Reverse Osmosis Tank Size Options | Full Overview

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Picking the right-sized RO tank is vital so that you and your family have enough filtered water to drink and cook with.

There are a few considerations to make when sizing your reverse osmosis tank, and today we will look at a complete overview of them so you don’t end up with something that doesn’t suit your needs.

Let’s go!

Key Takeaways

Due to not being standardized, there are many different reverse osmosis tank sizes. Also, because air is contained in an RO tank, the actual water capacity of the tank will be less than the gallon value listed:

  • 5-gallon tank – about 0.3-0.4 gallons of water
  • 0-gallon tank – about 0.6-0.8 gallons of water
  • 0-gallon tank– about 1.0-1.2 gallons of water
  • 0-gallon tank – about 1.7-2.2 gallons of water
  • 0-gallon tank – about 2.5-3.2 gallons of water
  • 0-gallon tank– about 5.4-7.2 gallons of water
  • 0-gallon tank – about 6.6-8.8 gallons of water
  • 0-gallon tank – about 8.4-11.2 gallons of water

Reverse Osmosis Tank Sizes

There is no standardized size for a reverse osmosis storage tank. The most common home under sink units you will find are around 3 gallons but may be larger if there is the space and need for it.

The tank size also does not mean your tank will hold that amount of water, as there needs to be room for the bladder and the air too.

Nominal Tank Size True Capacity Height Diameter
0.5 gal ~0.3-0.4 gal 7.5” 5”
1.0 gal ~0.6-0.8 gal 9.5” 6.5”
2.0 gal ~1.0-1.2 gal 11.5” 7.5”
3.0 gal ~1.7-2.2 gal 14” 8.5”
4.0 gal ~2.5-3.2 gal 14” 11.5”
9.0 gal ~5.4-7.2 gal 21” 13”
11.0 gal ~6.6-8.8 gal 22” 15”
14.0 gal ~8.4-11.2 gal 23” 15.5”

Gallon Capacity: How Much Water Does a Reverse Osmosis Tank Really Hold?

RO storage tanks contain a certain amount of pressurized air in order to be able to function. The only time it may be filled with 100% water is if the air bladder ruptures; if that happens, you will need to replace the whole tank.

There are three factors that influence the actual water storage capacity of your RO tank.

Under Sink Reverse Osmosis System

Tank Air Pressure

The higher the air pressure of your storage tank, the less water it will hold, and the sooner the ASO valve will switch off when it is filling. For this reason, it is important to make sure your empty tank psi sits at its recommended range. If your tank has air pressure that is too low, it will hold more water, but it won’t come out of your RO faucet at the force it should.

Feed Water Pressure

If you have strong feed water pressure, your reverse osmosis tank will hold more water. If your feed water pressure is too low, consider a booster pump to increase the efficacy of your RO tank.

Shutoff Pressure

Your RO tank’s ASO valve is usually triggered when the internal storage tank pressure reaches about two-thirds of the feed water pressure. But this can vary.

How to Choose the Right-Sized RO Storage Tank

It is important to assess your household needs to figure out the right-sized water storage tank for you. If the tank is too small, it might mean you often run out of water by the time everyone had a chance to use it. The same goes if you are using it for multiple things, like cooking and washing dishes.

The following are some important considerations when sizing your RO system and tank.

Your Water Demand at Times of Peak Consumption

Does everyone in your household want to drink a quarter gallon of water when they get home from work or school? If you have a 2-gallon tank and a large family, that means your tank is going to be drained pretty fast, and you may need a bigger one, or consider storing some water in the fridge for peak times.

Average Water Use within 24 hours

RO tanks take a long time to fill. Let’s say your 3-gallon tank has the capacity to fill three times within a 24-hour period; then your maximum household consumption of RO water needs to be less than 9 gallons a day.


There often isn’t a lot of room under the sink. This will be a deciding factor in the size of the tank you want to purchase vs. what is actually realistic. If you know your water consumption will be more than the storage tank you can fit, you may want to look into a tankless RO system instead.

If you have any questions about RO tank sizes please don’t hesitate to leave a comment below!

About the Author Alexandra Uta

Alex is a content writer with an affinity for research and a methodical attention to detail. Since 2020, she has fully immersed herself into the home water treatment industry only to become an expert herself. Alex has been using water filters and similar products for years which has gained her lots of hands-on experience.
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