Water filters are the clearest example of a product where the failure is invisible. A saturated carbon cartridge passes water at the same rate as a new one, and the water tastes about the same, because taste improvement is the first thing a filter does and the first thing it stops doing quietly. Nothing on the outside changes, which is why so many households run filters for a year and assume they are fine.
Capacity, not calendar
Almost every filter is certified by volume. A typical pitcher cartridge is rated for 150 litres, a fridge cartridge for 700, an under sink carbon block for 1,500 to 4,000. The months printed on the box are the manufacturer dividing that capacity by an assumed household. If you drink more than the assumption, your filter expires earlier.
| Filter | Rated capacity | 1 to 2 people | 4 or more people |
|---|---|---|---|
| Pitcher cartridge | 150 litres | 60 days | 30 to 40 days |
| Fridge inline filter | 700 litres | 6 months | 3 to 4 months |
| Under sink carbon block | 1,500 to 4,000 litres | 12 months | 6 to 8 months |
| Reverse osmosis prefilter | varies | 6 months | 4 months |
| Reverse osmosis membrane | varies | 2 years | 18 months |
Intervals by filter type
Pitcher filters
The shortest cycle and the easiest to neglect, because the pitcher sits in the fridge door looking the same every day. Sixty days is the right default for one or two people. If you fill it three times a day, treat it as heavy use and change it every six weeks.
Fridge filters
Six months is the standard rating. Two things change it: an ice maker, which doubles throughput invisibly, and hard water, which loads the sediment stage faster. If your fridge has both, four months is a safer interval.
Under sink and reverse osmosis
These systems have stages with different lives, and the common mistake is replacing the cheap prefilter on schedule while leaving the expensive membrane until performance collapses. Track each stage as its own item with its own date.
A filter does not tell you when it is finished. That is the whole product problem, and it is why a date beats a judgement.
Signs a cartridge is finished
- Taste drifts back. Chlorine flavour returning is the clearest signal, and it means capacity ran out some time ago.
- Flow slows. A slower pour usually means the sediment stage is loaded rather than the carbon.
- Cloudiness after standing. Often harmless air, but worth a change if it is new.
- Long idle periods. A filter left wet and unused for weeks should be replaced regardless of volume.
Hard water and well water
Hardness does not consume carbon, but scale clogs the mechanical stages, so effective life falls. In a hard water area, cut every interval above by about 30 percent. Well water adds sediment and sometimes iron, both of which shorten prefilter life dramatically. If you are on a well, measure once with a test kit rather than guessing, then set your schedule from the result.
A practical routine
Pick the date, not the volume, and let something else do the counting. Buy cartridges two at a time so a replacement is always in the cupboard, write the install date on the cartridge with a marker as a backup, and keep each stage as a separate line in your tracker.
Freshy ships pitcher filters at 60 days and fridge filters at 6 months out of the box, applies your intensity setting, and reminds you three days before the date so the replacement can arrive first. Logging the swap takes one tap and starts the next cycle.
Questions
How often should you change a pitcher filter?
About every 60 days for one or two people, roughly 150 litres. Six weeks under heavy use or hard water.
How long does a fridge filter last?
Six months on the standard rating, or four if you use the ice maker and dispenser daily.
What happens if you never change it?
Filtration effectively stops while flow and taste stay acceptable, and a saturated cartridge can host bacterial growth.
Can you rinse and reuse a cartridge?
No. Rinsing clears surface sediment but adsorption capacity is spent and cannot be restored at home.