5 Best Under Sink Water Filter Systems We’ve Tested

Our Testing & Selection Process

How we tested these under-sink filters: We installed and lived with each system, then sent before-and-after water samples to an accredited third-party laboratory to measure exactly what each filter removed, and just as importantly, whether it added anything. Each system earns a weighted overall score out of 10 across six factors: contaminant reduction (weighted most heavily), filtration rate, design, setup, maintenance, and company policies. Because we test the filtered output and not just the manufacturer’s claims, our scoring captures real-world results that manufacturer testing may miss, including two systems in this guide that introduced new contaminants we did not start with. We have personally tested more than 70 water filters to date. Brian Campbell, who leads our testing, is a WQA Certified Water Specialist with 10+ years in water treatment.

Article Update History

• 06/05/2026 – Cloud RO scores recalculated for accuracy

The Best Under-Sink Water Filters of 2026

System	Cloud RO	Waterdrop G3P800	Clearly Filtered	Waterdrop X Series	NU Aqua
Score	9.24	9.21	9.18	8.42	8.32
Type	Conventional RO	Tankless RO	Direct Connect	Tankless RO	Tankless RO
Price	$649	$539 – $999	$550	$1,200	$469.95
Certifications or Testing	NSF 42, 53, 58	NSF/ANSI 58, 372	NSF 42, 372	NSF 58	Not Certified
Filter Capacity	500 / 1,500 gal.	550/ 1100/ 2900 gal.	2,000 Gallons	1,100/ 550/ 2,200 gal.	550 / 1,100 / 3,200 gal.
Cost per Gallon	$0.26	$0.11	$0.22	$0.14	$0.15
Warranty	1 year	1 year	2 years	2 years	3 years
Read review	Cloud RO Review	Waterdrop G3P800 Review	Clearly Filtered Under Sink Review	Waterdrop X Series Review	NU Aqua Review

Cloud RO: Best Overall Under-Sink Filter (9.24/10)

Cloud RO is the highest-scoring and top-performing under-sink water filtration system we tested, achieving an overall weighted score of 9.24. In our testing, this tank-based reverse osmosis system reduced or removed all contaminants with health effects in our water, and impressed us with its modern, space-saving take on the conventional, clunky under-sink RO design.  

Overall Score: 9.24

How We Test & Score

9.42 Contaminant Reduction

10.00 Design

10.00 Maintenance

8.50 Filtration Rate

7.50 Setup

9.20 Company

Best For

Folks looking for an under-sink reverse osmosis filter that’ll purify their tap water on demand, without the water waste and pH reduction associated with conventional systems.

Product Specs

Price	$649
Contaminants Reduced	200+
Certifications	NSF 58
Process	Carbon + Reverse Osmosis + Remineralization
Filter Capacity	Pre/Post Filters 500 gallons, RO membrane 1,500 gallons
Annual Cost	~$200
Warranty	1 year

View at cloudwaterfilters.com

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🎬 Video Review

We tested Cloud twice with two different municipal water supplies in Colorado. For each test, we sent two samples to Tap Score for testing: one baseline sample of unfiltered tap water, and one of water purified and dispensed from Cloud RO. 

In test 1, a number of contaminants with possible health effects were detected in our unfiltered water, including fluoride and two types of disinfection byproducts (both detected above the HGL), as well as copper and aluminum.  

Cloud performed exactly as expected for an RO system, eliminating fluoride and both disinfection byproducts. It also removed 100% aluminum, copper, manganese, molybdenum, and strontium, and reduced sulfate by 94% and barium by 78%.

In test 2, our baseline sample had a TDS reading of 707 PPM — that’s almost 10x the dissolved solids concentration of our test 1 water source.

Barium, nitrate, nitrite, three disinfection byproducts, total chromium, copper, molybdenum, fluoride, manganese, boron, and uranium were among the contaminants with health effects detected, and all three DBPs were detected above the HGL.

This time, Cloud eliminated fluoride, all three disinfection byproducts, uranium, copper, strontium, selenium, manganese, potassium, molybdenum, and nitrate. It also greatly reduced barium by 97%, sulfate by 95%, and total chromium by 93%.

The system has an NSF certification for TDS reduction, which is a bit of a cop-out certification compared to specific contaminants we’re concerned about, but still helped bring its contaminant reduction score up to 9.42. 

In all, it removed contaminants more thoroughly than any other under-sink system we tested, and the included remineralization filter prevented the negative aesthetic effects of RO purification, increasing the pH of our water from 8.4 to 9.7 (very alkaline).

How easy is Cloud RO to install and maintain?

We installed and used Cloud RO in our own home to see how it performed under the typical customer use conditions. 

