How often should you change a water filter cartridge? Jugs, faucet, under-sink and RO

Replacement by type of filter

Start with what kind of filter you have, then go by the gallons (or litres) or months printed on the cartridge, whichever comes first. Some typical manufacturer ratings:

Filter type	Rated by	Example
Jug / pitcher	Weeks or gallons	BRITA UK: at least every 4 weeks. Brita US Standard: 40 gallons, about 2 months. Brita US Elite: 120 gallons, about 6 months
Faucet-mounted	Gallons or months	Brita US faucet systems: 100 gallons, about 4 months
Under-sink reverse osmosis (RO)	Each stage separately	One A. O. Smith RO system: stages 1 and 3 every 365 gallons or 6 months; membrane every 730 gallons or 12 months

Around six months is a common rhythm for under-sink pre-filters. When Hong Kong’s Housing Authority arranged filter changes for public-housing tenants in 2016, it said that for most filter brands the cartridges needed changing once every six months, while another brand needed cleaning every three months and replacing within 12 months. Stages in the same system often run on different schedules, so check the manual instead of swapping only the first one.

Why “it still tastes fine” isn’t the test

Most problems with an old cartridge can’t be tasted. The CDC says any home filter has to be maintained to keep it working properly and to prevent germs from growing in it, and that includes changing it regularly according to the manufacturer’s recommendations. Brita adds that its Standard pitcher filter’s performance is not certified after 40 gallons.

It also helps to know what your filter is for. The CDC points out that pitcher filters and the filters built into refrigerators usually use activated carbon, and their main purpose is to improve taste and smell rather than the safety of the water. If you need to remove germs or a specific contaminant, check the product label for what it’s certified to remove — keeping an old cartridge in longer won’t add that.

Gallons, months and hard water

The month figures assume average use. Brita bases its timing on 11 glasses a day for an average household, so a big family or a jug that also fills the kettle and the cooking pot will reach the gallon rating well before the months are up. Brita also says that with hard water you may need to change filters more often.

So keep an eye on both numbers and change at whichever comes first. If the jug or system has an indicator — BRITA’s MEMO, for instance, shows the remaining weeks as bars — reset it every time you fit a new cartridge.

Changing a cartridge

1. Check which cartridge or stage is due. On a multi-stage system, note which ones you’re replacing.
2. Wear gloves, as the CDC recommends, and wash your hands when you’re done.
3. Follow the manual for shutting off the water, fitting the cartridge and preparing it before first use — the steps differ between jugs, faucet filters and under-sink systems.
4. Use the right replacement. A. O. Smith, for example, says to use certified, genuine replacement filters for continued contaminant removal and system performance.
5. Reset the indicator or write the date down.

Common mistakes

• Counting months but not gallons: heavy use reaches the gallon rating first.
• Changing only the first stage: RO and multi-stage systems have different schedules for each cartridge.
• Skipping the indicator reset: the light keeps counting the old cartridge.
• Expecting a jug to make unsafe water safe: carbon jug filters are mainly about taste and smell. Brita says its systems are designed for treated tap water or water from a private supply that’s been approved as safe to drink.
• Changing it bare-handed: wear gloves and wash up afterward.

How to remember

A cartridge lasts long enough that its due date is forgotten long before it arrives, and a multi-stage system adds the question of which stages were done last time. Tracking “change water filter” as a recurring item, with a note on each entry saying which stages you replaced, means you won’t have to open the housing to find out.

Kitchen upkeep tends to come in pairs: cleaning the fridge fits the same weekend. And like a filter cartridge, a toothbrush head and an air purifier filter look fine long after they should be replaced.