A filter that works well in one kitchen can be the wrong choice in the next, and the difference usually comes down to format. Faucet-mounted units, pitchers, countertop units, under-sink systems and whole-house filters each fit a different sink, a different amount of water, and a different person who has to change the cartridge. Start with the space and the demand, and only then compare products.

Measure the space before you read a product page

Take the measurements yourself. Under the sink, measure the clear width and depth at the height where the housing would sit, then leave room for the cartridge to be pulled out or the housing to be unscrewed. Note where the supply pipes and drain come up, and whether the cabinet door clears the housing when it is open. For a faucet-mounted unit, check the faucet type: a single-hole faucet, a pull-down spout, and the thread on the aerator each affect what will attach. For a countertop unit, measure the counter depth and the height under any upper cabinet.

Match the format to how the water is used

Ask who drinks the water and for what. A household that only needs drinking and cooking water has a very different demand from one that fills a bathtub or runs a dishwasher. A pitcher or faucet-mounted unit serves drinking and cooking water, and its limits are how fast it can filter and how often you have to refill it. An under-sink system serves one or two taps. A whole-house filter treats every tap at once, which means it has to handle the household's peak flow, not just the kitchen sink.

Check the flow rating on the label

Every filter that moves water has a rated flow. Find it on the product sheet or packaging and compare it with the way you plan to use the filter. If the sheet lists a flow in gallons per minute, ask whether that figure is a maximum or a typical rate. A filter run above its rating will usually deliver water faster than it can treat it, so check whether the product says what happens at higher flow. Do not guess. Ask the seller or the manufacturer to state the rated figure in writing before you buy.

Ask who changes the cartridge and how

A format that fits the sink on day one can still be a poor fit if the cartridge is hard to reach. Ask how the cartridge is removed, whether you must shut off the water first, whether a wrench or special tool is needed, and how much space the old cartridge needs to come out. Ask whether replacement cartridges are sold for that exact model and where you would buy them. If the cartridge sits behind the pipes, plan for that before you install, not after.

Compare the capacity claim with your own change schedule

Most filters state a capacity in gallons or a time period. Set that figure against your own household's use: how many glasses of water pass through a pitcher each day, and how much cooking water passes through an under-sink unit. Then work out how often the cartridge needs changing and whether you will remember. A format that needs changing more often than you can manage is a poor fit even if the sink measurements work.

Before you order

Write down the sink measurements, the faucet type, the rated flow, the cartridge replacement route and the stated capacity. Put those numbers next to each product you are considering. To see how independent ratings weigh these factors, read How Grades Work: Criteria, Weights, and the A to F Scale. Then choose the format that fits your sink, your water use, and the person who will do the cartridge changes.