Every reverse osmosis system gets a score from 0 to 100% built from six 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. Systems are grouped by type, tankless and with a tank; certification, filter cost, water wasted and warranty use the same scales for both, while output and price have their own reference points for each type.
| Factor | Measures | Weight |
|---|---|---|
| Certification | certified to NSF/ANSI 58 for reverse osmosis | 25% |
| Filter cost | replacement filters a year | 20% |
| Water wasted | drain water per gallon of purified water | 15% |
| Output | rated gallons a day | 10% |
| Price | price of the system | 15% |
| Warranty | warranty length | 15% |
Where the numbers come from
Output, drain ratio, stages, tank size and warranty come from the maker’s product page, spec sheet, performance data sheet or manual. Certification comes from the certifiers’ own directories (NSF, WQA, IAPMO and CSA), checked by exact model; a maker’s claim that a system is “tested to” or “meets” a standard doesn’t count. Filter cost adds up the maker’s replacement filter prices at the maker’s recommended change intervals for one year. Prices are checked at Amazon and the maker’s own store, and the lowest is used.
The factors
Certification is full marks for a system certified to NSF/ANSI 58, the reverse osmosis standard, by an accredited body. A system certified only to other NSF/ANSI standards, such as 42, 53 or 372, scores 30%, and one with no third-party certification scores zero.
Filter cost is what the replacement filters and membrane cost for one year at the maker’s schedule, on a log scale, so halving the cost always counts the same. $250 a year scores zero and $50 full marks.
Water wasted is how many gallons go down the drain for each gallon of purified water, from the maker’s stated pure to drain ratio. 4 gallons (1:4) scores zero and 0.33 gallons (3:1) full marks.
Output is the maker’s rated gallons a day. For tankless systems 300 gpd scores zero and 1,000 gpd full marks; for systems with a tank 50 gpd scores zero and 100 gpd full marks.
Price is the current price, on a log scale. For tankless systems $900 scores zero and $250 full marks; for systems with a tank $500 scores zero and $150 full marks.
Warranty is the maker’s warranty: 1 year scores zero and 5 years full marks.
What isn’t scored
The number of stages, a remineralization stage, tank size, a smart faucet that shows TDS or filter life, a booster pump and the release date are listed but not scored.
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. Where a maker gives no pure to drain ratio, it isn’t worked out from other figures, so that factor is neutral.
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.