How often should you change water filter cartridges?
The short answer: sediment every three to six months, carbon every six to twelve, reverse osmosis membranes every two to three years.
The longer answer is that time is a proxy for volume, and volume is what actually exhausts a cartridge. A household of five gets through capacity roughly twice as fast as a couple, and a café gets through it in a fraction of the time either does.
By cartridge type
| Cartridge | Typical interval | The real driver |
|---|---|---|
| Sediment (pleated or spun) | 3–6 months | How dirty the supply is |
| Granular carbon | 6–12 months | Volume through it |
| Carbon block | 6–12 months | Volume through it |
| Inline fridge / coffee filter | 6 months | Manufacturer rating |
| Fridge appliance cartridge | 6 months | Usually a timer, not a measurement |
| RO pre-filters | 6–12 months | Protecting the membrane |
| RO membrane | 2–3 years | TDS rejection ratio |
| Phosphate scale inhibitor | 12 months | Dosing rate |
| UV lamp | 12 months | Output decay, not failure |
| Shower filter | 6 months or less | High flow rate |
How to tell it is due without guessing
Flow has dropped. The commonest and most reliable sign on sediment and carbon block. If the filtered tap runs noticeably slower than it did, the cartridge is loading up.
Taste has come back. Carbon exhausts gradually, and chlorine taste returning is the signal. By the time you notice, it has usually been past its best for a while.
TDS has crept up. The right tool for reverse osmosis. Compare the feed reading with the RO tap reading — when the membrane's rejection ratio starts falling, it is going. This gives you weeks of warning rather than none.
A pressure gauge. On whole-house installations, gauges either side of the housing tell you exactly how much the cartridge is restricting flow. Far better than a calendar.
What happens if you leave it too long
It stops working, which most people expect. But an exhausted carbon cartridge is not merely inert — it is a warm, damp, high-surface-area medium loaded with organic matter it has spent months collecting, and it can begin to release some of that back into the water. Bacterial growth in long-neglected cartridges is well documented.
This is the one genuinely important reason not to stretch the interval. A filter that has not been changed in three years is doing something worse than nothing.
On reverse osmosis, neglecting the cheap pre-filters ruins the expensive membrane. It is the clearest false economy in domestic water treatment.
Making yourself actually do it
Write the date on the housing in permanent marker at every change. It survives house moves, changes of mind and forgotten calendars.
Set a recurring calendar reminder the day you fit them, not the day you intend to change them.
Buy cartridges in multipacks. Having the next one already in the cupboard removes the ordering step, which is where most intervals actually slip.
If you have a service contract on a commercial system, keep the records. In a food business they form part of your food safety documentation.