Under Sink Reverse Osmosis System: How It Works, Costs, and What to Check Before You Buy

By Filtiva Editorial Team · Updated Oct 31, 2026 · 14 min read

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Under-sink RO system with three filter housings, a membrane, and a storage tank below a sink with a separate RO faucet
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Quick answer

**An under sink reverse osmosis system pushes your cold tap water through several filters and a fine membrane, then sends clean water to its own small faucet and the rejected water down the drain.** HomeGuide puts a typical system at $300 to $950 installed. Before you buy, check two things: that the system is certified under NSF/ANSI 58 for the *specific* contaminant you care about, and how much water it sends to the drain.

Reverse osmosis (RO) is one of the most thorough ways to treat drinking water at home. It is also more than many homes need. The EPA notes that a simple tap filter or pitcher may be all you need if you only want to fix taste or reduce one contaminant (EPA WaterSense).

This guide explains how an under sink RO system works, the stages inside it, tank versus tankless designs, what certification really proves, how much water it wastes, how installation goes, and what it costs to own. It is the first guide in our reverse osmosis guides section.

Safety first: RO systems are meant for water that is already safe to drink from a germ point of view. One manufacturer's manual warns not to use its system "with water that is microbiologically unsafe or of unknown quality without adequate disinfection" (Pentair manual). If you are on a private well, the CDC recommends testing at least once a year with a state-certified lab (CDC).

Do you need an RO system? Start with your water

The CDC's first piece of advice on home filters is to learn what is in your water before you buy anything. You may not need a filter at all. Taste and smell also don't tell you whether water is safe (CDC).

  • City water: Read your annual water quality report, called a Consumer Confidence Report (CCR). EPA says utilities must deliver it by July 1 each year (EPA CCR).
  • Private well: Test with a state-certified lab. EPA's selection guide points well owners to lists of certified labs (EPA guide).

RO makes the most sense when your report or lab test shows something a basic carbon filter does not handle well. Examples are high dissolved solids, sodium, nitrate, arsenic, or fluoride. If you are still choosing between technologies, our under sink water purifier guide compares carbon, RO, and UV side by side.

What RO removes, and what it doesn't

The CDC says RO systems remove common chemical contaminants such as lead, copper, chromium, chloride, and sodium. It says they may also reduce arsenic, fluoride, radium, sulfate, calcium, magnesium, potassium, nitrate, and phosphorus. EPA adds that RO can reduce lead, volatile organic compounds (VOCs), PFAS, arsenic, bacteria, and viruses (EPA WaterSense).

There are trade-offs. EPA's selection guide says RO does not remove all contaminants, removes beneficial minerals such as calcium, and can be costly to install and maintain (EPA guide). The CDC also notes that filters can remove fluoride that your utility adds to protect teeth.

Question What the sources say
Removes lead, copper, chromium, sodium? Yes, per CDC
Reduces arsenic, nitrate, fluoride, radium? May reduce, per CDC; check the certified claim
Reduces PFAS? Can, per EPA; look for a PFAS claim under NSF/ANSI 58 or 53
Kills germs in unsafe water? Not its job; disinfect first or use a UV system
Keeps healthy minerals? No; removes calcium and other minerals

Sources: CDC, U.S. EPA WaterSense, EPA PFAS filter guidance, Pentair manual.

How an under sink RO system works

Inside the membrane, water is pushed against a very fine sheet. Clean water passes through. Most dissolved minerals and contaminants are rejected. EPA describes the output as two streams: treated water (permeate) and a concentrate or brine stream that goes to the drain (EPA WaterSense).

Under-sink systems are the most common type of point-of-use RO, according to EPA. They connect to your cold water line and feed a separate faucet on the sink (EPA WaterSense).

