Best Tankless Under Counter Reverse Osmosis Systems

TL;DR: The Waterdrop G3P600 delivers 8‑stage, 600 GPD filtration in a tankless design that saves 70 % of under‑sink space, but its fluoride removal stops at about 85‑98 %.

The standout feature is its 8‑stage filtration train, which includes a 0.0001 µm RO membrane, UV sterilization, and a smart display faucet that offers real‑time TDS monitoring. At 600 GPD and a pure‑to‑drain ratio of 2:1, the system can keep a household of 5‑7 people supplied with filtered water continuously, while the tankless form factor trims the unit’s depth to 18.06 inches and claims a 70 % space‑saving benefit.

Compared with the seven other systems in this roundup, the G3P600 is heavier than the Frizzlife PX500‑A (about 1 lb) and iSpring RO5004F (about 15 lb), but lighter than the competing Waterdrop Reverse Osmosis System (34.8 lb). Its flow of 0.42 GPM is slower than iSpring’s RO800G (0.6 GPM), yet the G3P600 still outpaces the VonWater 800GPD model in terms of integrated smart features and UV sterilization, which the latter does not list.

Professional reviewers applaud the system’s comprehensive contaminant reduction—covering lead, PFAS, chlorine, and microplastics—and its quick 3‑second filter‑change mechanism. Consumer feedback frequently praises the space‑saving design and the convenience of the smart faucet, while noting that fluoride removal caps at roughly 85‑98 % and that installation demands drilling and a nearby electrical outlet.

Additional technical details reinforce its premium positioning: the CF filter lasts up to 6 months or 550 gallons, the faucet cord stretches 59 inches, the power cord reaches 90 inches, and the system’s overall weight is 32.0 pounds. Integrated waterways and a smart leakage protector add layers of safety, and the real‑time TDS readout lets you monitor water quality at a glance.

Best for: Households of 5‑7 people that want high‑volume, space‑saving RO filtration with smart monitoring features.

Avoid if: You need complete fluoride removal or prefer a non‑electric, plug‑and‑play system.

TL;DR: The Waterdrop G2 delivers 400 GPD of filtered water with a 7‑stage system, 1:1 waste ratio and fast 12‑second cup delivery, all for $209.99.

The standout spec is its 400 gallons‑per‑day production rate combined with a pure‑to‑drain ratio of 1:1, meaning you waste no more water than you drink. Seven filtration stages—including a 0.0001 micron RO membrane—remove 10 listed contaminants, and the optional PCC remineralization adds calcium, magnesium and other minerals back into the water.

Compared with the other seven units, the G2 sits in the middle of the flow‑rate range: Waterdrop’s T3 pushes 450 GPD, while iSpring’s RO5004F reaches 500 GPD. In height, the G2’s 13.98 inches is shorter than the T3’s 17.95 inches and iSpring’s 16.6 inches, making it a good fit for tighter cabinets. It is heavier than the T3 (25.0 lb vs 19.6 lb) but lighter than the Frizzlife PX500‑A (27.6 lb) and the larger Waterdrop Reverse Osmosis System (34.8 lb). Its width of 5.91 inches is narrower than the T3’s 6.22 inches and iSpring’s 6.1 inches, yet wider than the PX500‑A’s 5.0 inches.

Reviewers consistently praise the rapid 12‑second cup fill and the noticeable improvement in taste, attributing it to the 99 % impurity elimination and 90 % TDS reduction. Professional reviewers also highlight the comprehensive filter set and the convenience of an included brushed‑nickel faucet. The main trade‑off noted is the need for a nearby electrical outlet, which limits placement options, and the system’s indoor‑only design, ruling out outdoor or RV use.

While the Waterdrop G3P600 claims the top spot in our rankings with a broader spec list, the G2 still offers a solid blend of performance and price. Its BPA‑free plastic cover, automatic shutoff, and included accessories (cover, filters, tubing, power cable) give entry‑level buyers a complete package without extra add‑ons.

