Two filters can have the same sticker price and very different yearly costs. The number that matters is the total you pay over twelve months, including every cartridge, shipping charge, and special item. Comparing one cartridge at a time hides the cost you will actually feel.

Start with the replacement schedule

Find the change interval the maker prints for each cartridge, in months or in gallons. Note whether the interval is time-based, volume-based, or whichever comes first. A cartridge that lasts longer in your home may cost less per year, even when its unit price is higher.

Count the cartridges per year

Count every cartridge the system uses, including each stage and any pre-filter or post-filter. A housing with three stages means three line items at each change, not one. Multiply each stage by how many times a year it must be changed under the schedule you found.

Price the items that come with it

Add shipping, minimum order requirements, and any tax or fee. Check whether a bundle includes spares, and whether the price assumes you buy a full set at once. Note whether a seller charges extra for a particular certification or claim, and whether that certification covers the contaminant you care about. Check the label on the cartridge against the claim you want, rather than trusting the product name.

What pushes the yearly total up or down

Several things move the total. The number of stages and the media type in each stage set the price of each change. The interval set by the maker sets how often you pay. Your water use sets how fast a cartridge is used up. Certification and third-party claims often add cost. Bulk buying and subscription offers can lower the unit price but commit you to a schedule. Missing a change can cost more later if you end up replacing a housing or retesting the water.

Build a one-year budget before you buy

Write down each cartridge, its interval, the number of changes in twelve months, the unit price, and the shipping. Add the figures for each option. Keep the sheet with the manual so the next change is planned rather than guessed. If the total feels high, ask whether a different model uses fewer stages or a longer interval for your water.