Every whole-house water filter gets a score from 0 to 100% built from five factors, which maps onto the 5-point rating shown on the site (a 0% score would be 1.5 stars; a perfect score, 5). Each factor is scored against fixed reference points, not against the other products in the list, so adding a new model never moves another model’s rating. Filters are grouped by build, cartridge and tank, on the same scales except price.
| Factor | Measures | Weight |
|---|---|---|
| Running cost | replacement filters or media per 1,000 gallons | 25% |
| Certification | NSF/ANSI certification by an accredited body | 25% |
| Flow | service flow rate | 15% |
| Price | price of the system | 20% |
| Warranty | warranty length | 15% |
Where the numbers come from
Capacity, flow, port size, stages and warranty come from the maker’s product page, data sheet, manual or the maker’s copy on its store listings. Certifications are counted only when they appear in an accredited body’s public listing, such as the NSF certified products database. Prices are checked at Amazon and the maker’s own store, and the lowest is used. Replacement filter and media prices are checked the same way.
The factors
Running cost is the price of the replacement filters or media for one rated capacity, divided by that capacity, per 1,000 gallons, on a log scale, so halving the cost always counts the same. $3 per 1,000 gallons scores zero and $0.20 full marks. The capacity is the maker’s gallon rating for the shortest lived filter in the set. For a tank system whose maker sells no media refill, the whole system price stands in for the refill.
Certification is 60% for certification to any NSF/ANSI drinking water standard by an accredited body, and the full score when that includes NSF/ANSI 53, 58 or 401, which cover health related contaminants rather than taste and odor. A filter the maker says is tested to a standard, but not certified, scores 30%. One with neither scores zero, since a certification that isn’t in a public listing can’t be checked. Certification of materials only, such as NSF/ANSI 372 for lead free parts or NSF/ANSI 61, isn’t counted.
Flow is the maker’s service flow rate: 5 gpm scores zero and 15 gpm full marks. When a maker gives both, the rated service flow is used rather than the peak.
Price is the current price on a log scale, against the build’s own range: for cartridge systems $800 scores zero and $150 full marks; for tank systems $3,000 scores zero and $700 full marks.
Warranty is the maker’s warranty on the system: 1 year scores zero and 10 years full marks.
What isn’t scored
Capacity on its own, the number of stages, port size, chloramine and PFAS reduction claims, backwashing and a UV stage are listed but not scored. Chloramine claims have their own list.
Missing data
If a figure isn’t published, that factor gets the neutral middle score (50) and the product page marks it as estimated. The exception is warranty: a maker that doesn’t publish one gets no warranty counted, so that factor scores zero, the same as the shortest warranty on the scale, and is marked estimated. A filter with no gallon rating or no replacement price gets the neutral score for running cost.
Ties and discontinued models
Products are ranked by score within each group. Discontinued models stay listed but rank below every current model and can be hidden with the “Hide discontinued” filter.