Cloud consists of two units: the main unit (containing four filter cartridges and the battery pack), and a 2.8-gallon purified water holding tank. Most components are made from BPA-free food-grade plastic, and the unit has a materials safety certification as part of its NSF 58 performance certification. We were impressed by its design, awarding a top score of 10.00 in this category.

The system works by purifying water and storing it in the tank, then using pressure to deliver it on demand to the dedicated faucet. It has a decent flow of 80 GPD (gallons per day), and in our tests, at a feed water temperature of 57.5°F and a static water pressure of 50 PSI, it had a filtration rate of approximately 65.17 GPD. This earned it a score of 8.50.

As a reverse osmosis system, Cloud does waste water, but its pure-to-wastewater ratio of 1:1 means it’s far more efficient than conventional systems, which typically waste up to 4 gallons of water for every 1 gallon purified. This was corroborated by our own tests, which recorded a pure-to-wastewater ratio of approximately 1:1.3 when purifying 12 ounces, and 1:1.03 when purifying 64 ounces.

We also tested the installed system for TDS creep, and Cloud had a 1st draw TDS reading of 284 PPM. At first glance, this is indicative of TDS creep, especially when we compared it to our second draw sample reading, which we took after running water through the system for 2 minutes: 68 PPM.

But our contact at Cloud explained that this was actually caused by minerals leaching from the new remineralization media and should normalize after a few weeks.

As for setup, the unit was easy to install, and all the fittings and filters are pre-installed, but it still received a slightly lower score of 7.50 due to the extra installation requirements for an RO system (including installing the dedicated faucet and drain line). 

It redeemed itself on the maintenance front, with its score of 10.00 reflecting its affordable ongoing spend of $0.26/ gallon and its easy filter changes, with built-in sensors and a dedicated app that tracks water filter lifespan.

Waterdrop G3 P800: Best Tankless RO Under-Sink System (9.21/10)

The Waterdrop G3 P800 comes in close second place behind Cloud, with an overall weighted score of 9.21. It’s another reverse osmosis system, but with a tankless design (meaning purified water bypasses a tank and is sent straight to the faucet). It performed very similarly to Cloud and is certified to remove more contaminants, but the setup process was slightly trickier and its shipping and returns policies aren’t as impressive.

Overall Score: 9.21

How We Test & Score

9.28 Contaminant Reduction

9.70 Design

10.00 Maintenance

10.00 Filtration Rate

7.00 Setup

8.65 Company

Best For

Anyone looking for the best under-sink RO filtration system that’s more compact than a tank-based model but still offers the same comprehensive purification.

Product Specs

Price	$539 – $999
Contaminants Removed	1,000+
Certifications	NSF/ANSI 58, 372
Process	RO
Filtration Capacity	550/ 1100/ 2900 gallons
Annual Cost	~$145
Warranty	1 year

View at waterdropfilter.com

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What did the Waterdrop G3 P800 remove in lab testing?

We tested the Waterdrop G3 P800 with city water in Parker, Colorado. Before filtration, 9 contaminants with health effects were detected, including lead and arsenic, both detected at concentrations above the Tap Score HGL.

When we tested it without the optional remineralization filter, the G3 P800 completely removed 6 of these (including lead and arsenic) and greatly reduced barium, nitrate, and strontium.

With remineralization, our results were even better: the G3 eliminated all contaminants detected above the HGL, including fluoride. It completely removed barium, copper, selenium, molybdenum, and vanadium. Additionally, strontium was reduced by 97%, and total chromium by 88%.

The system is also IAPMO certified to NSF 42, for removing chlorine, and NSF 58 and NSF 53, for reducing TDS, PFOA/PFOS, fluoride, cadmium, hexavalent chromium, and selenium, barium, and radium. Without the remineralization filter, calcium and magnesium were almost completely removed, and the pH of our water dropped from 7.3 to 6.3. With remineralization, pH was only reduced from 8.0 to 7.5.

Is the Waterdrop G3 P800 certified for materials safety?

Waterdrop is similar to Cloud when it comes to smart design features, receiving a 9.70 design score. It’s made from polypropylene and ABS, with a space-saving design featuring built-in TDS meters, a smart faucet, and a UV purifier (to protect against bacteria growth within the filters).   

Like Cloud, the G3 has a materials safety certification as a component of its NSF performance certifications. Plus, it has an IAPMO certification to NSF Standard 372 for lead-free design.

We were able to install the system ourselves, and the installation process was similar to installing Cloud, but without the water-holding tank. The G3 received a 7.00 score for setup, being more time-consuming and requiring more effort to install than the non-RO systems we tested. 

