Buying a reverse osmosis system can be confusing. There are tank-based systems, tankless systems, countertop units, systems with booster pumps, remineralization filters, UV stages, different gallon-per-day ratings, and a wide range of contaminant-reduction claims.

The best reverse osmosis system is not necessarily the one with the most filtration stages or the highest price. The right system is the one that matches your water quality, household needs, installation space, budget, and specific contaminant concerns.

This Reverse Osmosis Buying Guide explains what to look for before purchasing an RO system and how to compare the features that actually matter.

For a complete explanation of how reverse osmosis works, what contaminants it can reduce, and the different types of RO systems available, visit RevOsmo’s The Ultimate Guide to Reverse Osmosis Systems.

Quick Answer: What Should You Look for When Buying a Reverse Osmosis System?

Before buying an RO system, evaluate these factors:

Your water quality and contaminants
Third-party certification and contaminant claims
RO system type
Water production capacity
Water efficiency and reject-water ratio
Tank vs. tankless design
Available installation space
Water pressure requirements
Filter and membrane replacement costs
Maintenance requirements
Additional features such as remineralization or UV
Warranty and replacement-part availability

The most important rule is:

Choose the system based on what you need removed from your water—not based on the number of filtration stages listed on the box.

NSF/ANSI 58 covers reverse osmosis drinking-water treatment systems and includes requirements for areas such as material safety, structural integrity, TDS reduction, efficiency, recovery, and contaminant-reduction performance. Specific optional contaminant claims can include substances such as lead, arsenic, fluoride, nitrate/nitrite, chromium, radium, VOCs, and others. (NSF)

Step 1: Test Your Water Before Buying an RO System

The best starting point is to understand what is actually in your water.

Don’t assume you need reverse osmosis simply because:

Your water tastes bad
Your neighbor owns an RO system
Your TDS meter shows a high number
A product advertisement says RO provides “pure water”

Start by identifying your water source.

Municipal water

If you receive water from a public utility, review your local Consumer Confidence Report and consider testing your water at the tap if you have concerns about plumbing, lead, taste, or a specific contaminant.

Private well water

Well owners should pay particular attention to water testing because water quality can vary significantly based on local geology, agricultural activity, plumbing, and other factors.

Potential concerns can include:

Arsenic
Nitrate
Nitrite
Iron
Manganese
Radionuclides
Bacteria
Hardness
Other dissolved contaminants

Once you know what you’re trying to reduce, you can select the appropriate technology.

For a broader discussion of when RO makes sense, link readers to your article:

Do You Need Reverse Osmosis?

Step 2: Check Exactly What the System Is Certified to Reduce

This is arguably the most important buying factor.

Don’t simply look for:

“NSF Certified”

Instead, find out what the complete system is certified or tested to reduce.

NSF/ANSI 58 includes required TDS-reduction performance and optional contaminant-reduction claims, which may include:

Lead
Arsenic
Fluoride
Nitrate/nitrite
Hexavalent chromium
Trivalent chromium
Cadmium
Barium
Copper
Selenium
Radium 226/228
VOCs
Perchlorate
Cysts
Turbidity
Asbestos

The available claims depend on the specific product and testing. (NSF)

Important: Certification Is Product-Specific

Two RO systems can both use an RO membrane but have very different certified contaminant claims.

For example, current NSF listings can show one system with claims for a broad group of contaminants while another is listed only for TDS reduction. That is why buyers should verify the exact model and its claims instead of assuming every RO system has identical performance. (NSF International)

What to look for

If your primary concern is:

Lead → Verify lead reduction.
Fluoride → Verify fluoride reduction.
Nitrate → Verify nitrate/nitrite reduction.
Arsenic → Verify the appropriate arsenic claim.
Chromium-6 → Verify hexavalent chromium reduction.
Radium → Verify radium 226/228 reduction.
VOCs → Verify VOC reduction.
High TDS → Compare TDS-reduction performance.

Never assume that an RO system removes a particular contaminant just because another RO system does.

Step 3: Choose the Right Type of Reverse Osmosis System

There are several major types of RO systems.

Under-Sink RO Systems

This is the most common residential option.

The system is typically installed beneath the kitchen sink and supplies treated water through a dedicated faucet or compatible dispensing setup.

Best for:

Homeowners
Families
Long-term installations
Drinking and cooking water
Higher daily water demand

Advantages:

Continuous access to treated water
Often lower cost per gallon than bottled water
Can offer multiple filtration stages
Large selection of systems

Disadvantages:

Requires installation
Uses cabinet space
Requires periodic maintenance

Tank-Based RO Systems

Traditional RO systems use a storage tank to hold treated water.