Best for: Budget‑conscious buyers seeking a reliable entry‑level option — the Waterdrop G2 is available at $209.99

Avoid if: You need an outdoor or portable solution, or lack an electrical outlet under the sink

TL;DR: The iSpring RO800G delivers 800 GPD of RO water at a 0.6 GPM flow rate from a compact, tankless unit that weighs 23.60 lb, but it relies on a dedicated power source and only two filtration stages.

This system’s standout spec is its 800 gallons‑per‑day capacity paired with a 0.6 gallon‑per‑minute flow, meaning an 8 oz cup fills in just 6.5 seconds. The tankless design measures 17.75 inches tall and wide, and 6.50 inches deep, fitting easily under most kitchen cabinets while still handling high‑demand usage.

Compared with the other seven entries, the iSpring RO800G is lighter than the Waterdrop G3P600 (32.0 lb) and the Frizzlife PX500‑A (27.6 lb), yet heavier than the VonWater 800GPD (18.82 lb). Its 6.50‑inch width is far narrower than the Waterdrop Reverse Osmosis System’s 17.5‑inch width, giving it a clear space‑saving edge. Height‑wise it sits between the Waterdrop G2 (13.98 inches) and the Frizzlife PX500‑A (18.0 inches), making it a good middle‑ground for most under‑sink footprints.

Reviewers consistently praise the rapid fill speed and the high daily output, noting that the system “fills an 8 oz cup in 6.5 seconds” and “handles large families or office use without bottlenecking.” Professional reviewers also highlight the tool‑free, click‑and‑lock filter replacement as a convenience. On the downside, users point out that only two filtration stages are provided, which may leave some specific contaminants unaddressed, and the mandatory power supply can be a hurdle for installations without an outlet.

Beyond speed, the RO800G includes an automatic pre‑flush, a built‑in booster pump option, and a filter‑life indicator that alerts you when a change is due. The pure‑to‑drain ratio of 2.5:1 helps conserve water, and an optional LED UV add‑on is available for extra microbial protection. All components, including a brushed‑nickel dedicated faucet, come pre‑installed, so you won’t need to hunt for extra parts.

Best for: Buyers willing to invest in a proven, high‑quality option — the iSpring RO800G offers Flow Rate: 0.6 GPM and 800 GPD capacity for high‑demand settings.

Avoid if: You need advanced multi‑stage filtration beyond two stages or cannot provide an electrical outlet for the booster pump.

TL;DR: The Waterdrop Reverse Osmosis System packs a 600 GPD flow rate, 5‑stage filtration and a smart LED faucet with hot‑water dispensing into a compact 6.7‑inch depth unit, but it carries a premium price tag.

The standout spec is the 600 GPD daily production rate, which outpaces several peers such as the iSpring RO5004F (500 GPD) and the Waterdrop T3 (450 GPD). Coupled with five filtration stages—including RO membrane, carbon block and PP cotton—the system promises thorough contaminant reduction while delivering hot water on demand within a temperature range of 104 °F‑203 °F.

In size terms, the unit is deeper‑friendly at just 6.7 inches, making it shallower than the Waterdrop G3P600 (18.06 in depth) and the Frizzlife PX500‑A (18 in depth). However, its 17.5‑inch width makes it considerably wider than the iSpring RO800G (6.5 in) or the VonWater 800GPD (4.9 in). At 34.8 Pounds, it is heavier than all seven competitors, which range from about 15 lb to 32 lb. The waste‑to‑pure water ratio of 2:1 is comparable to typical RO systems, and the tankless design frees up floor space that a traditional storage tank would occupy.

Reviewers consistently highlight the smart LED faucet and the built‑in TDS monitor as convenient, modern touches, and many appreciate the child‑lock safety feature. The hot‑water function also earns praise for adding instant boiling capability without a separate appliance. The main trade‑off mentioned is the higher price point, which sits above most of the other models in this lineup.

