Life is a mass, not a duration
Every published cartridge life is a fiction until it meets a particular water supply. What the cartridge actually has is a dirt holding capacity — a mass of solids it can retain before the pressure loss becomes unacceptable — and how long that lasts is arithmetic:
Days = capacity in grams ÷ (litres per day × mg/L ÷ 1000)
A household putting 300 gallons a day through a housing is passing about 1,136 litres. At one milligram per litre of suspended solids that is 1.14 grams a day. A cartridge holding 60 grams fills in about 53 days. Double the solids and it halves; halve the water use and it doubles. There is nothing else in it.
What makes the answer feel unpredictable is that nobody knows their solids figure. It is not on a utility water quality report in a useful form and it is not on a home test strip, and on a well it varies through the year — it climbs after heavy rain, after the pump has been off for a while, and dramatically if the well is drawing from a formation that produces fines. So the honest version of this calculation for most households runs the other way, from the change history.
Running it backwards from your own history
If the last cartridge lasted 45 days in a 60 gram housing on 300 gallons a day, then it was catching 60 ÷ 45 = 1.33 grams a day, which back-converts to about 1.17 mg/L. That number is worth more than a laboratory figure for this specific purpose, because it includes everything that actually shortened the cartridge life rather than only the fraction a solids test measures — biological film, iron that oxidised in the housing, a cartridge that was slowly collapsing.
It also gives you something to reason with. Once you have a loading rate for your water, you can ask what a 90 gram cartridge would give you, or two in parallel, or what happens if the household grows, without changing anything and waiting to find out. Re-derive it each time you change one and it converges on something reliable.
The teaching point: a finer micron rating does not shorten cartridge life, it lengthens the water heater life
The most common reaction to a cartridge that fills quickly is to fit a coarser one. That does work, in the narrow sense that the cartridge lasts longer, and it works for exactly one reason: more of the sediment is now going past it into the house.
The capacity of a cartridge is a property of the cartridge, not of the micron rating in isolation. Going from 20 micron to 5 micron does not reduce the grams it can hold. It increases the fraction of the incoming solids stream that gets captured, so the same 60 grams accumulate faster. The material that used to pass through is now in the cartridge instead of in the bottom of the water heater, in the aerators, and in the seats of every valve in the house.
So a shortened interval after fitting a finer cartridge is the filter doing its job. The right response, if the interval is genuinely inconvenient, is more capacity rather than less filtration: a bigger housing, a longer cartridge, or two in parallel. The wrong response is to reason from the interval alone.
| Change | Effect on interval | Effect on pressure loss |
|---|---|---|
| Coarser cartridge, same housing | Longer | Lower |
| Finer cartridge, same housing | Shorter | Higher |
| Longer cartridge, bigger housing | Longer roughly in proportion | Lower |
| Two housings in parallel | Roughly doubled | Roughly quartered |
| Pleated instead of spun, same size | Longer | Lower at the same rating |
The last row is the one that gets overlooked. A pleated cartridge folds several times the surface area into the same envelope, so at a given micron rating it holds more and loses less pressure. What it does not do as well is depth filtration, so a spun or melt-blown cartridge sometimes still wins on water carrying a wide spread of particle sizes.
Two housings in parallel, and why it is the best value fix
Splitting the flow between two identical housings doubles the dirt capacity, which is the obvious half. The less obvious half is the pressure. Loss through a cartridge scales close to the square of the flow through it, so halving the flow through each housing cuts the loss through each to about a quarter. You get twice the interval and most of your pressure back in one move.
That interaction is worked out in the whole house filter flow rate calculator, which takes the peak simultaneous flow of the house and scales each device loss to it. It is worth running before deciding that a filter is the reason the upstairs shower is disappointing.
Where sediment filtration sits in a train
Sediment comes first, because everything downstream is damaged by it. A softener resin bed fouls with silt, a carbon bed channels around it, and a UV sleeve gets shaded by it — that last one matters more than people expect, since a UV chamber depends entirely on light reaching the water. The UV dose calculator shows how quickly sleeve transmission and water clarity move the dose.
What a sediment cartridge does not do is remove anything dissolved. Iron in solution passes straight through it and only appears once it has oxidised somewhere downstream, which is why iron problems so often look like sediment problems arriving at the wrong place. Hardness likewise passes; that is the softener question and the arithmetic there is grains rather than grams. Every water quality number on this page — hardness, iron, solids, transmittance, a concentration, a target dose — is something you enter from a laboratory test of your own water. Use a state-certified laboratory; your state or county health department maintains the list and will usually tell you what a well in your area is commonly tested for. This calculator has no idea what is in your water, will not tell you whether a result is acceptable, and does not select equipment. Treatment follows the report, and a device that removes one thing does not remove another.
Questions people ask
How often should I change my whole house sediment filter?
When it has caught as much material as it can hold, which is a mass divided by a rate rather than a fixed number of months. A 60 gram cartridge on water carrying 1 mg/L of solids, in a house using 300 gallons a day, fills in around 53 days. The same cartridge on cleaner water might go six months, and on a well producing fines after heavy rain it might go three weeks. If you have no solids figure, use your own change history: the interval the last one gave you is a measurement of your water, and it is a better one than any general answer.
Why did my filter cartridge last three months last year and six weeks this year?
Something about the water changed rather than something about the cartridge. On a well the usual candidates are seasonal: heavy rain moving fines through a formation, a falling water table drawing from a different level, or a pump that has started stirring sediment from the bottom of the casing. A sudden change is worth taking seriously rather than absorbing by buying more cartridges, because a well that has begun producing sand is telling you something about the well. That is a conversation with a licensed well contractor, and any change in water quality is a reason to have it tested by a certified laboratory.
Does a 5 micron cartridge last less time than a 20 micron one?
Yes, on the same water, and that is the filter working rather than failing. The dirt holding capacity is a property of the cartridge and does not change with the rating; what changes is the fraction of the incoming solids the cartridge captures rather than passes. A finer cartridge takes out material that a coarser one lets through to the water heater, the aerators and the valve seats. If the shorter interval is genuinely a problem, add capacity — a bigger housing or a second in parallel — rather than going coarser and calling it solved.
Should I change on pressure drop instead of on a schedule?
Pressure is the more direct trigger and it is what most installations with a gauge either side actually use. It tells you the real state of the cartridge including things a solids calculation cannot see, such as biological film or a cartridge that has partly collapsed. The value of running the arithmetic alongside it is that it tells you what to expect, so a cartridge whose loss climbs far sooner than predicted is a signal that something other than ordinary sediment is happening. Use whichever comes first, and treat a large disagreement between the two as information.
Can I clean and reuse a sediment cartridge?
A pleated cartridge is sometimes rinsed and put back, and it does recover some flow. What it does not recover is the material driven into the depth of the medium, and every rinse leaves the pleat pack a little more damaged and a little more likely to bypass. Washable screen filters and spin-down separators are a different product designed for exactly that duty and they are the right answer where flushing is the plan. For a disposable cartridge the honest position is that rinsing buys some time at some cost in performance, and the manufacturer instructions are what govern whether it is intended to be done at all.