Advantages:

Stored water is immediately available
Often lower initial cost
Proven and widely available design

Disadvantages:

Requires more under-sink space
Can have slower refill times
Tank takes up cabinet room

Tank-based systems remain a practical option for many households.

Tankless RO Systems

Tankless systems produce treated water on demand.

Advantages:

Smaller footprint
No large storage tank
Often higher production rates
Modern, streamlined design

Disadvantages:

Usually higher upfront cost
May require electricity
Replacement filters may cost more
Performance depends heavily on water pressure and system design

Tankless RO is particularly appealing if under-sink space is limited.

Countertop RO Systems

Countertop RO systems are designed for users who don’t want permanent under-sink installation.

Best for:

Renters
Apartments
Temporary housing
Users who want portability

Advantages:

Little or no plumbing modification
Portable
Easy to move

Disadvantages:

Smaller capacity
Counter space required
Some designs require manual filling
Performance and capacity vary substantially

Step 4: Understand RO Production Capacity

RO systems are often rated in gallons per day (GPD).

Traditional systems may have ratings such as:

50 GPD
75 GPD
100 GPD

Tankless systems may advertise much higher production capacities.

But don’t choose a system based on GPD alone.

A higher GPD rating doesn’t automatically mean:

Better contaminant reduction
Better efficiency
Better water quality
Lower operating costs

Instead, consider how much treated water your household actually uses.

For most households, RO is primarily used for:

Drinking
Cooking
Coffee
Tea
Ice

You usually don’t need enough RO capacity to supply your entire home’s showers, toilets, laundry, and irrigation.

Step 5: Compare Water Efficiency

Water efficiency is one of the biggest differences between RO systems.

RO creates:

Permeate: the treated water you use
Reject water: water carrying concentrated rejected substances away from the membrane

Traditional point-of-use RO systems can send significant quantities of water down the drain. EPA states that a typical system may generate five gallons or more of reject water for every gallon of treated water produced, while WaterSense-labeled systems must meet stricter efficiency criteria and send no more than 2.3 gallons of water down the drain per gallon of treated water produced. (US EPA)

What to compare

When evaluating systems, look for:

Efficiency rating
Recovery rating
Water waste ratio
WaterSense labeling
Automatic shutoff

EPA’s WaterSense specification requires qualifying point-of-use RO systems to be certified to NSF/ANSI 58 and meet additional efficiency and performance requirements. (US EPA)

Buying tip

If two systems offer comparable contaminant-reduction performance, the more water-efficient model may provide better long-term value.

Step 6: Decide Between a Tank and Tankless Design

This decision largely comes down to space, convenience, and budget.

Feature	Tank-Based RO	Tankless RO
Under-sink space	More required	Usually less required
Water storage	Yes	No large tank
Initial cost	Often lower	Often higher
Production	Stored and replenished	On demand
Electricity	Often not required	Often required
Modern compact design	Less common	Common
Installation complexity	Moderate	Moderate
Flow rate	Depends on tank/system	Depends on model

Choose a tank-based system if:

You want a traditional design.
You prefer stored water availability.
You want to minimize upfront cost.
You have enough cabinet space.

Choose a tankless system if:

Under-sink space is limited.
You prefer a compact design.
You want high on-demand production.
You don’t mind a potentially higher initial investment.

Step 7: Measure Your Under-Sink Space

Before purchasing an RO system, measure:

Width
Depth
Height
Plumbing location
Electrical outlet availability
Garbage disposal location
Existing filtration equipment

This is especially important for tank-based systems.

A system may technically fit beneath your sink but leave insufficient room for:

Filter replacement
Maintenance
Tubing
Drain connections

For tankless systems, also check whether you need an electrical outlet nearby.

Step 8: Check Your Water Pressure

RO systems depend on pressure to move water through the membrane.

Low water pressure can affect:

Production speed
Water efficiency
System performance

Some systems use an integrated booster pump to improve performance.

Before buying, compare:

Minimum operating pressure
Maximum operating pressure
Recommended pressure range
Whether a booster pump is included
Whether the system requires electricity

If your home’s water pressure is unusually low, a system designed for low-pressure operation may be a better choice.

Step 9: Don’t Be Fooled by the Number of Filtration Stages

A common marketing tactic is to emphasize:

“5-stage filtration”
“7-stage filtration”
“10-stage filtration”

More stages do not automatically mean better water.

For example, one system might count:

Sediment filter
Carbon filter
Carbon filter
RO membrane
Post-carbon filter

Another may add stages for:

Remineralization
UV treatment
Alkaline media
Additional carbon
Other polishing filters

The number of stages doesn’t tell you the actual contaminant-reduction performance.

