A countertop or pitcher filter looks simple to buy, but most disappointments come from a mismatch between how much water the household pours and how much the unit can hold or filter between refills. Before you compare models, measure the draw you actually have.
Count what the kitchen really uses
For one ordinary week, keep a rough tally of how many glasses of filtered water are poured, and separately note the water used for coffee, tea, rice, soup and anything that goes into a pot. Ask who fills the pitcher and how often. A household where one person drinks a glass or two a day has a very different need from a household where several people pour from the same jug at dinner.
Match the reservoir to the refill habit
For a pitcher, check the reservoir size on the product page and ask whether the filter sits in the lid or the body, since that affects how fast it pours. If the pitcher empties before the next meal, the reservoir is too small for your habit, even if the filter itself is fine. For a faucet-mount unit, the constraint is different: it filters only while the tap runs, so the question is whether the flow it allows is comfortable for filling a pot or a large glass.
Read the replacement rating carefully
Manufacturers often state cartridge life in gallons, in months, or both. Gallons are the more honest measure if you know your use, because a cartridge used lightly for a year and one used heavily for a month can carry the same label. If a unit lists only months, ask how the estimate was made. Then ask what a replacement cartridge costs and how often you will need one, and add that figure to the price of the unit before you compare.
Check the fit at the faucet
For a faucet-mount filter, confirm the faucet's thread type and whether it has a diverter or a pull-out spray. Check that the unit does not block the spray head or the aerator in a way that makes the tap hard to use, and confirm that the switch between filtered and unfiltered water works before you rely on it.
Decide who refills and who changes the cartridge
Name the person who refills the pitcher, notices when the filter light or the flow slows, and orders and installs the replacement. A filter that nobody owns usually runs past its rated life. Write the change date on the cartridge or the calendar, and keep the replacement cartridges in the same cupboard as the spare filter so the swap does not depend on memory.
Questions to ask before you buy
Ask the seller which volume the rating refers to, what the replacement interval is based on, what a replacement cartridge costs, and whether the unit has been checked against the claims on our Review Methodology and Editorial Criteria page. If the answers are vague, the unit may suit a single occasional drinker but not a busy kitchen.