A micron chart is a useful orientation and a poor specification. It tells you roughly what size range a particle falls into and therefore what rating might stop it, and it leaves out the two facts that actually determine whether a filter does what you hoped: whether the rating is nominal or absolute, and whether the mechanism is straining at all.
What a chart is telling you
Charts arrange familiar things by size: sand and grit at the coarse end, then silt, then the range where protozoan cysts and bacteria sit, then viruses and dissolved material at the fine end. That ordering is genuinely useful for setting expectations, because it shows immediately that a coarse cartridge cannot stop an organism and that no straining filter of any rating stops something dissolved. Use it to rule options out, which is what it does well.
Nominal against absolute, which charts rarely show
Two cartridges printed with the same figure can perform quite differently. A nominal rating means most particles at that size are captured and some pass. An absolute rating means essentially all are, and it is the only kind that supports a specific claim such as cyst reduction. A chart that maps a contaminant to a micron figure implies that any cartridge at that figure will do, and it will not. Check which type of rating the product states, and look the model up in a certifier's listing where a health claim is involved.
Straining is not the only mechanism
Charts describe mechanical filtration, and a great deal of household treatment does not work that way. Carbon adsorbs molecules onto a surface, so its performance depends on contact time rather than pore size. Ion exchange swaps dissolved ions. Membranes reject dissolved material. Ultraviolet inactivates organisms without removing anything. A chart cannot represent any of those, which is why reading one and concluding that a fine cartridge covers everything is a common and expensive mistake.
Using it to stage a system
Where a chart earns its place is in staging. It shows why a coarse cartridge belongs ahead of a fine one, since most of the mass in gritty water sits in the large particles and the fine element lasts many times longer for having them removed cheaply. Match the coarse stage to what the water actually carries and the fine stage to what the next component needs, rather than fitting the finest available everywhere. Our well filtration guide covers ordering the stages.
Questions people ask about micron filtration chart
Does a micron chart tell me which filter to buy?
Only roughly. It does not show whether a rating is nominal or absolute, which is what decides whether a specific claim holds.
What is the difference between nominal and absolute?
Nominal captures most particles at the rated size and lets some through; absolute captures essentially all of them.
Can a fine enough filter remove dissolved contaminants?
No. Straining cannot remove anything dissolved, whatever the rating. That needs a membrane, ion exchange or adsorption.
How should I use a chart in practice?
To stage a system: a coarse cartridge first to take the bulk of the load, then a finer element that lasts far longer as a result.