Diagram of an under-sink RO system: sediment and carbon pre-filters, membrane, tank, post-filter, faucet, and drain line

The stages

NSF notes that most RO systems add other filters before or after the membrane (NSF). EPA's PFAS guidance says RO units typically have three to five stages (EPA PFAS). A common layout looks like this:

Stage Job Why it matters
Sediment pre-filter Catches sand, rust, and silt Keeps grit off the membrane
Carbon pre-filter Reduces chlorine and some chemicals Membranes have chlorine limits (one manual: under 2 ppm)
RO membrane Rejects most dissolved solids Does the heavy lifting
Storage tank (tank models) Holds treated water Gives fast flow at the faucet
Carbon post-filter Final carbon stage before the faucet Last step for taste

Sources: NSF, U.S. EPA PFAS filter guidance, iSpring support FAQ and Pentair manual (manufacturers).

Some systems add a UV stage; iSpring, for example, lists a UV lamp among its replacement parts. Treat extra stages as extras, and check what each one is certified to do.

What the membrane needs to work well

Membranes have limits on what they can handle. One manufacturer's specification sheet is a good example of what to look for. For its tankless model, Pentair lists a feed pressure range of 10 to 60 psi, a water temperature of 40–100°F, total dissolved solids under 2,000 ppm, and hardness under 10 grains per gallon. Above that hardness, the manual says scale builds up and shortens membrane life (Pentair manual). These are manufacturer specs for one model, so check the sheet for any system you consider.

If your water is very hard, test it first. Our guide on how to test water hardness covers strips, kits, and lab tests. Some homes soften water before it reaches the RO. Our best water softener buyer's guide explains those options.

Tank vs tankless RO systems

This is the biggest design choice. EPA summarizes it plainly: tankless systems take up less space under the sink, but they may need a booster pump or take a little longer to fill a glass (EPA WaterSense).

Feature Tank system Tankless system
Typical installed cost $300–$700 $450–$950
Unit-only price $150–$350 $300–$600
Rated production 50–400 gallons per day 400–1,200 gallons per day
Space under sink More (separate tank) Less
Power Varies by model Often needs an outlet
Upkeep extra Drain tank about every 2 weeks No tank to drain

Sources: HomeGuide (cost, price, and production ranges); U.S. EPA WaterSense (space, pump, tank draining); Pentair manual (outlet requirement).

Pros

  • Tank systems give a strong, steady flow from stored water
  • Tank systems usually cost less up front
  • Tankless systems free up cabinet space
  • Tankless systems have no tank to drain or sanitize

Cons

  • Tank systems take more space
  • EPA advises draining the tank about every two weeks
  • Tankless systems cost more and often need a power outlet
  • Tankless systems may fill a glass more slowly

As one example of the power need, Pentair's tankless manual asks for a grounded 110-volt outlet within 5 feet of the system (Pentair manual). Look at your cabinet before you buy a tankless RO system.

A note on "gallons per day"

Box ratings and certified numbers often differ. EPA explains that daily production rate is measured under set test conditions (EPA WaterSense). In NSF's RO listings, many systems sold as "50" or "75" gallon-per-day models show certified daily production rates closer to 11–15 gallons per day (NSF listings). Your real output also depends on your water pressure, temperature, and dissolved solids.

NSF/ANSI 58: what certification proves, and what to verify

There are no federal rules for home water filters, according to NSF. Instead, independent standards cover them. NSF/ANSI 58 is the standard for reverse osmosis systems (NSF).

The catch is that a certification does not mean a system reduces every contaminant. NSF says to check which specific claims a product is certified for. In NSF's own RO listings, many systems list only "TDS Reduction." Others add claims such as arsenic, barium, cadmium, chromium, copper, cyst, fluoride, lead, nitrate/nitrite, radium, selenium, or turbidity (NSF listings).

Here is a five-step check before you buy:

  1. Find the model in a certifier's database. Use the listing on the certifier's own site, not a screenshot. NSF warns that fake copies of its listings exist.
  2. Read the claims list. Make sure your contaminant is named. A "58" logo alone only tells you it passed the RO standard.
  3. Read the fine print. Arsenic claims, for example, often say they apply only to water at or below a set concentration and type of arsenic.
  4. Check the performance data sheet. EPA says the packaging should include a summary table and data sheet listing certified contaminants (EPA WaterSense).
  5. Match the replacement parts. NSF listings name the replacement filters that keep the system in its certified setup.

