How long do water filters last — and when to change yours

A jug water filter cartridge typically lasts about four weeks – or a defined litre capacity, whichever comes first. One distinction helps from the start: the cartridge is the working part that does the filtering; the jug is simply the vessel around it. When people talk about water filter replacement, it is almost always the cartridge they mean – a water filter cartridge replacement takes under a minute, and the jug itself stays in service for years.

The time limit is not arbitrary. Inside every cartridge sits an activated carbon filter paired with ion-exchange material. Activated carbon works by adsorption: an enormous internal surface area captures chlorine, certain impurities and taste-affecting substances as water passes through. That surface is finite. With every litre filtered, more binding sites are occupied, until the carbon is saturated and filtration performance declines — gradually and invisibly. An expired cartridge does not make water unsafe; it simply stops improving it.

Four weeks is a working average, not a law of nature. Water hardness and daily volume shift the real interval: in hard-water areas, or in households filtering many litres a day, the capacity is reached sooner. The same logic applies at every scale – household water filter systems plumbed into the mains also run on replaceable media with defined capacities; a jug simply makes the exchange something you can do at the kitchen counter.

Hard water is also the reason many households filter in the first place – the full picture of what limescale does, and does not do, is covered in the article Is limescale bad for you? What filtering really changes.

When exactly should you change it?