An under-sink reverse osmosis (RO) system gives you filtered drinking water from a dedicated faucet. Models differ in filter stages, flow, certifications and how much water goes to the drain. This guide compares six systems and explains what to check before you buy.
These picks are based on desk research of product listings and category buying criteria. We have not hands-on tested these water filters. Details marked "per listing" come from the seller's product title and are not independently verified. This is not advice about your local water quality; check your utility's water quality report or have your water tested.
- 6 products compared
- Research-based picks
- No paid placements
- Updated
Top picks at a glance
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Best OverallPick 1: Waterdrop G3P600 Tankless Reverse Osmosis System
Popular under-sink RO pick
Buy on Amazon for Waterdrop G3P600 Tankless Reverse Osmosis System (opens in a new tab, affiliate link) Why we picked it (Waterdrop G3P600 Tankless Reverse Osmosis System)About $539.00 on Amazon as of
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Best High-FlowPick 2: Waterdrop G3P800 Fast-Flow Reverse Osmosis System
Faster-flow RO for bigger households
Buy on Amazon for Waterdrop G3P800 Fast-Flow Reverse Osmosis System (opens in a new tab, affiliate link) Why we picked it (Waterdrop G3P800 Fast-Flow Reverse Osmosis System)About $849.00 on Amazon as of
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Best Alkaline MineralPick 3: Waterdrop X12 Alkaline Mineral Reverse Osmosis System
Remineralizing RO
Buy on Amazon for Waterdrop X12 Alkaline Mineral Reverse Osmosis System (opens in a new tab, affiliate link) Why we picked it (Waterdrop X12 Alkaline Mineral Reverse Osmosis System)About $1,099.00 on Amazon as of
Compare all 6 picks
Order reflects our current research-based ranking; see How We Rate for the criteria. Highlights are taken from seller listings and are not verified.
| Pick | Best for | Listing highlights (per listing, unverified) | Where to buy |
|---|---|---|---|
Waterdrop G3P600 Tankless Reverse Osmosis SystemBest Overall |
Households that want under-sink RO drinking water with a smart faucet, without a storage tank. |
|
About $539.00 on Amazon as of |
Waterdrop G3P800 Fast-Flow Reverse Osmosis SystemBest High-Flow |
Larger households that fill pitchers or pots often and want faster flow. |
|
About $849.00 on Amazon as of |
Waterdrop X12 Alkaline Mineral Reverse Osmosis SystemBest Alkaline Mineral |
Shoppers who want an RO system that adds minerals back after filtering. |
|
About $1,099.00 on Amazon as of |
 Waterdrop G2P600 Tankless Reverse Osmosis SystemSimpler 7-Stage Option |
Shoppers who want a simpler 7-stage under-sink RO system. |
|
About $299.00 on Amazon as of |
 Waterdrop G5P500A Reverse Osmosis SystemG5 Series Option |
Shoppers comparing the G5 series; the listing title gives few details, so check the full listing. |
|
About $259.99 on Amazon as of |
 Waterdrop G5P700A Reverse Osmosis SystemAlso Consider |
Shoppers comparing the G5 series who want the other version in the range; check the full listing for differences. |
|
About $359.99 on Amazon as of |
How we picked
Research-based, not hands-on tested. We compared listings against the buying criteria in this guide and explain who each pick suits. All six picks are from one brand, Waterdrop, because those are the water filtration systems available through our affiliate partner. This makes it a comparison within one lineup rather than a survey of the whole market, so compare with other brands before you buy. We left out refrigerator replacement cartridges, which only fit specific fridges. How we rate · What our labels mean
Our 6 ranked picks
Pick 1: Waterdrop G3P600 Tankless Reverse Osmosis System
Popular under-sink RO pick
About $539.00 on Amazon as of
Best for: Households that want under-sink RO drinking water with a smart faucet, without a storage tank.
Highlights from the product listing
- 8-stage tankless reverse osmosis (per listing)
- NSF/ANSI 42, 58 and 372 certification claim (per listing)
- Under-sink installation (per listing)
- 2:1 pure-to-drain ratio (per listing)
- Smart LED faucet (per listing)
Taken from the seller's listing, not tested or independently verified. Confirm specifications and current price on the retailer page before buying.
What we like
- Reverse osmosis is one of the most thorough home filtration methods and is widely used for drinking water.