Maintenance was much easier, with the built-in filter change reminder and low ongoing spend of $0.11/ gallon contributing to the unit’s 10.00 score in this category. 

A big advantage of the G3 P800 is that it uses an internal pump to push water straight through the system to your faucet, giving it a fast water production rate of 800 gallons per day (10x the speed of Cloud’s filtration!) and a high score of 10.00. In our own testing, at a feed water temperature of 58.3°F, we measured the G3’s flow rate as approximately 886.54 GPD.

It’s also more efficient than Cloud, with a 3:1 pure-to-wastewater ratio.  When we tested efficiency ourselves, the unit had a recovery rate of 1:0.6 when purifying 12 ounces of water, and 1:0.45 when purifying 64 ounces of water.

Tankless RO systems are susceptible to TDS creep, which may impact their efficiency and water output. But in our on-site tests, the unit’s first-draw reading was higher than the second-draw reading by just 33 PPM, which is not indicative of TDS creep.

Clearly Filtered: Best Non-RO Under-Sink Filter (9.18/10)

The Clearly Filtered Under Sink system was the highest-scoring non-RO under-sink filter we’ve tested so far, with a few advantages versus the reverse osmosis systems: it costs less, is quicker and easier to install, and doesn’t waste water or reduce pH and alkalinity. We were impressed with how it performed in our lab tests, but it doesn’t quite match up to RO performance, and it’s only certified to reduce 1 contaminant. 

Overall Score: 9.18

How We Test & Score

8.98 Contaminant Reduction

9.70 Design

9.75 Maintenance

9.00 Filtration Rate

9.50 Setup

9.00 Company

Best For

Anyone looking for a non-RO under-sink system with a comprehensive contaminant reduction performance, easy setup, affordable maintenance, and no water waste.

Product Specs

Price	$550
Contaminants Reduced	232+
Certifications	NSF 42, 372
Process	Carbon + Ion Exchange
Filter Capacity	2,000 gallons
Annual Cost	~$440
Warranty	2 years

View at clearlyfiltered.com

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What did the Clearly Filtered under-sink system remove?

We tested the Clearly Filtered 3-Stage Under the Sink System on a shared, treated well water supply containing traces of contaminants including uranium, fluoride, copper, and chloroform (a disinfection byproduct), detected at concentrations exceeding the Tap Score HGL. 

The unit eliminated uranium, fluoride, and chloroform, and reduced copper by 99.39%. It also removed 100% of barium and strontium (two contaminants detected below the HGL), but it only reduced 23% nitrate, 5.6% molybdenum, and 21.25% sulfate. 

Unexpectedly, sodium increased by 107% and potassium by over 1,000%, while calcium decreased by 92% and magnesium by 6%, pointing to the use of a sodium- and potassium-loaded ion exchange resin. In spite of this, the pH of our water increased from 7.3 to 9.3 (alkaline).

These testing results, paired with the filter’s WQA performance certification to NSF/ANSI Standard 42 for chlorine reduction, resulted in a contaminant reduction score of 8.98.

Is the Clearly Filtered under sink filter easy to install?

If you’re looking for an under-sink filter with a more DIY-friendly install process, this Clearly Filtered model should appeal. We awarded it a setup score of 9.50 as we found it significantly quicker and easier to install than the RO systems, with only a few parts and no drain line or dedicated faucet. The process took us just 15 minutes from unboxing to completion. 

We measure flow rate for non-RO under-sink systems in gallons per minute, and the Clearly Filtered model had a recorded flow rate of 1.08 GPM. That’s around half the flow that you’d usually get from your kitchen faucet and is pretty decent for a filter in this category, resulting in a score of 9.00.

While the system doesn’t offer the same high-tech features as an under-sink RO unit, it still has a solid, well-thought-out design, with its sturdy construction and multiple design certifications (for materials safety and lead-free design) contributing to a design score of 9.70.

All three filters have the same projected lifespan, which simplifies maintenance, although you’ll have to make your own calendar notes to change the filters, as there’s no built-in reminder. Ongoing servicing is affordable at around $0.22/ gallon (slightly cheaper than Cloud but not as affordable as Waterdrop), translating to an overall maintenance score of 9.75.

Waterdrop X Series: Best for High Flow Rate (8.42/10)

Coming in behind Clearly Filtered, with an overall score of 8.42, is another tankless under-sink RO system: the Waterdrop X Series (specifically the X12). It’s IAPMO performance certified and boasts a super fast 1,200 GPD filtration rate, offering built-in remineralization (giving it an edge over the G3). But fluoride, sulfate, & strontium weren’t reduced by as much as we’d hoped, and it also introduced an unwanted contaminant into our water, hence why it didn’t land higher in this list.

