How we research and rate water filters

Some older pages on this site say products were tested in person. That wording is wrong, and we are correcting it page by page. This page describes what we actually do.

Where our information comes from

• NSF/ANSI certifications. The question that matters most is which contaminants a filter is certified to reduce, and under which standard. NSF/ANSI 42 covers aesthetic claims such as chlorine taste and odor. NSF/ANSI 53 covers health claims such as lead, PFOA/PFOS and many VOCs. NSF/ANSI 58 covers reverse osmosis systems, and NSF/ANSI 401 covers some emerging contaminants. Certifications can come from NSF, WQA or IAPMO, and each publishes a searchable listing.
• Certified versus claimed. A certification applies to the specific contaminants listed for that model, not to everything the box mentions. When a filter's claim comes from the manufacturer's own lab report or a lab it hired, we say so, and we don't present it as a certification.
• Manufacturer specifications. Rated flow rate, filter capacity in gallons, replacement interval, replacement filter price, dimensions and installation requirements, from the manufacturer's product page, manual or performance data sheet.
• Public health and water quality data. For contaminants, health limits and tap water quality we cite the EPA, the CDC, state agencies and peer-reviewed studies, and link to the primary source so you can check it.
• Prices. Amazon prices come from the Amazon Creators API, Amazon's catalogue feed for Associates. A price is accurate only as of the date shown next to it and is subject to change. The price on Amazon at checkout is the one that applies. We are adding an as-of date to every price on the site; if you find one without a date, treat it as approximate.

Our scores don't draw on customer star ratings or review counts from any retailer, and we don't show them on our pages.

What we calculate

Some figures on our pages are our own arithmetic from published specs. The main one is cost of ownership: the purchase price plus replacement filters over one, three and five years, using the manufacturer's replacement interval. These are calculations, not measurements, and your costs will differ if your water or usage wears filters out faster than rated.

What our scores mean

Each product has a score out of 10 and sub-scores. Every product has performance, value and features, and most also have filtration, flow rate, filter life or installation. These are editorial scores. They are not test results, and they are not customer ratings.

• Filtration and performance reflect which contaminants the filter is certified to reduce, and under which standards. A manufacturer-only claim counts for less than a certification.
• Flow rate and filter life reflect the rated flow and rated capacity in the manufacturer's spec sheet.
• Installation reflects what the manufacturer says installation involves: tools, plumbing changes, drilling and space under the sink.
• Value reflects the price and replacement filter cost against what the filter is certified to do.
• Features reflects things like filter-change indicators, remineralization, tank size and countertop footprint.

Our editors assign the scores after comparing a product with others in the same category. The overall score is a judgment across the sub-scores, not a fixed formula, so treat a gap of a few tenths as close to a tie. A score can't tell you what a filter will remove from your water. Start from which certifications you need, then read who each pick is best for.

How pages are written, reviewed and updated

• Pages are researched and written by the Clean Water Critic editorial team.
• Before a page is published, its product specs are checked against the manufacturer's source. Automated checks run before every site update to catch broken links, missing images and faulty structured data.
• We revise pages when a manufacturer changes a spec, a certification is added or withdrawn, a model is discontinued, or prices move. Each page shows the date it was last updated.
• When an affiliate link stops working or a product is no longer sold, we repoint or remove it.

How our affiliate links work

Most product links go to Amazon and carry our Associates tag. Others go to the manufacturer's own store, and some of those are affiliate links too. If you buy through one, we may earn a commission at no extra cost to you. That is how this site is funded.

A commission doesn't change a score or a ranking, and no manufacturer pays to be included. If the right filter for a situation comes from a brand with no affiliate program, we still recommend it. Read our full affiliate disclosure.

Found an error?

We want to know about any certification, spec, price or claim on this site that doesn't match its source. We don't have a public reader inbox yet, and we would rather say so than list an address nobody reads. When one is set up, it will be posted here.

What we don't do

• We don't send water for lab analysis or measure contaminant reduction ourselves.
• We don't measure flow rates, filter lifespan or durability.
• We don't accept payment for coverage or placement.

Read more about Clean Water Critic, see our guide to choosing a water filter, or compare picks in our buyer's guides.