Best for: Users who need a high‑capacity system and value smart features— the Waterdrop Reverse Osmosis System offers Flow Rate: 600 GPD.

Avoid if: You're on a tight budget or only need basic functionality.

TL;DR: The Frizzlife PX500-A delivers 500 GPD of 99.99% contaminant‑free water from a compact, tankless unit that fits under the sink, offers alkaline remineralization, and saves up to 60% of space.

What really sets this system apart is its tankless design that claims up to 60% space savings while still producing 500 gallons per day. A 7‑stage filtration train removes 99.99% of more than 1,000 contaminants, and the integrated alkaline filter pushes the pH above 7.5 for smoother‑tasting water.

At 27.6 lb, the PX500‑A is heavier than the iSpring RO5004F (about 15 lb) but lighter than the Waterdrop G3P600 (32 lb). Its 5.0‑inch width is narrower than Waterdrop’s 5.68‑inch width, while the 18.0‑inch height is a touch taller than the G3P600’s 17.76 inches. Compared with the Waterdrop G2 (25 lb, 13.98‑inch height, 5.91‑inch width) and the Waterdrop T3 (19.6 lb, 17.95‑inch height, 6.22‑inch width), the Frizzlife unit offers a balance of capacity and footprint.

Professional reviewers applaud the built‑in pump for maintaining consistent flow, yet they note the system needs a minimum inlet pressure of 20 psi and that colder water can slow the 0.34 gallon/minute flow rate. Users also appreciate the two‑second filter‑change indicator, which flashes blue or red and beeps when it’s time to swap the 6‑month‑interval pre‑filter or the yearly post‑filter.

Best for: Households with limited under‑sink space that need high‑capacity filtration – the Frizzlife PX500‑A offers a 0.34 gallon/minute flow rate and 500 GPD production.

Avoid if: Your home’s water pressure stays below 20 psi or you live in a cold climate where low water temperature could hinder performance.

TL;DR: The VonWater 800GPD delivers 800 GPD of 99.99%‑pure water from a compact, tankless 7‑stage system for $149.99, but it needs a nearby power outlet and a modest DIY install.

The standout spec is its 800 gallons‑per‑day production rate paired with a 0.56 gallon‑per‑minute flow, meaning you can fill a large pitcher in under a minute. Its 7‑stage filtration tackles nine listed contaminants—including PFAS, lead, and TDS—at a 99.99% removal rate, and the built‑in filter‑life reminder plus auto‑flush keep upkeep simple.

Compared with the seven other systems in this roundup, the VonWater sits near the lightest end at 18.82 lb, lighter than the Waterdrop G3P600 (32 lb) and the Waterdrop Reverse Osmosis model (34.8 lb), yet a touch heavier than the iSpring RO5004F (15 lb). Its 4.9‑inch width is narrower than the Waterdrop T3 (6.22 in) and the iSpring RO5004F (6.1 in), making it one of the slimmest options for cramped cabinets. Flow‑rate wise, it outpaces the Waterdrop T3 (450 GPD) and iSpring RO5004F (500 GPD) while matching the higher‑capacity iSpring RO800G’s 0.6 GPM figure.

Reviewers consistently note the system’s effective contaminant reduction, with many users reporting noticeable drops in TDS after installation. Professional reviewers highlight the “exceptional filtration efficiency combined with space optimization” and praise the 2:1 pure‑to‑drain ratio as a strong water‑waste metric. On the downside, users mention the need for an electrical outlet and the drilling of holes in the countertop and drain pipe, which can be a hurdle for renters or those uncomfortable with plumbing work.

Additional technical context includes a 6‑12‑month lifespan for the pre‑carbon (PC) filter and 18‑24 months for the RO membrane, a stainless‑steel 304 faucet that rotates 360°, and automatic shutoff to prevent over‑run. The system operates on standard 110‑120 V AC at 80 W, and its feed‑water pressure range of 14.5‑87 psi (0.1‑0.6 MPa) covers most household supplies.

