Under the sink RO is bought on filtration claims and regretted on physical fit. The cabinet is already crowded, the tank is bigger than the photograph suggests, and low feed pressure quietly halves what the membrane produces. Three checks made before you order, with a tape measure and a pressure gauge, decide whether the system you want will work in your kitchen.
Measure the cabinet honestly
Open the doors and note what is already in there: the trap, the angle stops, the dishwasher supply, a garbage disposal, sometimes a hot water dispenser and a bin. Then measure the clear height at the back where the tank will stand, the clear width for the filter bank, and the depth to the door. A standard storage tank stands taller than most people expect and must sit upright. If the space is genuinely tight, look at tankless units, which are far more compact, or plan to put the tank in an adjacent cabinet or a basement below with a longer run of tubing.
Check your feed pressure before anything else
Reverse osmosis production depends on pushing water against the membrane, so feed pressure determines both how much water you get and how much goes to drain. A house at the low end of normal supply pressure will see noticeably reduced output, slow tank refill and a faucet that runs out during a dinner party. Fit a cheap gauge to an outside tap and read it, ideally at a busy time of day. If pressure is marginal, budget for a booster pump or a permeate pump, which recovers energy from the drain stream and improves both output and efficiency.
Confirm the drain and the faucet hole exist
Every membrane system needs a drain connection, normally a saddle clamp on the sink tailpiece above the trap and clear of a garbage disposal outlet. If your sink drains into a wall behind cabinetry with no accessible tailpiece, that is a plumbing job to solve before ordering. You also need a hole for the dedicated faucet: a spare knockout is easy, a stainless sink can be drilled, and a stone countertop usually means a fabricator. Discovering either problem after delivery is the most common reason a system sits in its box.
Tank capacity against how you actually draw water
A membrane produces slowly, so the tank is the buffer between production and the way a household uses water in bursts. If you fill large pots, run a coffee machine and feed a refrigerator ice maker from the same system, a small tank empties and then trickles. Choose a larger tank or a higher production membrane if the fridge is on the line. Note also that a tank is a pressurized vessel with an air charge behind a diaphragm, and that charge needs checking occasionally, because a flat tank gives a weak faucet even with a perfect membrane.
Serviceability is part of fit
You will change filters several times a year for the life of the system, so the cartridges must be reachable without disconnecting the tank or emptying the cabinet. Quick change designs, where the whole cartridge twists out with its housing, are far more pleasant in a cramped space than sump housings needing a wrench and clearance below. Leave enough slack in the tubing to pull the manifold forward. Our under sink RO comparison guide covers which formats stay serviceable in a tight cabinet.
Questions people ask about under the sink ro
How much cabinet space does an under sink RO system need?
Enough clear height for an upright storage tank plus width for the filter bank and access to change cartridges, which is more than most product photos suggest.
Does low water pressure affect an RO system?
Considerably. Lower feed pressure reduces how much water the membrane produces and increases the proportion sent to drain, which a booster or permeate pump can correct.
Can the storage tank go somewhere other than under the sink?
Yes, a basement or an adjacent cabinet works with a longer tubing run, though a long run costs some pressure at the faucet.
Are tankless RO systems a good option for small cabinets?
They save a lot of space by producing on demand rather than storing water, at the cost of needing power and delivering a slower fill for large pots.