When to replace a water filter, and how to stop forgetting
Manufacturers rate filters in two ways: a volume of water and a number of months. Whichever you reach first is the replacement point, and for most households the months arrive first because nobody is measuring litres. The numbers below are typical ranges — the label on your own cartridge always wins.
Typical intervals by filter type
- Pitcher and dispenser filters: roughly every two months, or around 40 gallons / 150 litres for common carbon cartridges. Long-life versions exist and say so on the box.
- Faucet-mounted filters: commonly two to three months, often rated around 100 gallons / 380 litres.
- Refrigerator filters: usually about every six months, or 200–300 gallons. Most fridges have an indicator light, which counts water dispensed and elapsed time rather than actually testing the water.
- Under-sink carbon blocks: typically six to twelve months.
- Reverse-osmosis systems: staged — sediment and carbon pre-filters roughly every six to twelve months, the RO membrane every two to five years, the post-carbon polishing filter about once a year.
- Whole-house sediment cartridges: often three to six months, much sooner on well water or after work on the mains.
Why your interval may be shorter than the box says
Ratings assume average water and average use. Adjust down if you have hard water or visible sediment, if you are on a private well, if your household is large or you fill bottles constantly, or if the filter feeds an ice maker as well as a tap. Adjust up only if the unit genuinely sits unused — and even then, see the next point.
An idle filter is not a fresh filter
A wet cartridge that has sat unused for weeks is a damp, room-temperature environment, which is why labels give a time limit alongside the volume. After a holiday, run the tap for a while before drinking, and if a filter has been standing far past its date, replace it rather than trying to flush it back to life.
Signs you are already overdue
- Noticeably slower flow, or a pitcher that takes much longer to drain than when the cartridge was new.
- Taste or smell returning — chlorine, metallic or musty notes.
- Visible particles, or cloudiness that does not clear when the glass settles.
- Fine grey carbon dust that never stops appearing, well past the initial flush.
Waiting for these is the wrong strategy. A carbon filter that is past its capacity does not fail loudly; it quietly stops removing what it used to remove, and taste is the last thing to change. Note also that a standard carbon filter is for taste, odour and certain contaminants — it is not a substitute for boiling or a treatment device if your supply is known to be unsafe. If you have real concerns about your water, have it tested.
A routine that survives
- Write the install date on the cartridge itself with a marker, or on a strip of tape on the housing.
- Keep one spare in the cupboard, so the replacement takes two minutes rather than a shopping trip.
- Flush a new cartridge as the instructions say — usually a few minutes or a couple of discarded fills.
- Set a reminder the day you install it, not the day you notice the taste.
Agoly
A filter change every six months is the classic task nobody remembers: too rare for a habit tracker, not urgent enough to survive on a to-do list, and by the time you think about it you no longer know the date. Agoly shows the days since you last changed each filter and tells you when the interval you set has passed. Keep one task per filter, since the fridge and the shower head do not share a schedule, and change any interval later without losing the record. The history shows the real average gap between replacements. Reminders, the home screen widget and the history are all in the free app, with no account and nothing leaving your phone.