Methodology

How we check claims and compute costs

A reduction counts as certified only when a certifier's public database lists the exact product with that contaminant in its claims. Running cost = replacement filters per year × their price (plus drain water for reverse osmosis), for a household's water use, with every input sourced.

1. "Does it remove X?"

• Certified: the product appears in the listings of NSF, WQA (Gold Seal), IAPMO R&T or CSA, and the contaminant is one of its listed reduction claims. We link the listing. Certifiers test to NSF/ANSI standards (42, 53, 58, 401, 473 and others) and audit production.
• Brand test only: the brand publishes or cites a lab report for the claim, but no certifier lists it. We link the brand's evidence.
• No claim: neither. It doesn't prove the filter does nothing; it means nobody has shown it.
• We match the certifier's wording to the contaminant (for example "PFOA/PFOS" counts for PFAS). A listing for one PFAS compound is shown as it is listed; it is not a claim for every PFAS.

2. What it costs to run

Replacements per year for each filter or stage = the larger of (gallons a year ÷ rated gallons) and (12 ÷ rated months): a cartridge is changed when it reaches either limit.

Gallons a year = people × daily use × 365. Daily use is 0.28 gallons per person for drinking and cooking (point-of-use filters), EPA's average adult intake of tap water including water used to prepare food and drinks (source), and 58.6 gallons per person for all indoor use (whole-house filters) (source).

Reverse osmosis also sends water to the drain: drain gallons = filtered gallons ÷ the pure-to-drain ratio, priced at $21.03 per 1,000 gallons for water and sewer, the average in Ohio EPA's 2023 rate survey (we found no free national figure; your utility bill shows your own rate) (source).

Prices are each brand's own list price for the unit and its replacement filters, with the date checked. Subscriptions and multi-packs are often cheaper; we use the single-filter price so every filter is compared the same way. Missing data makes a result "unknown", never zero.

3. Rankings

"Best for" pages apply a fixed rule (certified for the contaminant, or of the right type) and then rank by first-year cost: purchase price plus one year of filters for a household of 2. Commissions never change a ranking.

4. What we don't cover

• Health effects and medical advice. For what is in your water, read your utility's Consumer Confidence Report or test a private well with a certified lab.
• Water softeners and UV disinfection units, for now.