PFAS claims

EPA says its research found RO, carbon, and ion exchange home units can greatly reduce PFAS if they are maintained. It advises looking for a PFAS reduction claim under NSF/ANSI 58 or 53, from certifiers such as CSA, IAPMO, NSF, UL, or WQA. EPA also cautions that, as of its April 2024 review, current certifications did not yet show removal down to its new drinking water standard (EPA PFAS). If PFAS is in your water report, ask your state agency or utility for guidance too.

Waste water and the WaterSense label

Every RO system sends some water to the drain. That water carries away what the membrane rejected. The amount varies a lot.

EPA says a typical point-of-use RO system sends five gallons or more to the drain for every gallon of treated water. Some inefficient units send up to 10 gallons. Newer membrane technology can bring that down to one or two gallons (EPA WaterSense).

Bar chart of RO drain water per gallon treated: up to 10 for some units, 5 or more typical, 2.3 or less with WaterSense

In November 2024, EPA released a WaterSense label for RO systems. To earn it, a system must:

  • Have an efficiency of at least 30%. Efficiency is treated water divided by the total water supplied.
  • Send no more than 2.3 gallons to the drain per gallon of treated water.
  • Be certified to the same contaminant reduction standards as other systems.

Source: U.S. EPA WaterSense.

EPA estimates that a household that picks a WaterSense labeled system over a typical one can save more than 3,100 gallons a year, close to 200 showers' worth, and nearly $50 a year on water and sewer bills. It also notes that retrofit kits can reuse reject water (EPA WaterSense).

Manufacturers publish their own ratios too. For example, Pentair says its tankless model sends less than 1 gallon to the drain per gallon produced, and its data sheet lists a 54.90% efficiency rating (Pentair manual). That is a manufacturer claim. The WaterSense label is the easier way to compare.

Installing an under sink RO system

Most RO systems are sold as DIY kits, but the job touches your cold water line and your drain. HomeGuide puts professional labor at about $150 to $350, or $45 to $150 an hour for a plumber (HomeGuide).

A typical install follows these steps. This outline draws on one manufacturer's manual (Pentair manual); always follow your own system's instructions.

  1. Shut off the cold water and open the faucet to release pressure.
  2. Mount the faucet. Use a spare sink hole or soap-dispenser hole. If you need a new hole, the manual calls for a 3/4" hole in most sinks.
  3. Install the drain clamp on a vertical section of the drain pipe, about 6 inches above the trap. Drill through one side of the pipe at the clamp's opening.
  4. Connect the feed to the cold water line with the supplied valve.
  5. Mount the unit, and the tank if there is one, and run the tubing without loops or kinks.
  6. Flush the system as the manual directs before drinking the water, and check every fitting for leaks.

Keep the system somewhere it can't freeze. The Pentair manual says frozen water can damage the system.

When to hire a pro: if your sink is stone or porcelain, if you have no spare sink hole, if there's no outlet for a tankless unit, or if you are not comfortable drilling into a drain pipe. HomeGuide suggests hiring someone licensed, bonded, and insured, ideally WQA-certified, and getting a written estimate (HomeGuide).

Cost and maintenance

Upfront cost

HomeGuide's figures give a realistic starting budget for anyone asking how much it costs to install a reverse osmosis system:

Item Typical cost
Under-sink RO, installed $300–$950
Tank system, installed $300–$700
Tankless system, installed $450–$950
Professional labor $150–$350
Home test kit $10–$30
Lab water test $100–$300

Source: HomeGuide, Reverse Osmosis System Cost (updated Jul 2025).

For comparison, HomeGuide lists countertop RO at $200 to $450 and whole-house RO at $4,800 to $8,000. If you are weighing whole-home treatment instead, see our whole house water filtration system cost guide.

Ongoing cost

HomeGuide estimates RO maintenance at $50 to $250 a year. Replacement filters run $20 to $100 every 6 to 12 months, and membranes last 2 to 5 years. It works out to roughly $0.03 to $0.10 per gallon of treated water (HomeGuide).

