Everything You Need to Know About Reverse Osmosis Flow Rate

Everything You Need to Know About Reverse Osmosis Flow Rate

If you’re a homeowner who has a reverse osmosis (RO) filter or is considering purchasing one, it’s important to understand that they produce water at a flow rate that is much lower than water coming directly from your tap. There are many factors that affect the flow rate of an RO system, including the size of the unit, your water pressure, how well maintained your system is, and the quality of the filter (you get what you pay for). Having a tank to store treated water will also give you a higher short-term flow rate for things like cooking or getting a glass of water.

In this blog post, we’ll discuss what affects reverse osmosis flow rate and how you can choose the right system for your needs. We’ll also provide some tips on how to maintain your reverse osmosis system so that you get the best flow performance possible.

While it may be frustrating to wait a minute or two for your RO water, remember that you’re getting pure water with all the contaminants removed. That’s worth the wait!

What Is Reverse Osmosis?How does reverse osmosis work

Reverse osmosis (RO) is a safe and reliable way to remove many common contaminants from drinking water. A RO water treatment system uses a semi-permeable membrane that allows water to pass through while blocking impurities such as heavy metals, VOCs, chlorine, and a lot more.

Osmosis is the process of water diffusing through a semi-permeable membrane in order to equalize solution strength by moving from lower concentration to one of higher concentration. Reverse osmosis is the opposite of this – pressure is used to force the water in the concentrated solution through the membrane. This separates the water from the contaminants.

In order for an RO system work, the applied pressure must be greater than the osmotic pressure produced by the soluble salts in your drinking water.

Read my articles on RO Systems:
Complete Guide to Under-sink Reverse Osmosis Filters
The Definitive Guide to Whole House Reverse Osmosis Filter Systems

Reverse Osmosis Flow RateUndersink reverse osmosis system

The flow rate of a residential RO filter is measured in gallons per day (gpd). One of the biggest factors that will affect your reverse osmosis flow rate is the size of your unit. If you have a smaller unit, it will likely have a slower flow rate than a larger unit. This is because the smaller unit has to work harder to filter out the same amount of water as the larger unit.

A reverse osmosis filter for a typical house should have a flow rate of at least 500 gpd. Any less than this, and it will take a long time to get a glass of water or fill a pot of water to cook with. RO filters with higher flow rates cost more initially, so you’ll want to be sure you select the right size for your needs.

If your RO filter is too small, you’ll spend all day waiting on water to be produced. If you oversize the system, then you’ll pay more than you need to.

Typical RO flow rates

The flow rates for RO water systems on the market today are:

  • 50 gpd
  • 75 gpd
  • 100 gpd
  • 400 gpd
  • 500 gpd
  • 600 gpd
  • 1,000 gpd

Reverse osmosis systems with higher flow rates cost more than lower flower flow units.

What RO flow rate is right for you?

Even with adequate water pressure feeding the system, the flow rate of filtering water is much slower than that of tap water directly from the plumbing. So, what is the best flow rate for a reverse osmosis treatment system?

Let’s do the math. 500 gallons per day is equal to 20.83 gallons per hour. This is the same as 0.347 gallons per minute.

Filling a pot with one quart of water will take you about 3/4 of a minute. Does 45 seconds seem like a long time to fill a pot of water to cook with? If so, you’ll want a RO filter with a higher flow rate.

Increasing the flow rate from 500 to 1,000 gpd will shorten the time to fill your cooking pot from 45 seconds to 22 seconds. This will also increase the cost of your RO system.

Water storage tank to boost flow rateReverse Osmosis Water Storage Tank

In order to get the same amount of water in a shorter time, you will need to increase the size of your storage tank. Another option is to add a water storage tank to allow you to quickly fill a glass or a pot without waiting on the filter to process the amount of water you need.

Many RO filters with a slow flow rate (less than 500 gpd) include a water storage tank. A typical RO water tank is approximately 5 gallons in size. With the air bladder, this means it has a usable capacity of about 2.5 gallons.

A 5-gallon tank will allow you to fill a pot with one quart of water in about 15 seconds. This is much faster than waiting for the RO filter to process the same amount of water.

If you have a household of five or less people, this will likely be more than enough.

What Causes a Low Flow Rate in an RO System?

Like any complicated system, RO treatment units rely on every part working together properly. Unfortunately, components can sometimes go bad, so it’s critical that your system be maintained on a regular basis. This can assist you in detecting little problems before they become larger issues that are more expensive and difficult to repair. A great example of this is a reduced flow rate.

Here are the top 5 reason your RO filter might have low flow.