Overall Score: 8.42

How We Test & Score

7.83 Contaminant Reduction

9.70 Design

10.00 Maintenance

10.00 Filtration Rate

7.00 Setup

8.85 Company

Best For

Folks who want a tankless under-sink RO system that comes with a built-in remineralization filter and have the budget for a more expensive model. 

Product Specs

Price	$1,200
Contaminants Reduced	1,000+
Certifications	NSF/ANSI 58
Process	Mechanical + Carbon + Reverse Osmosis + UV + Remineralization
Filter Capacity	1,100/ 550/ 2,200 gallons
Annual Cost	~$173
Warranty	2 years

View at waterdropfilter.com

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🎬 Video Review

Did the Waterdrop X12 add any contaminants?

We tested the X12 with municipal water in Westminster, Colorado. Our tests revealed that the unfiltered tap water contained 14 contaminants with health effects, including three types of disinfection byproducts, lead, and uranium (all present in concentrations exceeding the HGLs), as well as fluoride, uranium, and other heavy metals.

The Waterdrop X12 eliminated all the contaminants that exceeded the HGL, as well as 100% of cobalt, copper, molybdenum, and zinc. It also reduced 80% fluoride, 97.93% barium, 84.44% strontium, and 92.46% sulfate.

Our test reports did show that magnesium and calcium were nearly completely eliminated, but the pH of our water only decreased slightly (from 8.5 to 8.0). This suggests that the remineralization filter was doing its job in preventing a significant pH drop.  

But there was an unexpected result, too: 0.00299 PPM of antimony, a metal that has been linked to several health effects, was detected in our filtered water, exceeding Tap Score’s HGL of 0.001 PPM by 195%. It’s possible that antimony was introduced from the filters themselves or the internal plumbing in the system, as we’ve discussed in more detail in our full review. 

Regardless of the reason, this outcome affected our water quality and pulled the contaminant reduction score down to 7.83. This was even with its IAPMO certifications to NSF 58 for TDS reduction and NSF 42 for chlorine reduction. 

How hard is the Waterdrop X12 to install?

The Waterdrop X12 has a simple, minimalistic design, with on-demand filtration (no tank!) and a small footprint. It’s a plastic unit with a few smart features, along with IAPMO certifications for materials safety and lead-free design, resulting in a design score of 9.70. 

Just keep in mind that, as we mentioned in our G3 review, the X12 is susceptible to TDS creep due to its tankless design. 

And, as with the G3, installation was a bit tricky due to the additional plumbing jobs needed for an RO system. The X12 received a slightly lower score of 7.00 here, based on its long 2-hour installation process that requires a bit of DIY know-how.

On a positive note, the unit has an efficient 3:1 drain ratio, so only 1 gallon of water is rejected for every 3 gallons of water purified. It’s quick and easy to maintain and received a top score of 10.00, reflecting its low cost-per-gallon of $0.14 and helpful filter change indicators on the smart faucet.  

It also has a super-fast filtration rate of 1,200 GPD. This is actually excessive for most residential cases, given that most people use around 70 gallons of water a day for everything. Even so, its higher production rate earned it the highest score of 10.00 in this testing category.

NU Aqua Efficiency Series: Best Budget Under-Sink RO (8.32/10)

Rounding off this list of the top-performing under-sink water filters is the NU Aqua Efficiency Series, another tankless RO system. This unit earned an overall weighted score of 8.32, with its overall-pretty-good contaminant reduction performance and decent 2:1 efficiency ratio let down slightly by its lack of certifications, unusual testing outcomes, and inability to reduce fluoride in our testing. 

Overall Score: 8.32

How We Test & Score

7.89 Contaminant Reduction

8.10 Design

10.00 Maintenance

10.00 Filtration Rate

7.00 Setup

9.50 Company

Best For

People looking for a more affordable alternative to the Waterdrop G3 P800 that offers the same flow rate and a similar efficiency and overall performance

Product Specs

Price	$469.95
Contaminants Reduced	10+
Certifications	Not Certified
Process	GAC + RO + UV
Filter Capacity	550 / 1,100 / 3,200 gal.
Annual Cost	~$200
Warranty	3 years

View at nuaquasystems.com

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Is the NU Aqua Efficiency Series safe and certified?

We tested the NU Aqua Efficiency Series in a home on treated well water, which contained 3 contaminants exceeding the HGL: arsenic, uranium, and (most concerningly) lead. Copper, molybdenum, nitrate, phosphorus, iron, selenium, boron, and fluoride were also detected, but at concentrations below the HGL. 

