How Many Stages Should My Reverse Osmosis System Have?

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It’s time to look at purchasing a reverse osmosis system! The three-stage seems good. Small, functional, and it will fit under my sink easily! Oh, but wait. A 4-stage would also fit, and it isn’t that much more expensive. Hang on, if I’m thinking about the 4-stage, I may as well go all out and get the 5-stage, right? For super water purification?

This is a common thought process for many people, so we are here today to help you figure out how many stages your reverse osmosis system should have and what to consider before buying one.

Key Takeaways

  • A reverse osmosis system should have a minimum of 3 filtration stages: Carbon pre, RO membrane, and carbon post.
  • If your water is high in sediments, you should add a fourth stage: A sediment pre-filter.
  • Any other filter stages, such as remineralization and UV, are optional and should be added based on your water quality.

How Many Stages Should My Reverse Osmosis System Have?

So, how many stages should a reverse osmosis system have?

A reverse osmosis system should ideally be designed around the quality of your water. It is recommended to have a minimum of three stages, though: a carbon pre-filter, a reverse osmosis (RO) membrane, and a carbon post-filter.

The pre-filter plays a crucial role by removing chlorine and coarse dirt from the water, which prevents the fouling of the RO membrane, ensuring its longevity and optimal performance. The RO membrane itself eliminates almost all impurities from the water, while the carbon post-filter serves as the final stage to further enhance taste, especially when water has been stored in a tank.

That said, you may need more than these three stages!

Different Water Filter Cartridges and Membranes

Considerations When Choosing Your Ideal Number of Filter Stages

When choosing how many filtration stages you want in your RO system setup, there are a few things you need to consider.

Water Quality (Contamination)

This is the most crucial consideration when thinking about purchasing a reverse osmosis system. What is in your water, and in what quantities?

  • For example, if your water supply is very low in sediment, you might not need a sediment pre-filter, but if it is high in sediment, you might want one or two!
  • And if you are facing elevated levels of iron, an iron filter stage will come in handy.

Ideally, you would have your water professionally tested to see what is in it and then choose your number and type of filter stages accordingly.

What You Want to Achieve

What is your motivation for wanting to filter your water? Is there a particular smell you are trying to get rid of or a specific contaminant? While reverse osmosis systems will cover all your bases, if you want to simply eliminate the taste of chlorine from your water and nothing else, then you may not even need an RO system at all!

Available Space

Space considerations are another thing. The more filter stages you have, the more space the module will occupy under your kitchen sink. You may only have room for a three-stage!


Installation and maintenance requirements for 3, 4, or 5-stage reverse osmosis systems are relatively similar, though more stages mean a little more work and higher maintenance costs.

Good RO Filter Stages to Have

At a minimum, most reverse osmosis systems will come with a carbon pre-filter, the reverse osmosis membrane filter stage, and a carbon post filter. Aside from those, some other common filtration stages are sediment pre-filters, a UV water purifier, and a remineralization filter.

Sediment Pre-Filter

This pre-filter is good to have if your water contains high levels of dirt and debris. It will remove them prior to the water reaching the reverse osmosis membrane. If you have a lot of sediment in your water and do not remove it first, you risk the RO membrane becoming clogged.

Carbon Filters

Both the carbon pre and post-filters aid in reducing unpleasant tastes and odors in water, plus all kinds of organics. The carbon pre-filter also removes chlorine from the water before it damages the reverse osmosis membrane.

Reverse Osmosis Membrane

The reverse osmosis membrane is the heart of the system. It eliminates more water contaminants than any other filtration step, including salts, heavy metals, microorganisms, microplastics, asbestos, and various chemicals and compounds.


Reverse osmosis removes not only nasty impurities from water but also good ones, such as calcium and magnesium. Some prefer to have those minerals added back into the water after RO filtration for health and taste reasons. This is the purpose of a remineralization filter.

UV Disinfection

If the feed water contains microorganisms such as bacteria or cysts, an optional UV purifier can be a valuable addition. Utilizing ultraviolet light, this purification stage kills pathogens in the water by scrambling their DNA. It may not be necessary for homes that use treated municipal water, but it is very helpful if your home has well water!

under sink reverse osmosis system

3-Stage vs 4-Stage vs 5-Stage Reverse Osmosis Systems

Here’s a breakdown of the differences between the most popular at-home setups: 3-stage, 4-stage, and 5-stage RO systems:

3-Stage RO System

A 3-stage RO system consists of three filtration stages. The first stage is a carbon pre-filter that eliminates chlorine and some other chemicals from the water. The second stage involves the reverse osmosis membrane. Finally, the water moves to the carbon post-filter for a final polish.

4-Stage RO System

A 4-stage RO system includes additional filtration stages for enhanced purification. Before reaching the reverse osmosis membrane, a sediment pre-filter and a carbon pre-filter eliminate debris and chlorine. After the pre-filtration stages, the water passes through the reverse osmosis membrane. The final step involves caron post-filtration.

5-Stage RO System

A 5-stage RO system offers even more options for customization. Various filter configurations can be used. For example, you could have three pre-filters (2x sediment + 1x carbon), the RO membrane, and a carbon post-filter. Another option is using two pre-filters (1x sediment + 1x carbon), the RO membrane, a carbon post-filter, and a remineralization filter or a UV lamp for additional purification.

Are RO Systems with More Filter Stages Always Better?

Selecting the appropriate number of filter stages for a reverse osmosis system isn’t a simple decision. The effectiveness of the system relies on the specific filters utilized. A higher number of stages doesn’t automatically guarantee superior outcomes, especially if your water doesn’t need that much filtering. This is why having your water quality checked will be the best starting point for deciding your RO setup.

If you have any thoughts about the question, how many stages should my reverse osmosis system have, 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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