Maintenance schedule

Intervals depend on your water and the model. Here is what the sources say:

Task How often Source
Replace pre-filters About every 6 months U.S. EPA
Replace pre-filters Every 6–12 months iSpring (manufacturer)
Replace carbon post-filter Every 12 months iSpring (manufacturer)
Replace membrane Every 1–5 years U.S. EPA
Drain the storage tank About every 2 weeks U.S. EPA
Sanitize the system Yearly (many manufacturers) U.S. EPA

Sources: U.S. EPA WaterSense RO report; iSpring support FAQ (manufacturer).

EPA says a well-maintained system can last as long as 15 years (EPA WaterSense). The CDC adds that people with weakened immune systems should not change filters themselves if possible. If they must, they should wear gloves and wash their hands afterward (CDC).

A handheld TDS meter is a cheap way to spot a tired membrane. iSpring advises replacing the membrane sooner if the TDS of your RO water starts to rise (iSpring). Compare a reading from the RO faucet with one from the kitchen tap every few months.

For a schedule that covers every filter in your home, see our water filtration maintenance schedule.

Signs your RO system needs service

EPA lists a few warning signs: less water than usual, low pressure at the faucet, or a strange taste or smell. It also says that water running to the drain all the time can mean the automatic shutoff valve needs to be replaced (EPA WaterSense).

Check these first:

  • Slow flow: Pre-filters may be clogged. Replace any overdue filters first.
  • Water always running to the drain: Have the automatic shutoff valve checked.
  • Odd taste or smell: Check whether filters are due, and drain or sanitize the tank as your manual directs.
  • Leaks: Turn off the feed valve and check fittings and housings.

Our water filtration repair guide walks through these fixes step by step. If you are on a well, our guide to well water filtration systems covers the pretreatment that often comes before RO.

The bottom line

An under sink RO system is a strong choice when a water test shows dissolved contaminants that basic filters miss. Buy it based on your water report, not on a sales pitch. Confirm the exact contaminant claim in a certifier's listing. Favor a WaterSense labeled model to cut wasted water. Then budget for filters every 6 to 12 months and a membrane every few years.

FAQ

Frequently asked questions

How much does an under sink reverse osmosis system cost?
HomeGuide puts a typical under-sink RO system at $300 to $950 installed. Tank systems run about $300 to $700 and tankless systems about $450 to $950. Plan for $20 to $100 per replacement filter and $50 to $250 a year in upkeep.
Is a tankless RO system better than one with a tank?
Neither is better for everyone. EPA says tankless systems take up less space but may need a booster pump or take a little longer to fill a glass. Tank systems store treated water so it is ready on demand, but EPA advises draining the tank about every two weeks.
How much water does an under sink RO system waste?
EPA says a typical point-of-use RO system sends five gallons or more down the drain for every gallon of treated water, and some inefficient units send up to 10. WaterSense labeled systems must send no more than 2.3 gallons to the drain per gallon treated.
What does NSF/ANSI 58 certification mean?
It means the system was tested to the reverse osmosis standard. It does not mean it reduces every contaminant. Many systems in NSF's listing claim only TDS reduction, so check the listing or performance data sheet for the exact contaminant you need to remove, such as lead, arsenic, nitrate, or PFAS.
How often do you change RO filters and the membrane?
EPA says most RO systems need new pre-filters every six months and a new membrane every one to five years, depending on water quality and use. Follow your own system's manual, since manufacturer schedules differ.
Does reverse osmosis remove bacteria and viruses?
EPA says RO can reduce bacteria and viruses, but RO systems are labeled for water that is already microbiologically safe. If your water may contain germs, such as an untested private well, get it tested by a state-certified lab and treat or disinfect it first.

Sources

Filtiva earns commissions from some links on this page at no extra cost to you. Commissions never change our criteria. We have not hands-on tested the products named; all product facts come from the linked certification listings and manufacturer pages, checked on October 31, 2026.