A better question is:

What does the complete system independently verify that it reduces?

NSF/ANSI 58 focuses on measurable performance and can include specific contaminant-reduction claims rather than simply counting filter stages. (NSF)

Step 10: Consider Remineralization

Reverse osmosis reduces many dissolved minerals.

Some people prefer the taste of water with more mineral content.

A remineralization stage can add selected minerals back after RO treatment.

Consider remineralization if:

You prefer a less neutral taste.
You want mineralized RO water.
You want to modify the final water chemistry.

You may not need it if:

You prefer the taste of standard RO water.
You want the simplest system possible.

Remember that remineralization is a post-treatment preference, not evidence that the RO process itself is inadequate.

Step 11: Consider UV Treatment for Microbial Concerns

Some systems include UV treatment.

UV may be useful when microbial contamination is a specific concern, particularly with certain private water supplies.

However, UV isn’t automatically necessary for every household.

If you’re buying an RO system for well water with bacteria concerns:

Test the water.
Identify the microbial issue.
Select appropriate treatment.
Verify system specifications.

RO, UV, sediment filtration, and other treatment technologies each serve different purposes.

Step 12: Calculate the Real Cost of Ownership

Don’t compare RO systems based only on purchase price.

Calculate:

Initial cost

The price of the system and installation.

Annual filter cost

How often do prefilters and postfilters need replacement?

Membrane cost

How much does the RO membrane cost, and how long is it expected to last?

Electricity

Tankless and pumped systems may use electricity.

Water cost

Less-efficient systems send more water to the drain.

Installation costs

Do you need:

A plumber?
An electrical outlet?
Additional fittings?
A new faucet?

A $250 system with $200 in annual replacement filters may cost more over several years than a $400 system with lower maintenance costs.

Step 13: Check Replacement Filter Availability

This factor is often overlooked.

Before buying, verify:

Are replacement filters readily available?
Are they proprietary?
Can they be purchased from multiple retailers?
How much do they cost?
How frequently do they need replacement?
Is the membrane easy to obtain?

A system is only a good long-term investment if you can maintain it.

Step 14: Look at Filter Replacement Intervals

Manufacturers commonly specify different replacement schedules for:

Sediment filters
Carbon filters
RO membranes
Post-carbon filters
Remineralization cartridges
UV lamps

Don’t assume all filters need replacement at the same time.

Create a maintenance-cost estimate before purchasing.

For example:

Component	Typical Replacement Consideration
Sediment filter	Depends on water quality and manufacturer schedule
Carbon prefilter	Depends on chlorine/chloramine exposure and manufacturer schedule
RO membrane	Depends on water quality, use, and manufacturer schedule
Post filter	Depends on manufacturer schedule
Remineralization cartridge	Depends on media capacity and manufacturer schedule

Always follow the schedule for the specific model.

Step 15: Consider Installation Difficulty

Most under-sink RO systems require connections to:

Cold-water supply
RO system inlet
Drain line
Storage tank, if applicable
Dedicated faucet or dispensing connection

Some modern systems simplify installation with:

Push-to-connect fittings
Integrated filter housings
Tool-free filter replacement

Before buying, check the installation manual.

Ask:

Do I need to drill a faucet hole?
Can I connect to an existing soap-dispenser hole?
Is a drain saddle required?
Does the system require electricity?
Is professional installation recommended?

Renters should also consider whether countertop or non-permanent systems are a better fit.

Step 16: Choose Features That Match Your Needs

Some useful features include:

Leak detection

Can alert you to leaks or automatically shut off water.

Automatic shutoff

Helps stop water production when the system reaches its storage capacity.

Filter-life indicators

Can simplify maintenance scheduling.

TDS monitoring

Can provide an indication of changes in dissolved-solids performance.

However, remember that a TDS meter doesn’t identify individual contaminants.

Booster pump

Useful for systems operating with insufficient incoming water pressure.

Remineralization

Changes final mineral content and taste.

UV

May be appropriate for specific microbial treatment needs.

Choose features because they solve a real problem—not simply because they increase the number of features on the product page.

Step 17: Check the Warranty and Manufacturer Support

A good RO system should have clear information about:

Warranty period
What components are covered
Replacement parts
Filter availability
Customer support
Installation instructions
Maintenance instructions

Be cautious when a manufacturer doesn’t clearly explain how to maintain its system.

Step 18: Compare RO Systems Using This Checklist

Before purchasing, compare your finalists using this scorecard.

Water Quality

I know my water source.
I have reviewed available water-quality information.
I have tested for contaminants relevant to my situation.

Contaminant Reduction

The system addresses my specific contaminants.
I verified the exact model’s claims.
I checked third-party certification where available.

