When to change a water filter: schedules, signs and simple habits

A filter does not announce when it is due. A cartridge does not always look different on the day it should be replaced, so the most reliable approach is a simple system: know the schedule, mark the date and check a few signs. This guide covers how to read the maker's schedule, what signs to watch for, how to change a cartridge carefully, and a simple calendar you can copy. It also explains what test kits and TDS meters can and cannot do.

Start with the maker's schedule

The instructions for your product are the authority on when to change each cartridge. Many makers give both a time and a volume, and you change at whichever you reach first. A busy household may reach the volume before the time runs out, and a household that is away for weeks may reach the time before the volume. If you cannot find the instructions, check the product page, ask the maker, or contact us.

Do not borrow a schedule from a different product. The schedule belongs to the exact cartridge. If your system has several stages, as reverse osmosis systems and many under-sink systems do, each part may have its own schedule, so write them all down.

Label the date, and use the reminders you have

On the day you fit a new cartridge, write the date on the housing, the pitcher lid or a sticker, and put the due date in your phone calendar. If the schedule is by volume, a rough estimate of how much your household uses will tell you which limit is likely to come first. Some products have a reminder light or a counter. If yours does, use it alongside the schedule, not instead of it.

Signs the filter may be due

• Water comes out more slowly than it used to, a pitcher takes much longer to filter, or the shower pressure has dropped.
• The taste or odor of the water has changed.
• You see particles in the water.
• A reminder indicator on the product says it is due.

Treat these as prompts to check the filter and the instructions, not as tests of your water. Taste and smell are not a reliable way to judge whether water is suitable to drink, because many things cannot be tasted or smelled, and a filter can be past its schedule without any change you notice. That is why the schedule matters more than your senses. Leaks or drips from a housing, hose or fitting are a separate matter: they need attention straight away, whatever the schedule says.

Changing a cartridge carefully

For anything connected to your plumbing, such as under-sink, reverse osmosis and whole-house systems, and some refrigerators, turn off the water supply to the system first and release the pressure as the instructions describe. Have a towel and a container ready, because housings hold water. Use the tool supplied for the housing and handle the seals as the instructions say. After you turn the water back on, check for leaks, and look again later in the day. If a system has a pump or other electrical parts, follow the instructions about switching it off first. If you are unsure at any point, ask a licensed plumber, and keep in mind that local plumbing rules can apply to some work.

Flush a new cartridge, and keep a spare

Many instructions tell you to run water through a new cartridge for a set time or volume before you drink from it. A pitcher cartridge may need to be soaked, rinsed or have its first batches discarded. Follow exactly what yours says, because the steps differ between products.

Keep a spare cartridge on a shelf in its sealed packaging, stored as the packaging says, so the change never has to wait. Replacement parts are specific to a model, so check the exact fit and certification first, as described in How to read a water filter label. You can browse Cartridges & Filters.

Pressure gauges on whole-house housings

Some whole-house housings come with, or can take, pressure gauges, one before the filter and one after. When a cartridge is new, the readings give you a baseline. A growing gap between the two, or a clear drop in pressure after the filter, can suggest the cartridge is clogging and needs attention. Note the readings on install day and look at them from time to time. The instructions explain which readings to expect and what to do. Never adjust fittings while the system is under pressure. See Whole-House Filters and Whole-House Cartridges.

Watching trends with test kits and TDS meters

A TDS meter reports a single number for total dissolved solids. It usually estimates that number by measuring how well the water conducts electricity. Used the same way each time, on the same tap, it can show whether readings drift over the weeks, and a change is a useful prompt to check your filter. Its honest limit is that it does not measure everything. It does not tell you what is dissolved, and it cannot check for many things you may care about.

A water test kit covers only the items it lists. Check its expiry date and follow its instructions exactly. Neither tool is a laboratory test, and neither tells you whether your water is suitable to drink. For that, or for any health concern, contact your local water utility (most publish an annual water quality report), a certified testing laboratory or a health professional. See TDS Meters, Water Test Kits and everything under Testing & upkeep.

A simple maintenance calendar

The intervals come from the instructions for your product, which is why the column below points you there. The notes are habits that make the routine easier.

Pick a regular day, such as the first of the month, to glance at the dates, look for drips and note any change in flow. If you are still choosing a system, Pitcher, faucet, under-sink or whole-house: which water filter fits your home? compares the types, including how much upkeep each one asks for.