An under-sink unit gets sized by two numbers most people never write down: how much water goes through the kitchen tap on a normal day, and how much room the unit needs under the cabinet. Pick a model before you have both, and you may end up with a tank that runs low at dinner or a housing that will not let the cabinet door close.
Count the draw, not the household
Household size is a weak guide. Two adults who cook from scratch every night and fill a pitcher each morning can draw more from the drinking and cooking tap than a family of four that buys bottled water for drinking. List what actually comes off the filtered tap: drinking glasses, the pitcher, cooking water, coffee, and any hand washing at that sink. Then check whether the refrigerator ice maker and the dishwasher connect upstream or downstream of the filter. That single detail decides whether the unit has to carry those loads too.
Time it yourself
Fill a marked container from the tap you plan to filter, at the rate you normally use it, and note how long it takes. Do this on a typical day, not during a holiday meal. Write the total down. Then put a direct question to each provider or manufacturer: at this daily volume, what tank size and flow rate does the unit give, and how often will the cartridge need replacing at this use? Ask them to show the math on paper. If they cannot, that tells you how much care went into the estimate.
Check the flow at the tap, not just the tank
A tank holds treated water for later use, but the stream you get at the faucet depends on the housing, the restriction in the cartridge, and the pressure in the supply line at the sink. Ask whether the unit still delivers a usable stream when the tank is low, and whether a fresh cartridge changes the flow. A slow trickle is the most common complaint about a small unit on a busy kitchen tap, and it tends to be noticed quickly. If you have a pressure gauge or can borrow one, note the reading at the sink. A low reading makes a weak stream more likely, and you should raise it before you buy.
Measure the cabinet for the install, not just the box
Under-sink fit is mostly about clearance. Measure the cabinet width, depth, and height at the back, the height from the floor to the underside of the sink, and anything in the way, such as the drain trap, the supply shutoff, or a garbage disposal. Then add the space you need to change cartridges. A housing that unscrews downward needs room below it, and a tank needs space to sit without pressing on the supply line. Check the door swing and any drawer below. Compare your numbers with the listed dimensions of the unit, and with the room needed to swap a cartridge, which some product listings leave out.
When the answer is a bigger system
If the kitchen draw is large, or the kitchen tap is the only one you want filtered, the honest answer may be that a point-of-use unit is the wrong size for the job. A whole-house system or a second unit may fit better. Keep in mind that a larger unit may not fit the same cabinet, so ask for the footprint in writing before you agree to any plan that depends on it.
What to bring to the estimate
Bring your written draw count, your cabinet measurements, a list of appliances that sit on the filtered line, and the model you are considering. Ask for the sizing recommendation in writing, including the tank size, the flow rate, the cartridge schedule, and the clearance the unit needs. For how we weigh provider claims, see our review methodology.