Capacity

The system produces enough water for my household.
I understand its GPD rating.
I understand whether it uses a tank.

Efficiency

I compared water-efficiency information.
I understand the reject-water ratio.
I considered a WaterSense-labeled system.

Installation

I measured my available space.
I checked water-pressure requirements.
I verified whether electricity is needed.
I reviewed the installation requirements.

Long-Term Cost

I calculated replacement-filter costs.
I checked membrane replacement costs.
I verified replacement-part availability.

Maintenance

I understand the maintenance schedule.
I can easily obtain replacement filters.
I am willing to maintain the system properly.

What Is the Best RO System for Your Home?

There isn’t one best system for everyone.

The best reverse osmosis system depends on whether you prioritize:

Best for contaminant reduction

Look for the specific independently verified claims relevant to your water.

Best for water efficiency

Compare WaterSense-labeled models and system efficiency.

EPA says WaterSense-labeled point-of-use RO systems are independently certified to meet EPA criteria for efficiency and performance, including requirements related to membrane life, TDS reduction, and any additional contaminant claims made by the manufacturer. (US EPA)

Best for apartments

Consider compact or countertop RO systems.

Best for renters

Prioritize systems requiring minimal permanent plumbing changes.

Best for families

Consider capacity, production rate, filter costs, and convenience.

Best for small kitchens

A compact tankless design may be preferable.

Best for tight budgets

Compare five-year ownership costs, not just the initial purchase price.

Common Mistakes to Avoid When Buying an RO System

Mistake #1: Buying the system with the most stages

Stage count is not a reliable measure of performance.

Mistake #2: Assuming every RO system removes the same contaminants

Verify the exact model’s contaminant claims.

Mistake #3: Ignoring water efficiency

Water waste can vary significantly between systems. (US EPA)

Mistake #4: Buying before testing your water

You may pay for treatment technology you don’t need.

Mistake #5: Ignoring replacement costs

Filters and membranes are recurring expenses.

Mistake #6: Choosing based only on GPD

Higher production doesn’t automatically mean better filtration.

Mistake #7: Forgetting to measure cabinet space

A traditional system may require considerably more space than expected.

Mistake #8: Ignoring water pressure

Low pressure can affect system operation and production.

Mistake #9: Confusing “NSF certified components” with a complete system certification

A membrane or component may meet only certain material or structural requirements, while a complete RO system can have separate performance claims. Verify exactly what has been certified. (NSF)

Reverse Osmosis Buying Guide: Frequently Asked Questions

How much should I spend on a reverse osmosis system?

The best price depends on the system type, capacity, certification, efficiency, and features.

Instead of setting a budget based solely on the purchase price, calculate the expected total cost of ownership.

Consider:

System price + installation + replacement filters + membrane + water use

Is tankless reverse osmosis better?

Not necessarily.

Tankless systems save space and can offer high on-demand production, but they may cost more and can require electricity.

A tank-based system may provide better value for some households.

Is NSF/ANSI 58 important?

Yes. It is a key standard covering reverse osmosis drinking-water treatment systems and addresses areas including TDS reduction, efficiency, recovery, structural integrity, and potential contaminant-reduction claims. (NSF)

However, always verify the certification and claims for the exact model you plan to buy.

How long does an RO membrane last?

Membrane life varies based on:

Water quality
Incoming pressure
Prefiltration
Water usage
Maintenance

Follow the manufacturer’s replacement schedule.

EPA’s WaterSense performance criteria require labeled systems to demonstrate a minimum membrane life of at least one year before replacement. (US EPA)

Should I buy an RO system with remineralization?

Choose remineralization if you prefer the resulting taste or want minerals added after RO treatment.

It isn’t automatically necessary for every buyer.

Should I buy a WaterSense-labeled RO system?

If you’re purchasing a new point-of-use RO system, it is worth considering.

EPA says WaterSense-labeled systems must meet efficiency and performance criteria, and qualifying systems send 2.3 gallons or less of water down the drain per gallon of treated water produced. (US EPA)

Final Verdict: How to Buy the Right Reverse Osmosis System

The best reverse osmosis system isn’t determined by:

The highest price
The most stages
The biggest GPD number
The flashiest marketing

Instead, choose based on:

Your water → Your contaminants → Verified performance → Capacity → Efficiency → Installation → Long-term cost

Start by understanding your water quality. Then identify the contaminants or treatment goals that matter to you. Finally, compare complete systems based on their specific, verifiable performance claims and practical ownership costs.

For the complete foundation behind this buying decision—including how RO works, what it removes, system components, and additional system comparisons—read RevOsmo’s The Ultimate Guide to Reverse Osmosis Systems.