When comparing reverse osmosis systems, certification is an important criterion to consider. Rick Andrew, director of the National Sanitation Foundation’s Global Water Program, advises that you select a reverse osmosis system that is tested and certified to treat the specific contaminants that are impacting the drinking water supply. “Testing and certification ensure that the product is safe and structurally sound and that it will treat the water according to the claims made by the manufacturer,” he says. We researched reverse osmosis systems from the top brands, taking into account the size of the system, speed, wastewater production, and ease of installation, as well as the taste of the water they produce. We also looked for systems that have been certified by the National Sanitation Foundation (NSF) or the  Water Quality Association (WQA). Our top pick, the Brondell H20+ Circle Reverse Osmosis Water Filtration System, is an under-sink system with a small footprint and a low wastewater production rate. It offers four stages of filtration and has long-lasting, easy-to-change filters. Here are the best reverse osmosis systems. Tank Size: 6 liters | Output: NA | Filtration Stages: 4 | Filtration Method: Carbon block, hollow fiber membrane, sediment | Warranty: 1 year You get eight levels of water filtration when you choose this tankless reverse osmosis system. It reduces TDS, chromium, per- and polyfluorinated substances (PFAS), fluoride, arsenic salt, iron, radium nitrate, calcium, particles, chloride, chlorine, and heavy metals such as lead. All of this filtration doesn’t slow the system down; it’s rated for 400 gallons per day and can fill a glass in about 12 seconds. Three composite filters are responsible for this system’s filtration activities and feature single-turn replacement.  While some reverse osmosis systems require no power supply, that’s not the case for this tankless model. You need to plug the unit into a standard 110V outlet underneath the sink.  Price at time of publish: $519 Tank Size: Not applicable | Output: 400 gallons per day | Filtration Stages: 8 | Filtration Method: Carbon block, pleated polyester | Warranty: 1 year The first five filtration stages are intended to remove large particles such as dust, dirt, and rust, plus contaminants that include lead, fluoride, and chlorine. The final filter uses mineral stones to restore healthy levels of calcium, potassium, and magnesium to the water. The storage tank is also pre-pressurized, which produces a more consistent flow when dispensing water and is also beneficial if you connect the water line to your refrigerator. Take note that waste water production is 3:1 with this system, making it unremarkable in terms of efficiency or water conservation. Installation can also be more challenging when compared with some other under-sink reverse osmosis systems, but it can be accomplished with basic tools and some understanding of home plumbing. A water line kit is included if you want to connect your refrigerator’s water line to the reverse osmosis system.  Price at time of publish: $217 Tank Size: 3.2 gallons | Output: 75 gallons per day | Filtration Stages: 6 | Filtration Method: Reverse osmosis, alkaline, activated carbon, carbon block | Warranty: 1 year Another reason to splurge on this reverse osmosis system is the dual-purpose faucet. One handle dispenses filtered water, free of sediment and chlorine, which is commonly used for cleaning or cooking. The handle on the other side of the faucet gives you a stream of purified drinking water. The LCD display on the front of the faucet provides you with a TDS reading and filter life status.  As with some other tankless reverse osmosis systems, you need to plug this unit into a standard electrical outlet. The single-unit design means that it doesn’t take up much space in your sink cabinet—you just need 16.6 inches of height clearance, 18.2 inches of depth, and 5.2 inches of width to make this unit work in your space. Price at time of publish: $432 Tank Size: Not applicable | Output: 600 gallons per day | Filtration Stages: 4 | Filtration Method: Sediment, carbon, carbon block | Warranty: 1 year The 4-gallon storage tank holds filtered water to be dispensed via the included countertop faucet. This system is rated for 50 gallons per day, which may not be sufficient for large families, but it should be plenty for households of three to four people. The wastewater-to-clean-water ratio is unremarkable at 3:1, but it’s on par with similarly priced under-sink reverse osmosis systems. You need to devote a significant amount of under-sink cabinet space for this unit. But in return, you enjoy fresh filtered water, long filter life, and simple operation.  Price at time of publish: $200 Tank Size: 4 gallons | Output: 50 gallons per day | Filtration Stages: 5 | Filtration Method: Multi-method | Warranty: 1 year In apartments, condos, or other small spaces, this type of reverse osmosis system is a great option. The under- sink system measures 14 inches tall, with a 6.75-inch footprint. It saves considerable space by condensing the filters into a single cartridge and forgoing a storage tank for filtered water. Instead, water is pumped to a countertop carafe that sits on display with an included base. A sensor automatically refills the carafe, ensuring that you never run dry. Note that the base of the carafe must be plugged in for operation. You’ll also need a path for the tube to bring water from the filtration system to the carafe.  Price at time of publish: $250 Tank Size: 2.5 liters | Output: Not listed | Filtration Stages: 3 | Filtration Method: Carbon block, sediment | Warranty: 1 year We recommend this model for its simple installation, tool-free filter changes, and four-stage filtration that targets contaminants such as lead, chlorine, bacteria, copper, fluoride, and iron. While most people install it for long-term use, this unit is designed in such a way that it can be easily transported and set up in a new location, making it a good pick for use in rentals, RVs, or other short-term situations. This countertop reverse osmosis system is designed to connect to most standard faucets, using a diverter valve. (It’s not compatible with specialty faucets such as pull-out or sprayer faucets.) Once connected, the system uses four filters, which, according to the manufacturer, remove 95 percent of lead, 96 percent of iron, and 100 percent of the chlorine present in tap water. It can filter up to 90 gallons per day, but you may find the flow rate to be slow if you’re filling a large container or water bottle. Price at time of publish: $250 Tank Size: Not applicable | Output: 90 gallons per day | Filtration Stages: 4 | Filtration Method: Alkaline or calcite, coconut shell carbon, sediment | Warranty: 1 year This is a three-stage reverse osmosis system that primarily targets lead and chlorine. Filtered water is available to you from a large, 5-gallon storage tank that pipes water up to the included faucet dispenser. The dispenser is basic by appearance but includes an indicator light to show you when it’s time to replace the filters.  The biggest recurring expense of any reverse osmosis system is filter replacement. With this system from Whirlpool, you can expect six months of viability from the two-part pre- and post-filter set, and one to three years of use from the reverse osmosis membrane filter. When it is time to change the filters, the encapsulated design makes the filter canisters easy to detach and remove without the need for tools.   The system takes up more space than some other options, due to its larger storage tank. In addition, no data is available from the manufacturer on wastewater production. If you’re interested in an efficient reverse osmosis system that minimizes water waste, other options may be a better fit. But to kick your bottled water habit and get filtered water on-demand from your sink, the Whirlpool WHAROS5 is a solid system that is straightforward to install and maintain.  Price at time of publish: $160 Tank Size: 5 gallons | Output: 18.46 gallons per day | Filtration Stages: 3 | Filtration Method: Activated carbon, hollow fiber membrane | Warranty: ‎Lifetime warranty available

