Five micron appears on more household water filters than any other rating, and the number by itself is less informative than it looks. It describes a particle size, and the claim attached to it varies between manufacturers in a way that matters when a specific result is being relied on rather than a general improvement.
The unit and the claim
A micron is one thousandth of a millimetre, far below what the eye resolves, and a filter rated at five microns is claiming to stop particles of about that size and larger. The important qualifier is how the claim is made. A nominal rating means most particles at the rated size are captured and some pass. An absolute rating means essentially all of them are. Two cartridges printed with the same number can therefore behave quite differently, and only absolute supports a specific claim.
What sits around that size
Sand and grit are far larger and are caught easily. Fine silt sits around and below it, so some passes. Protozoan cysts and bacteria are smaller, which is why a five micron sediment cartridge is not a barrier against organisms whatever else it does. Dissolved material has no size in this sense at all and passes regardless, which is why no rating however fine addresses hardness, dissolved metals, nitrate or salts.
Why it became the default
Because it is a good compromise. Fine enough to catch what damages appliances, softener valves and membranes and to remove the haze on many supplies. Coarse enough not to block every few weeks or to throttle a household. That balance is why it is the standard whole house sediment rating and the standard first prefilter in a reverse osmosis stack, and why most cartridge ranges are built around it.
When to move away from it
Go coarser when a five micron cartridge is blocking within weeks, which means the sediment load is too heavy for that rating as a first stage. Go finer only where the next stage or a specific claim requires it, and then check that the rating is absolute rather than nominal. And stage rather than simply choosing: a coarse cartridge first with a five micron element behind it outlasts either alone on gritty water. Our quote page covers specifying a train from a test.
Questions people ask about 5 micron
How small is five microns?
About one two hundredth of a millimetre, far below what the eye can resolve, so the rating is a claim rather than something visible.
Does five micron stop bacteria?
No. Bacteria and protozoan cysts are smaller, so a five micron sediment cartridge is not a barrier against organisms.
Why is five micron so common?
It balances catching what damages equipment against not blocking constantly, which suits both whole house and prefilter duty.
Do all five micron filters perform the same?
No. A nominal rating lets some particles at that size through; only an absolute rating supports a specific claim.