  1. Fouled or clogged filters: All RO systems have particle filters to remove suspended solids, rust, and other particles from the water. Many RO units also have carbon filters to remove chlorine and other impurities These filters become dirty and have to be replaced regularly. Generally, they should be replaced annually. Some manufacturers recommend replacing them every 6 months. Read my article How to Measure Reverse Osmosis Filter System Removal Effectiveness
  2. Fouled RO membrane: The RO membrane is the heart of the system and is responsible for removing dissolved contaminants. The membrane will eventually become fouled with scale, bacteria, and other deposits. A fouled membrane will reduce flow and production of treated water. In general, RO membranes should be replaced every two years. However, turbid water, high mineral content, or improper maintenance of filters can shorten the lifespan of the membrane.
  3. Low air pressure in the RO water tank: If your RO system has a water storage tank, then the air pressure may be low. These tanks have a bladder that is filled with compressed air to maintain water pressure. Over time, the air can leak out, resulting in low water flow. Check the air pressure with a gauge. It should be between 7 and 8 psi. If it’s low, add more compressed air to restore the pressure.
  4. Ruptured or leaking tank bladder: Sometimes, the air bladder inside the RO water tank can rupture. This will also result in low water pressure and reduced flow. You’ll need to replace the entire water tank if this happens because the air bladder can’t be replaced or repaired.
  5. Kinked water tubing: All residential RO filters use flexible tubing to connect all of the system components. If any of these tubes are kinked, it can restrict water flow and cause low pressure. Check all of the tubing to make sure there are no kinks or blockages. Check for closed or partially closed valves, too.
  6. Low water pressure: The water pressure supplied to your home can sometimes decrease. This can be due to a problem with the municipal water supply or problems with your home’s plumbing. Check your home’s water pressure with a gauge. It should be between 40 and 60 psi. If it’s lower than that, you’ll need to have your plumbing checked for leaks or other problems.

You may be interested in my other article – Reverse Osmosis Troubleshooting Guide: 10 Common Problems.

How to Determine Your RO System’s Flow Rate

If you already have a reverse osmosis system, you can measure the flow rate with these five steps.

Step 1: Verify the water to your RO system is “ON”. If you have a water storage tank, close the valve on top of the tank “OFF”.

Step 2: If you have a filtered water faucet, turn it “ON” by opening the valve or flipping the handle. Lock it in open position so the water flows continuously.

Step 3: Allow the water to run for a few minutes until it slows to a slow drip from the faucet. This is the rate of flow that your RO filter is producing treated water. Under normal conditions, this water would be filling the storage tank (it is now closed from Step 2 above). If you don’t have any flow, then your reverse osmosis system is not producing water.

Measure the volume of water from RO filterStep 4: Fill a measuring cup from the RO faucet for 60 seconds. Use a timer for accuracy!

Step 5: Multiply the ounces of water collected by 11.25. This is the number of gallons per day your RO filter is producing.

If your flow rate is less than 500 gpd, then you may have a unit that is too small for your needs.


How do you increase reverse osmosis water flow?

If you have a reverse osmosis system and the flow rate is too low, there are a few things you can check. First, make sure that there is no blockage in the tubing or at any of the valves. Next, check the pressure of the air bladder inside the water tank (if your system has one). If it’s low, you can add more air. Finally, check the municipal water supply to see if the pressure is low. If it is, you’ll need to have your plumbing checked for leaks or other problems.

Why is my reverse osmosis system so slow?

There are a few potential causes of slow reverse osmosis flow: blockages in the tubing or valves, low air pressure in the water tank, a ruptured or leaking tank bladder, or low municipal water pressure.

How do you size a reverse osmosis system?

The size of the reverse osmosis system you need depends on the amount of water you use each day. A family of four, for example, will need a system that can produce at least 500 gallons per day.


If you’re having trouble with your reverse osmosis system’s flow rate, these are the things you can check. Low water pressure, a ruptured tank bladder, or kinked tubing can all restrict the flow of water. You can also increase the flow rate by adding more air pressure to the water tank or increasing the municipal water pressure. If your system is too small for your needs, you may need to upgrade to a larger model.

If you have any questions about reverse osmosis flow rate or how to size a system, please leave a comment below! Thanks for reading!

Chief Guru

Richard Boch is a chemical engineer responsible for designing water filtration systems for industrial and residential customers. He has more than 20 years of experience with ion exchange, activated carbon, and reverse osmosis. Richard's expertise has made him a go-to source for municipalities and businesses looking to improve their water quality. When he's not working, Richard enjoys spending time with his wife and two young children. You can also follow him on LinkedIn, Twitter and Facebook.

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