Best for: Entry‑level buyers or those on a tight budget — the VonWater 800GPD is available at $149.99

TL;DR: The Waterdrop T3 delivers tankless reverse osmosis with 450 GPD flow, 8‑stage filtration and a sub‑8‑second cup fill, all for $189.99, making it a strong budget choice.

At the heart of the T3 is a 0.0001μm RO membrane that tackles heavy metals, chlorine, PFOA and PFOS. Combined with an NSF/ANSI 372 certification, the system guarantees lead‑free material safety. Its 450 GPD flow rate and a pure‑to‑drain ratio of 2:1 mean you get a full 200 ml cup in roughly 8 seconds without waiting for a storage tank.

Weighing 19.6 lb, the T3 is lighter than the Waterdrop G2’s 25.0 lb but a touch heavier than the VonWater 800GPD’s 18.82 lb. Its height of 17.95 in fits comfortably under most sinks, standing taller than the VonWater unit’s 13.9 in height while remaining shorter than the iSpring RO800G’s 17.75 in. Width‑wise, 6.22 in makes it a bit wider than the Frizzlife PX500‑A’s 5.0 in but comparable to the iSpring RO5004F’s 6.1 in. Overall, the T3 balances a compact footprint with a solid build.

Customers appreciate the lightning‑quick fill speed and the high‑volume 450 GPD delivery, noting that it eliminates the typical wait associated with tanked RO systems. Reviewers also point out the convenience of smart alerts, LED status lights and automatic shutoff. The main complaint centers on the need for a dedicated electrical outlet, which can limit placement options in kitchens without nearby power.

Beyond speed, the T3 offers eight filtration stages and an automatic shutoff feature that helps conserve water. Smart alerts let you know when it’s time to replace the all‑in‑one cartridge, which the manufacturer rates for 6‑24 months of use depending on water quality. Although it doesn’t include a remineralization stage, the system’s pure‑to‑drain efficiency and certified material safety make it a reliable everyday solution.

Best for: Entry-level buyers or those on a tight budget — the Waterdrop T3 offers Flow Rate: 450 GPD

Avoid if: You want the best value-per-dollar in its price range

TL;DR: The iSpring RO5004F is a tankless, under‑sink RO system that delivers 500 GPD of 99 %‑pure water, packs six filtration stages including UV, and fits in a compact 16.6‑inch‑high footprint for about $280.

What really sets the RO5004F apart is its six‑layer filtration train plus a built‑in UV light that together knock out more than 99 % of contaminants, including PFAS, lead, chlorine and arsenic. A twist‑to‑unlock filter mechanism and a filter‑life monitor make maintenance straightforward, while the built‑in pump pushes water fast enough to fill a cup in just 10 seconds.

At 16.6 inches tall and 6.1 inches wide, the unit sits lower than the Frizzlife PX500‑A (18.0 in) and shorter than the Waterdrop T3 (17.95 in), yet it’s taller than the Waterdrop G2 (13.98 in). Weighing 15.00 lb, it’s lighter than the VonWater 800GPD (18.82 lb) and considerably lighter than the iSpring RO800G (23.60 lb). Its 500 GPD flow rate outpaces Waterdrop T3’s 450 GPD but trails the 800 GPD capacity of both the VonWater and iSpring RO800G models.

Professional reviewers highlight the high flow and space‑saving tankless design as major wins for families that need plenty of drinking water on demand. Users also appreciate the UV protection and self‑cleaning features, noting that the UV lamp is rated for 8,000 hours of operation. The main trade‑off is the 2:1 pure‑to‑drain water ratio, which means more waste compared with newer low‑waste systems, and the need for an electrical outlet to power the pump and UV lamp.

Best for: Most users seeking a balanced, mainstream option — the iSpring RO5004F offers Flow Rate: 500 GPD

Avoid if: You want the best value‑per‑dollar in its price range