How we rate water filters
Every product on this site carries a score out of 100, built from four measurable inputs: owner ratings weighted by review volume (45 points), third-party certification (20), filtration stages (20) and warranty length (15). Each is scored against what is achievable in that product's category, not against a perfect product that does not exist. The same formula runs on everything.
We do not lab-test filters. Every input comes from published specifications and verified owner ratings, and we say so rather than implying testing we have not done.
The four inputs
Owner ratings, weighted by volume — 45 points
The star rating, scaled across the range Amazon products actually occupy, then multiplied by a confidence factor based on how many people rated it.
Why it counts: A 4.8 from 12 buyers is a weaker signal than a 4.5 from 3,000. Weighting by volume stops a handful of reviews outranking years of evidence.
NSF/ANSI certification — 20 points
Whether an independent body has certified the filtration claims to a named standard — NSF/ANSI 42 for taste and chlorine, 53 for health contaminants, 58 for reverse osmosis. Scored in three tiers, because "certified" and "tested" are not the same claim.
Why it counts: Any manufacturer can print "removes 99% of contaminants" on a box. Certification is the only part of that claim someone else has checked.
Filtration stages — 20 points
How many distinct filtration stages the system runs, measured against the number normal for its category.
Why it counts: A three-stage whole house system is normal; a three-stage reverse osmosis system is thin. The same number means different things, so it is judged per category rather than in the abstract.
Warranty — 15 points
Length of the manufacturer warranty, against the length standard for the category.
Why it counts: How long a manufacturer will stand behind a product is a reasonable proxy for how long they expect it to last.
How the certification tier is decided
This is the part of the formula we got wrong first, and it is worth explaining because almost every water filter site treats it as a yes/no. It is not. Our own research found that 68.8% of water filtration systems invoke NSF or ANSI while only 21.5% state a certification to a standard covering what they remove — so a binary check counts the other 47% as certified when they are not.
| What the listing states | Scores |
|---|---|
| Certified to two or more performance standards, named and checkable | 1.00 |
| Certified to one performance standard, named and checkable | 0.70 |
| "Tested by a third party to meet NSF/ANSI standards" — no certifier, no standard number, nothing to look up | 0.25 |
| No third-party language at all | 0.00 |
The 0.25 tier is the one that matters. Somebody did commission a test, so it is worth more than silence — and it is not a certification, so it is worth a great deal less than one. Every product card on this site states which tier it falls into, including when the answer is "none".
Scored against the category, not against perfection
This is the part we got most wrong, twice, and it is worth explaining because it changes what a score means.
The first version scored against theoretical ranges — ratings out of five, stages out of six, warranty out of five years — and gave a well-reviewed system 48 out of 100. Everything landed between 40 and 50. The second version fixed the rating range but still measured certification against a perfect world, and rated the best whole house filter on the market 62.
A reader seeing 62 concludes "mediocre, keep looking". But 62 was not measuring the product. It was measuring the distance between that product and an imaginary certified one that does not exist in this category — and it applied the identical penalty to every candidate, so it did not even rank them. It was noise dressed as rigour.
A score answers "how good is this choice?", and a choice can only be made among things that exist. So each input is now scored against what is achievable in the category. Meet the category norm and you score full marks on that input; fall short and you lose points in proportion.
The benchmarks, published
| Category | Normal stages | Normal warranty | Best certification available |
|---|---|---|---|
| Whole house systems | 3 | 1 yr | Third-party tested only (0 of 21 certified) |
| Under-sink and countertop | 5 | 1 yr | Certified to a performance standard (29.2% certified) |
| Pitcher filters | 1 | 1 yr | Certified to a performance standard (75.0% certified) |
Those last two columns come from our own survey of 93 systems, not from guesswork. Because no whole house system in that survey states a performance certification, certification cannot separate whole house products and is not allowed to drag them all down — the fact that the category lacks it is stated in prose on the page instead, which tells you something true. A uniform penalty told you nothing.
The consequence is worth being explicit about: a whole house system scoring 92 is not making the same claim as an under-sink system scoring 92. The first is excellent among uncertified options; the second cleared a bar that includes certification. Compare scores within a category, and read the certification line on every card — it is always stated, even when the answer is "not certified".
What the bands mean
| Score | Reads as |
|---|---|
| 88–100 | Excellent — at or near the best available in its category |
| 75–87 | Good — a sound choice with a clear trade-off somewhere |
| 60–74 | Fair — works, but something better exists at a similar price |
| Below 60 | Weak — falls short of category norms on more than one input |
What we do not do
- We do not lab-test. We have no water testing facility. Anyone claiming independent contaminant testing should be able to show you the lab and the method; we would rather tell you what we actually do.
- We do not score on price. Prices move constantly, and a score that shifts with a sale is not measuring the product. Value is a judgement we make in words, not a number.
- We do not take payment for placement. Brands cannot buy a score, a position, or a "best for" label. We earn a commission when you buy through our links, which is disclosed on every page, and it does not depend on which product you choose.
Where the data comes from
Specifications, certifications, warranty terms and category rankings come from Amazon's product data. Owner ratings and review counts come from the same source. Both are refreshed when pages are rebuilt, and every product card shows the date its data was last updated.
Where a manufacturer's claim cannot be verified against a certification or a published specification, we say so on the product rather than scoring it as though it were confirmed.