Plumbing Requirements

Most reverse osmosis systems have minimal plumbing requirements. You need to connect under-sink systems to the cold-water line; most kits include saddle valves for this purpose. You also need to connect the RO system to the drain line, to expel wastewater. Finally, a countertop-mounted faucet needs to be installed to dispense the filtered water, unless you choose a countertop RO system or one that directly fills a pitcher or water dispenser.

Water Waste

Unlike traditional water filters, not all the water pumped through a reverse osmosis filter comes out as drinkable water. Only 50 percent or less is filtered, and the rest is considered waste. When possible, avoid units with 75 percent or more waste, especially if you are treating a high volume of water per day. This unit is typically measured in ratio form, so a tank that produces 75 percent waste has a filtered-water-to-wastewater ratio of 1:3.

Tank Size

If you have a large household, pay attention to tank size. Filtered water is stored inside of the tank and dispensed on-demand when you fill a glass or water bottle. As the water is depleted from the tank, the system filters water to refill it. However, demand can outpace that process, which is typically slow. For this reason, most families can appreciate a water storage tank of 3 to 5 gallons. At the same time, the larger the water tank, the more space the reverse osmosis system occupies in your cabinet under the sink. 

Water Output Capacity

Water output capacity of reverse osmosis systems is measured in gallons per day (GPD). Some systems for light-duty use may be rated 50 GPD, but if you use your water filtration system frequently, look for a system rated 400 to 800 GPD. Usually, higher water capacity output translates into faster fill times, too. For example, some high-output reverse systems can fill a water glass in 12 seconds or less.

Why Trust The Spruce?

This article was written by Erica Puisis, a freelance home writer who has been contributing to The Spruce since 2017. When researching picks for this list, she compared various types of RO systems, including options with and without a storage tank. She evaluated the most popular models based on the wastewater of each system, filtration speed, filter maintenance, and storage tank considerations. All of our recommendations include at least three stages of filtration and have been certified by the NSF or WQA. Every reverse osmosis system consists of an RO membrane, a sediment filter, a carbon filter, and several stages of filtration. When water first enters the system, it goes through pre-filtration, which usually removes sediment and or chlorine, which could clog up the membrane. Water then goes through the membrane, which further removes dissolved particles. Once the filtering is complete, the water goes to the system’s storage tank, where it is ready for use. When you turn on your faucet, the filtered water comes from the storage tank through another filter in the system that polishes it for drinking. As part of the research process, Erica also interviewed Rick Andrew, director of the NSF Global Water Program. Rick provided insight into how reverse osmosis systems function and elaborated on the importance of choosing certified systems.