The Efficiency Series reduced most contaminants by at least 70%, including complete eliminations of copper, lead, molybdenum, phosphorus, and uranium. The only letdown was fluoride, which remained the same (0.004 PPM) post-filtration, possibly removed and reintroduced by the remineralization media.

Concerningly, a number of contaminants were detected post-filtration when they weren’t present in our baseline sample. These included:

• Antimony
• Barium
• Boron
• Cobalt
• Coliform bacteria
• Iron
• Manganese
• Nickel
• Selenium
• Strontium
• Thallium
• Toluene

We suspect that the bacteria came from the homeowner accidentally contaminating the RO faucet when they put it in their water bottle to fill it, and was then imparted to the sample vial. The toluene may have been leaching from the plumbing or (more concerningly) the RO system itself. Everything else most likely came from the remineralization filter and was detected at concentrations not quantifiable by the testing lab and/or below the HGL.

Speaking of remineralization, we saw some significant increases in minerals like calcium, magnesium, and potassium, and an increase in pH from 7.6 to 7.8. 

Ultimately, though, the presence of new contaminants and the filter’s inability to remove fluoride, along with its lack of performance certifications, resulted in the lowest contaminant score so far: 7.89.

How easy is the Nu Aqua filter to install?

The Efficiency Series has a tankless design, with three filters stored inside the main unit, a separate post-remineralization filter and UV sterilizer, and a dedicated faucet to install at the sink. While it seemed sturdy enough, its lack of a materials safety certification pulled its design score down to 8.10. 

Setup was similar to the Waterdrop G3 and Cloud systems, with the tedious tasks of installing the faucet and drainline leading to a lower score of 7.00. But once installed, the unit’s filtration rate of 800 GPD meant we had instant access to purified water, putting it neck-and-neck with Waterdrop and earning it a top score of 10.00.

Where this system does fall short is performance efficiency. It only wastes 1 gallon of water per every 2 gallons purified, which still makes it highly efficient, but slightly more wasteful than Cloud and Waterdrop. 

But maintenance was as easy as for all the other under-sink RO systems in this list, with the system’s affordable cost-per-gallon of $0.15 and filter change lights earning it a score of 10.00.

Frequently Asked Questions

Q: What is the best under-sink water filter?

A: Based on our hands-on testing and accredited lab analysis, the best under-sink water filter is the Cloud RO (9.24/10). It removed or reduced every health-related contaminant in our test water, earned the highest contaminant reduction score of any under-sink system we have tested, and delivers purified water on demand from its tank with a 1:1 efficiency ratio.

Q: Are under-sink reverse osmosis systems better than non-RO filters?

A: For thoroughness, yes. The RO systems we tested removed more contaminants more completely than non-RO filters. The tradeoff is that RO wastes some water, can lower pH unless remineralization is included, and takes longer to install. The best non-RO option we tested, the Clearly Filtered Under Sink (9.18), still eliminated uranium, fluoride, and chloroform while installing in about 15 minutes with no water waste, so it is a strong choice if you want simplicity over maximum contaminant reduction.

Q: Can an under-sink water filter add contaminants to your water?

A: It can, and this is exactly why we lab test the filtered output rather than trusting marketing claims. Two systems in this guide introduced contaminants that were not in the source water: the Waterdrop X12 added trace antimony at 195% above the health guideline level, and the NU Aqua introduced twelve contaminants post-filtration, including coliform bacteria and toluene. Both findings lowered those products’ scores significantly.

Q: Does an under-sink filter need to be NSF certified?

A: Certification is reassuring but not legally required. We distinguish between a system that is genuinely certified for the contaminants you care about and one that carries a narrow or marketing hype certification. For example, the Waterdrop G3 P800 is certified for 100% of its reduction claims, while another system in this guide is certified for under 1% of its claims, and one is not certified at all. We always recommend matching the filter to the contaminants in your water, ideally confirmed with a certified lab water test.

Q: How much does it cost to run an under-sink water filter?

A: Ongoing cost is primarily driven by filter replacements, which we standardize and express as cost per gallon. Among the systems we tested, this ranged from $0.11 per gallon (Waterdrop G3 P800) to $0.26 per gallon (Cloud RO). Annual filter costs in this guide run roughly $145 to $440 depending on the system and your water usage.

Q: How hard is it to install an under-sink water filter yourself?

A: It depends on the type. The non-RO Clearly Filtered system installed in about 15 minutes with no drain line or dedicated faucet, earning a 9.50 setup score. The RO systems took longer, around 2 hours for the Waterdrop X12, because they require installing a dedicated faucet and drain line, which is why RO units scored lower (7.00 to 7.50) for setup in our testing.