Under-sink water filters are installed directly into the water line under the sink. Some filter all of the cold water that flows through the faucet; others have a separate filter faucet -- which requires drilling a hole in the sink deck or countertop to install. The options and price ranges in this category vary dramatically, as does the cost of replacement filters. If you just need small amounts of filtered water for drinking, see our discussion of water filter pitchers elsewhere in this report. If you don't need to filter all of your water, but would like to filter larger quantities for filling coffee pots or drinking bottles, see our separate discussion of faucet water filters.
All reverse osmosis systems are under-sink water filters, and all require installing a separate faucet filter; however, they are the top choice if you need to purify, not just filter, your water. Water flows from the main plumbing line into the filter, which filters it and stores it in a tank. When you turn on the filter faucet, water comes from that tank of filtered water. You then use your regular faucet for water that you don't need to filter.
Among reverse osmosis water filters, the APEC ROES-50 (Est. $200) gets top reviews from users for its excellent performance in improving water quality. We saw many posts from owners who shared before and after tests they had done on their own water, showing a marked improvement in removing total dissolved solids (TDS). Beyond that, the vast majority rave about the noticeable improvement in both the color and taste of their water after installing the APEC ROES-50. The ROES-50 is made by a WQA (Water Quality Association) certified manufacturer and uses some NSF-certified parts.
The ROES-50 is a very good value for a reverse osmosis system. The filters cost about $20 and need to be replaced every six to 12 months, making the ongoing cost of ownership fairly low. The basic kit includes one faucet in chrome, but faucets with other finishes are available for purchase. An optional Icemaker Kit (Est. $17) is available as well.
While most owners find this under-sink filter easy to install, some had issues, but report that customer service is unusually responsive in helping to troubleshoot problems. This water filter is assembled in the USA and customer service is based in the USA as well, something that's extremely popular with reviewers.
The big downside with the ROES-50 is that it has a 3:1 ratio of waste water to drinking water; in other words, for every one gallon of filtered water, 3 gallons of waste water (also called brine) is discarded. This is a pretty standard ratio for reverse osmosis systems. Also, something we noted that some people seem to misunderstand: The water is not truly "wasted;" rather, it goes down the drain and back into the community water supply. It's just that you still have to pay for it. Many people reroute the brine tubing, collect the waste water, and use it in their gardens or for other purposes. The ROES-50 has a tank capacity of four gallons, and a total system capacity of 30 to 50 gallons per day, depending upon your water pressure.
Another highly rated RO water system is the iSpring RCC7 (Est. $200), a 5-stage system that is made with NSF certified parts. It can supply up to 75 gallons per day, with a pressurized tank capacity of 3.2 gallons. It has a lower waste water ratio of than the ROES-50 at only 2:1. iSpring customer service also gets very good reviews. All other things being equal, this is a good choice if you use a lot of water and prefer a better balance of waste vs. filtered water.
Not all under-sink water filters are reverse osmosis systems, some are just basic, solid carbon block filters such as those used in pitcher, countertop and faucet-mounted water filters. These filters offer the convenience of a permanent, under-the-sink mounting that can provide basic filtering of all of the water that comes from your tap. Among this style of filter, we saw nothing but praise from both experts and owners for the Multipure Aquaversa MP750SB (Est. $450). It's NSF-certified to remove a wide variety of contaminants, including the stricter, California standards. In one professional test this under-sink filter is rated as Excellent in removing lead and chloroform, and Very Good for flow rate and resistance to clogging.
Users say the MP750SB is very easy to install and use, although some who had issues with leaking after a self-install say it's worth hiring a plumber to be sure it's done right. Most note an immediate improvement in water taste and clarity after installation. Filters cost about $70 and should be replaced after 750 gallons. How often that is will depend upon how much water you use. There is a version of this filter available that increases capacity to 1200 gallons with electronic monitoring, and that costs about $50 more. There are also optional accessories available to hook the MP750SB up to an icemaker or convert it to a countertop model.
Elsewhere in this report:
Best-reviewed water filters: Whether you just want better tasting drinking water, or need to purify your entire water supply, these are the best water filters for improving water clarity, taste and safety.
Best water filter pitchers: A water filter pitcher gives you a supply of cold, filtered water in your fridge at all times. They come in a wide range of sizes and are affordable and easy to use.
Best faucet water filters: Countertop and faucet-mounted water filters give you unlimited amounts of filtered water without a permanent installation; these are the best, easiest to install faucet water filters.
Best whole house water treatments: If you need to treat your water problems at the source, whole house water treatment systems are the best choice. We discuss both water filters and water softeners.
Buying Guide: Confused as to what you need to know before shopping for a water filter? Here is the information you need to make an informed decision about the right water treatment for your home.
Our Sources: These are the expert and user reviews and recommendations we used to find the best water filters. They are ranked in order of credibility and usefulness.