- Tankless under-sink designs (listed) save cabinet space compared with systems that use a storage tank.
- Certification claims (listed) give you specific standards to look up before you buy.
Keep in mind
- Under-sink RO needs installation, a spare cabinet space and usually a drill hole or spare sink hole for the faucet.
- Reverse osmosis sends some water to the drain; check the listed pure-to-drain ratio.
- Filters need replacing on a schedule; check replacement filter cost and availability.
Pick 2: Waterdrop G3P800 Fast-Flow Reverse Osmosis System
Faster-flow RO for bigger households
About $849.00 on Amazon as of
Best for: Larger households that fill pitchers or pots often and want faster flow.
Highlights from the product listing
- 800 GPD fast flow (per listing)
- NSF/ANSI 42, 53, 58 and 372 certification claim (per listing)
- 3:1 pure-to-drain ratio (per listing)
- Tankless under-sink design (per listing)
- LED purifier display and smart faucet (per listing)
Taken from the seller's listing, not tested or independently verified. Confirm specifications and current price on the retailer page before buying.
What we like
- Reverse osmosis is one of the most thorough home filtration methods and is widely used for drinking water.
- Tankless under-sink designs (listed) save cabinet space compared with systems that use a storage tank.
- Certification claims (listed) give you specific standards to look up before you buy.
Keep in mind
- Under-sink RO needs installation, a spare cabinet space and usually a drill hole or spare sink hole for the faucet.
- Reverse osmosis sends some water to the drain; check the listed pure-to-drain ratio.
- Filters need replacing on a schedule; check replacement filter cost and availability.
Pick 3: Waterdrop X12 Alkaline Mineral Reverse Osmosis System
Remineralizing RO
About $1,099.00 on Amazon as of
Best for: Shoppers who want an RO system that adds minerals back after filtering.
Highlights from the product listing
- 11-stage filtration with alkaline mineral pH+ stage (per listing)
- 1200 GPD (per listing)
- NSF/ANSI 42, 58 and 372 certification claim (per listing)
- 3:1 pure-to-drain ratio (per listing)
- Tankless under-sink design, BPA free (per listing)
Taken from the seller's listing, not tested or independently verified. Confirm specifications and current price on the retailer page before buying.
What we like
- Reverse osmosis is one of the most thorough home filtration methods and is widely used for drinking water.
- Tankless under-sink designs (listed) save cabinet space compared with systems that use a storage tank.
- Certification claims (listed) give you specific standards to look up before you buy.
Keep in mind
- Under-sink RO needs installation, a spare cabinet space and usually a drill hole or spare sink hole for the faucet.
- Reverse osmosis sends some water to the drain; check the listed pure-to-drain ratio.
- Filters need replacing on a schedule; check replacement filter cost and availability.
Pick 4: Waterdrop G2P600 Tankless Reverse Osmosis System
Simpler 7-stage RO
About $299.00 on Amazon as of
Best for: Shoppers who want a simpler 7-stage under-sink RO system.
Highlights from the product listing
- 600 GPD (per listing)
- 7-stage tankless reverse osmosis (per listing)
- Under-sink installation (per listing)
- 2:1 pure-to-drain ratio (per listing)
- PFAS reduction claim (per listing)
Taken from the seller's listing, not tested or independently verified. Confirm specifications and current price on the retailer page before buying.
What we like
- Reverse osmosis is one of the most thorough home filtration methods and is widely used for drinking water.
- Tankless under-sink designs (listed) save cabinet space compared with systems that use a storage tank.
- Certification claims (listed) give you specific standards to look up before you buy.
Keep in mind
- Under-sink RO needs installation, a spare cabinet space and usually a drill hole or spare sink hole for the faucet.
- Reverse osmosis sends some water to the drain; check the listed pure-to-drain ratio.
- Filters need replacing on a schedule; check replacement filter cost and availability.
Pick 5: Waterdrop G5P500A Reverse Osmosis System
G5 series RO
About $259.99 on Amazon as of
Best for: Shoppers comparing the G5 series; the listing title gives few details, so check the full listing.
Highlights from the product listing
- Reverse osmosis system (per listing)
- Part of the Waterdrop G5 series (per listing)
Taken from the seller's listing, not tested or independently verified. Confirm specifications and current price on the retailer page before buying.
What we like
- Reverse osmosis is one of the most thorough home filtration methods and is widely used for drinking water.
