system:

(+ Extra)

This is where you look at the tubing, air pumps, and stones, etc. that are going to go inside the reservoir as well as any net pots or other plant holders that are coming down through the reservoir lid and guesstimate their total area in cubic feet.  If you haven’t purchased everything yet, you should be able to get dimensions for most products from their product description page.

Step 5:  Calculating it all:

Alright, ready, here is the formula:


This works because for every 2,000 tablespoons of water, you’ll need one cubic foot of space in your reservoir.  As an example, imagine you have 100 full-sized plants in a drip system that you only want to add water to once a week.  You estimate that between your plants and evaporation, you’ll say goodbye to around 10 tablespoons of water per day.  Additionally, you have an estimated .5 cubic feet of equipment to go inside your reservoir.  How big of a reservoir do you need?

100 plants X 10 tbsp X 7 days / 2,000 + 0.5 = 4 cubic feet

Not bad!  As you can see, one large storage container should do it for most systems.




Backup Hydroponic Reservoirs


Backup Hydroponic Reservoirs
Backup Hydroponic Reservoirs,



Some mega-successful people will tell you to never have a Plan B… it is Plan A or nothing.  Invariably, they are the people that got away with it… the ones that got nothing aren’t near famous or successful enough to talk to crowds or get millions of clicks associated with their name on YouTube.  A backup reservoir is a good Plan B for many people, so consider having one ready.

What a backup reservoir means for you might vary from one system to the next, especially depending on the size of your proposed system.  Smaller systems might appreciate having a backup reservoir that has all of the tubes pre-installed, but not fully assembled.  When needed, such as when the main reservoir has a problem (say, algae) the two can be quickly swapped and filled.  Larger system users might like the backup reservoirs to be already assembled and accessible.  They can even be prepared with liquids early and simply rotated during your weekly additions of nutrient mix






Mixing Hydroponics Reservoirs

Mixing Reservoir
Mixing Reservoir with Small mixing Pump inside,


At some point or another, you might realize that it would be beneficial to utilize multiple reservoirs in a single system, perhaps even using different reservoir styles or adding different nutritional compounds to each one separately.  There can be many reasons for doing this, but one large one is simply having more room for water and having the ability to support a larger system.

Remember that adding another reservoir, even of the same size, to your system does not automatically double your water capacity as each reservoir will have to be fitted with any piping, water pumps, etc. that are needed for the system.

The new formula would be:





Hydroponic Reservoirs Tips and Tricks:

Here are a few helpful reminders:

  • When estimating your water usage per plant, over-estimating is better than underestimating.
  • Don’t confuse (as the author of this article DEFINITELY didn’t) square feet and cubic feet.
  • Don’t overthink any of this… if space allows, getting something a bit bigger is almost always better!
  • At the same time, not every system will require three simultaneously running reservoirs with two backups. 
  • If you do end up getting multiple reservoirs, please remember that they will all need pumps, tubing, etc. which can reduce internal space and drive up costs.
  • Install a small mixing pump in larger systems or systems that have automatic nutrient dosing in order to keep the nutrients mixed well. Leaving unresolved nutrients in the main tank can lead to wild pH and EC swings.



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HI! I'm Bobby, an entrepreneur, inventor, green builder and urban farmer. I've been a custom home builder since a kid and an avid gardener. The critters, bugs and climate outside our new home forced me to check out some new ideas in gardening. I found out about Hydroponics and have spent years learning the best techniques to build the best, most efficient, home hydroponics systems. I built one of the first ever, small scale, mobile hydroponic systems. Would you like to know how I did it?