Under-sink water filters and reverse osmosis
An under-sink system treats only the water at one tap, which is why it can afford to filter far more thoroughly than a whole-house filter — and why reverse osmosis lives here rather than on the main line. RO is the most complete filtration available to a household: it removes dissolved contaminants that carbon cannot touch, including lead, fluoride, arsenic, nitrate and PFAS.
It is not automatically the right buy. RO sends water down the drain to make water, strips the minerals out along with everything else, and costs more to run. If your water report shows nothing dissolved that worries you, a carbon block under the same sink is cheaper, simpler and wastes nothing.
In this cluster
- Best under-sink reverse osmosis systems — scored and ranked, with every certification checked against the certifier's own database
- How water filters work — what reverse osmosis actually does, and the five other mechanisms it is often confused with
- Do water filters remove fluoride? — the question that sends most people to RO in the first place
- Our certification research — under-sink is the category where certification actually separates products
Reverse osmosis or a carbon filter?
This is the only decision that matters before you start comparing models, and it comes down to what is in your water rather than what is on the box.
A carbon block handles the things you can taste and smell — chlorine, chloramine, sediment, some organic chemicals — and a good one is certified to NSF/ANSI 42 for taste and odour, sometimes NSF/ANSI 53 for specific health contaminants like lead. It wastes no water, needs no power, fits in a smaller cabinet and costs less to own.
Reverse osmosis pushes water through a membrane fine enough to reject dissolved solids — the things a carbon filter passes straight through. That is the entire reason to buy one: fluoride, nitrate, arsenic, chromium, sodium, PFAS and total dissolved solids. If none of those appear in your water report, you are paying for capability you will not use.
The three numbers worth understanding
Drain ratio is how much water goes down the drain to make a litre of filtered water, written as pure-to-drain. A 2:1 system produces two litres for every one wasted; an older 1:3 system wastes three to make one. Modern tankless systems reach 2:1 or 3:1, and the figure on the box is a rated best case — in a household that draws small amounts often, the real number is usually worse, because each draw triggers a flush.
GPD — gallons per day — is the membrane's rated output, and it is really a measure of how fast the tap runs. An 800 GPD tankless system fills a glass in about five seconds. A 75 GPD tanked system fills a glass instantly from its stored tank, then refills that tank slowly.
Remineralisation matters because RO removes calcium and magnesium along with everything else, which is what makes untreated RO water taste flat. An alkaline or remineralising post-filter adds some back. It is a taste feature, not a health one, and it is worth having.
Tankless or tanked
Tanked systems store filtered water in a pressurised tank under the sink. They are cheaper, need no electricity, and have decades of field history — but the tank takes most of the cabinet, and once you have drawn it down it refills slowly.
Tankless systems filter on demand at high pressure. They free the cabinet, never serve you water that has sat in a bladder, and generally achieve better drain ratios. In exchange they need a power outlet under the sink, they contain a pump that makes noise, and they cost more.
Neither is better filtration — the membrane does that work in both. It is a question of space, noise and budget, and the ranked systems include both.
Under-sink or whole house?
They solve different problems and many homes want both. A whole house filter protects plumbing and appliances and improves every tap, but at a flow rate that rules out thorough filtration. An under-sink system does the thorough work at exactly one tap — the one you drink from. If you are choosing only one and your concern is drinking water, choose this one.