- Tankless under-sink designs (listed) save cabinet space compared with systems that use a storage tank.
- Certification claims (listed) give you specific standards to look up before you buy.
Keep in mind
- The listing title gives few specifications; confirm capacity, stages and certifications on the listing before buying.
- Under-sink RO needs installation, a spare cabinet space and usually a drill hole or spare sink hole for the faucet.
- Reverse osmosis sends some water to the drain; check the listed pure-to-drain ratio.
Pick 6: Waterdrop G5P700A Reverse Osmosis System
Another G5 series version
About $359.99 on Amazon as of
Best for: Shoppers comparing the G5 series who want the other version in the range; check the full listing for differences.
Highlights from the product listing
- Reverse osmosis system (per listing)
- Part of the Waterdrop G5 series (per listing)
Taken from the seller's listing, not tested or independently verified. Confirm specifications and current price on the retailer page before buying.
What we like
- Reverse osmosis is one of the most thorough home filtration methods and is widely used for drinking water.
- Tankless under-sink designs (listed) save cabinet space compared with systems that use a storage tank.
- Certification claims (listed) give you specific standards to look up before you buy.
Keep in mind
- The listing title gives few specifications; compare it with the G5P500A listing before buying.
- Under-sink RO needs installation, a spare cabinet space and usually a drill hole or spare sink hole for the faucet.
- Reverse osmosis sends some water to the drain; check the listed pure-to-drain ratio.
Buying guide: how to choose
Quick checklist before you buy
- Check your local water quality report so you know what you want to reduce.
- Measure the space under your sink and check for a spare faucet hole or whether you will need to drill one.
- Look up which NSF/ANSI standards the system claims, and what each standard covers.
- Compare the pure-to-drain ratio if water use matters to you.
- Check the replacement filter schedule and cost before you buy.
A good fit if…
- You drink and cook with a lot of tap water and want it filtered at a dedicated faucet.
- You have space under the kitchen sink and are comfortable with a DIY install, or will hire a plumber.
- You want to reduce a wider range of contaminants than a simple pitcher filter targets.
Maybe skip it if…
- You rent and cannot modify the sink or countertop.
- You only want better-tasting water occasionally; a pitcher or faucet-mount filter is simpler.
- You need whole-house filtration for showers and appliances; that is a different kind of system.
How reverse osmosis works
RO systems push water through a very fine membrane, usually with sediment and carbon stages before and after it. The membrane does most of the work, while the extra stages protect it and improve taste. Some systems add a remineralization stage to put minerals back after filtering.
Certifications: what NSF/ANSI numbers mean
NSF/ANSI standards are published certification standards. Broadly, 42 covers aesthetic effects such as chlorine taste and odor, 53 covers certain health-related contaminants, 58 is the standard for reverse osmosis systems, and 372 relates to lead content in materials. Check which standards a system claims and look them up in the certifier's database.
Flow rate and tankless design
GPD (gallons per day) describes how quickly a system can produce filtered water. Tankless systems make water on demand instead of storing it in a tank, which saves cabinet space. Higher flow helps if you fill large pots or pitchers often.
Pure-to-drain ratio
RO systems send some water to the drain while filtering. A 3:1 ratio means about one unit of water goes to the drain for every three units of filtered water; a 2:1 ratio sends a little more to the drain.
Installation and maintenance
Most under-sink systems connect to the cold water line and the drain, and need power for tankless models. Plan for replacement filters on the manufacturer's schedule; some systems show filter life on the faucet or unit.
Frequently asked questions
Do I need a reverse osmosis system?
It depends on your water and preferences. Check your local water quality report or test your water first. RO targets a wide range of contaminants, while simpler filters mainly improve taste and odor.
Does reverse osmosis waste water?
RO systems send some water to the drain. Systems list a pure-to-drain ratio, such as 2:1 or 3:1, which tells you how much water goes to the drain relative to filtered water.
Can I install an under-sink RO system myself?
Many people do, following the manual, but it involves connecting to the water supply and drain and possibly drilling a faucet hole. Hire a plumber if you are unsure.
Why are all the picks from one brand?
These are the water filtration systems available through our affiliate partner. We say so openly; compare with other brands before you buy.
Did you test these water filters?
No. These are research-based picks, not hands-on tests. See How We Rate for how we choose and what we do and do not claim.
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