Replacing your water filter: when and how often

Reading time: about 12 minutes

How often should you replace a water filter?

The question sounds simple, but it is put the wrong way round. "Water filter" is an umbrella term for at least five different things, and they have nothing in common except that water runs through them. On the first page of search results in September 2026, almost everything is about fridges and coffee machines: there Google quotes a range of 2 to 6 months. That figure is correct for those appliances, but completely useless if you have a reverse osmosis system under your sink.

Two things determine when a filter is spent. The first is the amount of water that has passed through it: a filter is a sponge with a finite absorption capacity, and at 8 litres a day it fills up twice as fast as at 4 litres. The second is the quality of the incoming water. If you live in an area where the mains water sits at around 8 °dH, the figure Waternet gives as the Dutch average and where water is only called truly hard from 12 °dH, a pre-filter will last longer than in an area at the top of the legal range.

That is why every serious manufacturer works with two figures side by side: an interval in months and a volume in litres. Whichever is reached first counts. It is precisely that second figure that is missing from most blogs, and it is the figure that determines your own situation.

The interval per filter type

The complete table is below. Look up what is in your cupboard or in the appliance, not the name of the product it was once sold under.

Why the membrane lasts so much longer

The factor of five between a carbon filter and a membrane surprises many people, but it makes sense. A carbon filter works by adsorption: substances attach themselves to the carbon surface and stay there until that surface is full. A membrane works with pressure and flushes most of what it holds back straight to the drain. So it becomes saturated far more slowly. The performance standard that exists for this, NSF/ANSI 58, describes exactly this operating principle for domestic reverse osmosis systems.

That is precisely why a complete system always has several stages in series, each with its own interval. If you want to see how that sequence fits together, our overview of which filter stages a reverse osmosis system contains sets out the full build-up. And the fact that the carbon after the membrane does something different from the carbon before it is covered in our article on what activated carbon does and does not take out of your water.

The filter in your coffee machine and your boiling water tap

This is the category most search queries are about, and where the intervals vary most widely. The machine manufacturers themselves set out differing limits, measured on the first page of search results in September 2026: one quotes 90 days or 40 litres, another a minimum of two months or 220 litres. That difference comes from the size of the cartridge, not from its quality. So follow the manual for your machine and not the tip on a sales site. What such a cartridge does and does not solve is set out in our article on the cartridge that belongs in your coffee machine.

A boiling water tap often has a comparable cartridge in the feed to the boiler. Worth knowing: such a filter limits limescale and taste, but it does not produce osmosis water. A boiling water tap and a reverse osmosis system are two different things, and that difference also determines what you may expect from the replacement.

The four signals that warn you sooner than the calendar

A filter does not stop working on an agreed date. It gradually gets worse, and that shows in four ways. Anyone who knows these four never has to guess again.

1. The taste changes

This is the first and the most reliable signal, because your own taste buds notice a shift sooner than any meter. If a chlorine taste returns, the carbon filter is saturated. If the water tastes flat or stale instead, it is often the post-filter or a period of standing still. Test the simple thing first: run the tap for a minute. Vitens advises, in the case of an unusual smell or taste, to open several taps at once for a few minutes. If the complaint remains after that, it is the filter.

2. The flow rate drops

If filling a carafe takes noticeably longer than six months ago, there is resistance somewhere. With a pre-filter that is almost always sediment clogging the cartridge. Note: pressure that is too low at the inlet gives exactly the same picture while the filter is still perfectly fine. Our article on how many bar a filter needs explains how to tell those two apart before you buy a new cartridge for nothing.

3. The TDS reading rises

This is the only hard measurement you can take yourself at home, and it is the designated signal for the membrane. With a TDS meter, measure your tap water first and then your filtered water, and note both values right after installation. If the filtered value climbs steadily towards the tap value over the following months, the membrane is letting more through. The background document from the WHO on total dissolved solids is useful context here: there is no health-based guideline value for TDS, and in the taste panel below 300 milligrams per litre counts as excellent and 900 to 1,200 as poor. So TDS says something about taste and about the condition of your membrane, not about safety. How to read those measurements exactly is set out in our article on what the ppm value on such a meter means.

4. The number of litres drawn

The unglamorous but most objective signal. A family of four drinking 6 litres a day and cooking with filtered water comes to a good 2,100 litres a year. A two-person household that only drinks it sits at around 1,100 litres. With a filter rated at 2,000 litres, that is nearly a year of difference in the replacement interval. Write the installation date on the housing with a permanent marker; that is the cheapest bookkeeping there is.

What happens if you wait too long

A filter past its date does not stop filtering; it first becomes less effective and then a problem in its own right. That last point is the one most web shops skip.

