An under the sink water filtration system is a set of filter stages mounted in the sink cabinet, fed from the cold line and delivering treated water either through a dedicated faucet or through your existing one. A complete system is more than the filters: it includes the feed adapter, tubing, mounting hardware and faucet, and the design choice that matters most is carbon-only versus membrane-based.
What ships in a complete system
Expect the filter manifold with its stages, a feed adapter that tees into the cold water shutoff, food-grade tubing, a mounting bracket for the cabinet wall, and either a dedicated drinking faucet or fittings to feed your existing tap. Membrane systems add a drain saddle and, on tank designs, the pressurized storage tank. Reading the box contents against your sink matters: a system without a matching faucet finish, or with a drain saddle that will not fit your pipe size, turns a simple afternoon into a second shopping trip. Vendors list every included part on their product pages, which is worth checking line by line.
Carbon systems or a membrane: the fork that defines the purchase
Multi-stage carbon systems polish taste, remove chlorine and carry model-specific certified claims for contaminants like lead, and they feed your existing faucet at full flow with no drain line. Membrane systems, reverse osmosis, go further: they reject dissolved solids wholesale, which is what it takes for contaminants carbon cannot hold. PFAS is the current example driving many purchases: the EPA's 2024 rule set enforceable limits of 4.0 parts per trillion for PFOA and PFOS in drinking water, and reverse osmosis is among the treatment approaches households use at the tap for these compounds. If your target list includes dissolved contaminants, the membrane side of the fork is the relevant one.
Dedicated faucet or your existing one
Feeding the existing faucet keeps the counter unchanged and treats all cold water at that sink, but it commits your busiest tap to filtered flow, so cartridges exhaust faster from dishwater and rinse water that never needed filtering. A dedicated faucet meters the filtered water to drinking and cooking, stretching every cartridge, at the cost of a hole in the counter and a second spout. Membrane systems require the dedicated faucet by design. There is no wrong answer, only a usage pattern to be honest about. Our best under sink water filter system comparison scores current systems across both designs.
Questions people ask about under the sink water filtration system
Does an under-sink system filter the whole house?
No, it treats the one sink it is installed under. Whole-house problems like scale in the shower or staining in the laundry need a point-of-entry system instead.
Do I need a plumber to install one?
Carbon systems feeding the existing faucet are a hand-tools job for most owners. Membrane systems add a drain connection and often a faucet hole, which is where hiring help starts to pay.
Which design handles PFAS at the tap?
Membrane-based reverse osmosis systems are the under-sink approach used for dissolved contaminants of this kind, alongside specific certified carbon cartridges. Check the published claims for the exact model.