Under the sink reverse osmosis systems all use the same underlying process, so the differences that matter are in the supporting parts: how many stages there are and what each one does, what the system is independently certified to reduce, how fast the membrane produces, and how much work the thing asks of you each year. Stage count is the least useful of those and the most heavily advertised.
What the stages actually do
A typical system runs sediment, then carbon, then the membrane, then a polishing carbon after the tank, with a remineralizing cartridge as an optional extra. Only the membrane does the reverse osmosis. The stages before it exist to protect it, because chlorine damages the common thin-film membrane and sediment blinds it, and the stage after it exists to clean up any taste picked up in the tank. A system advertising a high stage count has usually split those same functions across more housings, which means more cartridges to buy rather than better water. Judge the arrangement, not the number.
Certification is what separates claims from marketing
Every RO system reduces dissolved solids. Whether a specific system is certified to reduce lead, arsenic, PFOA and PFOS, or cysts is a separate question, and that certification attaches to a model and its cartridges rather than to a brand name. Look up the model in the certifier's own listing rather than trusting a badge on the box. If you are buying because a test result named a specific contaminant, the certification list is the only part of the specification that speaks to your actual problem, and a system without it is a system you are buying on faith.
Production rate, tank and tankless
Membranes are rated for a daily output at a reference pressure and temperature, and your kitchen supplies neither. Cold winter feed water and modest house pressure both reduce real output substantially, which is why a system that seems generously rated can still leave you waiting. A conventional system buffers this with a pressurized storage tank, so drawing a glass is instant regardless of how slowly it was produced. Tankless systems use a higher-output pumped membrane and produce on demand instead, freeing the cabinet floor at the cost of needing power. Vendors publish these output ratings on their product pages and they are worth comparing directly.
The maintenance load you are signing up for
Over a decade, cartridges and membranes account for most of what the system costs you and all of what it asks of you. Look for a manifold design where housings twist off by hand rather than needing a wrench in a cramped cabinet, for cartridges available from more than one supplier, and for a clearly published change interval. Add a way to verify performance, ideally a built-in solids meter or an inexpensive handheld one, so you replace the membrane on evidence rather than on a hunch. Our comparison of the best under-sink reverse osmosis systems weighs these against each other model by model.
Questions people ask about under the sink reverse osmosis systems
Are more stages better?
Not inherently. Extra stages usually split the same sediment, carbon and polishing functions across more housings, which means more cartridges to buy rather than cleaner water.
How do I check my membrane is still working?
Compare dissolved solids in the feed water against the treated water using a handheld meter. A steadily rising reading on the treated side means the membrane is failing.
Should I choose tank or tankless?
Tank systems store water so delivery is instant and no power is needed. Tankless systems free the cabinet floor and avoid stored water, but they need an outlet and a pumped membrane.
Does an RO system need a softener in front of it?
On hard water it helps considerably, since hardness scales the membrane and shortens its life. On a well, sediment and iron removal ahead of the system matter even more.