Water utility Vitens is explicit about it in its explanation of water filters and softeners: bacteria can grow on a filter, and the company advises using only appliances with a quality declaration recognised by the Dutch state, with proper backflow protection, and having the installation comply with the principles of NEN 1006. Waternet says something comparable about the carbon filter in home filters. In short: a saturated carbon filter is a warm, damp environment with organic material in it, and that is precisely not a place you want to run your drinking water past.

Dutch mains water is also of high quality before it enters your filter. According to Vitens, around 200,000 samples are taken each year from source to tap, about 750 a day in the lab, and 24 to 48 hours pass between production and delivery. The RIVM oversees the statutory quality requirements. In the Netherlands you therefore filter to add something, such as removing PFAS, limescale and taste compounds, not to repair something. That is exactly the reason to replace a filter on time: you lose the benefit as soon as the cartridge is full, and you get a risk back in return.

Replacing a water filter: the step-by-step plan

On an under-sink system this is ten to fifteen minutes of work, and you need no tools beyond a bucket and a cloth. The order does matter.

1. Close the water supply. That is the small valve on the branch to the system, usually right next to the main tap under the sink.
2. Release the pressure. Open the filtered tap until nothing more comes out. Skip this and you will spray the bottom of the cupboard at the next step.
3. Have a bucket and a cloth ready. There is always half a litre of water left in a filter housing, and that water is brown.
4. Change one cartridge at a time. Note the order or take a photo. Watch the arrow on the cartridge: it indicates the flow direction, and fitted the wrong way round a filter does not work.
5. Tighten by hand. With bayonet and screw fittings, hand force is enough. Over-tightening deforms the rubber sealing ring and actually causes leaks.
6. Flush through. Open the supply and let the first 3 to 10 litres run away until the water is clear and the air bubbles are gone. With a new carbon filter the first water may look slightly grey from carbon dust; that is normal and disappears within a few litres.
7. Check for leaks and note the date. Look under the sink again after half a day, and write the date on the housing or in your phone.

If you have doubts about the membrane or the connections, have it done. At PureAqua the installation is carried out by our own specialist, and anyone doing it themselves will find the full explanation, including the requirements for the drain and the protection, in our article on what is involved in connecting it under the sink.

What does filter replacement cost per year?

This is the item people forget when buying and suddenly run into a year later. Work it out in advance and there will be no surprise later.

At PureAqua a filter set for The Source costs 149 euros in the three-part version and 75 euros in the two-part set for The Source 50; a mineral filter costs 45 euros. Those are the amounts from our product snapshot; always check the current price on the product page itself. With one filter set a year, that works out at roughly 12 to 16 euros a month in consumables, or 29 euros a month if you take everything including the tap, system, installation and filters through the subscription.

Set the alternative beside it. A family buying 2 litres of bottled water a day spends 365 euros a year at 0.50 euros per litre, plus the lugging and the plastic. Tap water itself costs almost nothing: Vewin quotes a 2026 drinking water rate of 2.71 euros per cubic metre including taxes. A thousand litres of filtered drinking water therefore costs you less than 3 euros in mains water; the rest of the bill is the filter. What a litre of osmosis water costs all in has been calculated separately for anyone who wants to make the comparison with buying.

What is already included in a subscription

The main reason filters get replaced too late is not the price but forgetting. A cartridge under your sink asks for no attention, gives no error message and looks exactly the same from the outside after three years as it did on day one. That is why the subscription exists.

The PureAqua Carefree Pure Water subscription includes a pure water tap of your choice, the filter system, a filter set for twelve months and installation by a specialist, for 29 euros a month with 299 euros in start-up costs. So you do not have to keep track of a date yourself or order a separate set. If you would rather buy outright, take a pure water tap with the system included, or the standalone pure water appliance at 495 euros that sits on the worktop without installation and where you change the filters on time yourself.

Both routes lead to the same water. The difference is who keeps the calendar. If you would first like to understand what happens inside the system before choosing, our page on how reverse osmosis works explains step by step which filter does its job in which order.

Frequently asked questions

Conclusion

The question "how often should I replace my water filter" has no single answer, but it does have a simple rule: replace the pre-filters on the calendar and the membrane on the measurement. Pre-filters every 6 to 12 months, post-filter and mineral filter annually, the membrane only when the TDS reading rises steadily. The cartridge in a coffee machine or fridge follows the manual for the appliance, somewhere between 2 and 6 months.

Two things make the difference between a system that keeps working well for years and one that slowly slides. The first is writing down the installation date, because no filter tells you how old it is. The second is that one TDS measurement right after installation, because without a baseline you will never know later whether your membrane has lost anything. Anyone who would rather not keep track of either chooses the subscription with the filter set already included. Either way: pure water is not a one-off purchase but a small amount of maintenance. Drink different.