A drinking-water filter is rated in gallons or months, and it is easy to pick one from the box without knowing how much water your household actually draws through it. The useful question is not how big the cartridge is. It is how many of your taps feed the filter, and how much water those taps move in a typical week.

List the taps that feed the filter

Start with the faucet or line the unit will serve. Most point-of-use filters sit under the kitchen sink, which means the treated water may go to the cold tap, the refrigerator line, the ice maker, and sometimes a coffee machine. Write each one down. If the filter feeds only the sink, the draw is small. If it also runs the fridge dispenser and the ice maker, the draw can be several times larger even though the cartridge looks the same.

Count the draws, not the rooms

Estimate the water your household uses through the filtered line in a typical week. Pitcher refills, drinking glasses, cooking water, rinsing produce, making coffee or tea, and ice all count. For one week, make a tally mark on paper each time someone fills a container or runs the cold tap for drinking or cooking. Track the ice maker separately if you have one. Once you know roughly how much a pitcher or glass holds, the tally gives you a workable gallon figure.

Compare your number to the rated capacity

Look at the manufacturer's rated capacity for the cartridge, usually given in gallons, and check whether the same cartridge also carries a time limit in months. Those two numbers can disagree. A busy household may use up the gallon rating first, while a quiet household may reach the time limit first. Whichever limit comes first is the one you plan around. Divide your weekly gallon estimate into the rated capacity to see how many weeks the cartridge should last under your use. If that answer is shorter than the time period on the box, use the shorter number.

Be careful with the rating itself. Capacity figures come from the maker's test conditions, and your water pressure, temperature, and the amount of sediment in your supply can change how quickly a cartridge loses effectiveness. Ask the seller or maker which conditions the rating assumes and what they recommend for a home with your draw.

Check the pre-filter and the flow at the faucet

Many filters have a sediment pre-filter ahead of the main cartridge. A clogged pre-filter slows the flow and makes the main cartridge work harder. If your home already has a whole-house sediment filter, confirm it is still doing its job, because a point-of-use unit downstream will need changing sooner when the supply carries grit.

Also notice how the water comes out after a cartridge change. If the flow is much slower than before, the unit may be undersized for the draws you are asking of it, or a fitting may need attention. Do not treat a clear drop in flow as normal.

Set the change schedule by volume

Write the change date on the housing or in your household log, but base it on the gallons you counted rather than the calendar alone. Set the reminder for whichever limit comes first. Keep the cartridge's model number in the same place, so the replacement matches the housing you already have.

What to decide before you buy

Before you choose a filter, be able to answer four questions: which taps and appliances the unit will feed, how many gallons your household draws through that line in a typical week, which of the two rated limits comes first on the cartridge, and who will track the change date. If you cannot answer one of them, the number on the box will not tell you whether the filter fits your home.

For county-level context on your supply, see the St. Charles Water Ratings report card. The How Grades Work page explains the criteria behind those ratings.