Filtration sizing
Filter micron size chart: absolute vs nominal, and what each size stops
Quick answer
An absolute micron rating means at least 99.9 percent removal at the stated size; a nominal rating is not standardized at all and generally implies only around 85 percent removal at a threshold each manufacturer chooses. A cyst reduction claim needs 1 micron absolute, certified to NSF/ANSI 53.
A micron rating printed on a cartridge box looks like a single, comparable number, and it frequently is not one. The word next to it, absolute or nominal, changes what that number actually promises, and the two are not remotely equivalent even when the digit is identical. Two cartridges both marked "5 micron" can behave very differently at the tap, and the difference is not a manufacturing defect, it is the difference between a defined test and an undefined one.
This chart lays out the micron scale used across residential filtration, what absolute and nominal each actually mean, the real particles those sizes correspond to, and the coarse to fine staging order the trade uses on dirtier water. None of it substitutes for knowing what is actually in your water; a micron rating stops particles by size, and it has no opinion whatsoever about anything dissolved.
On this page
Absolute vs nominal: the distinction that decides whether a rating means anything
An ABSOLUTE micron rating is a defined test result: the cartridge removes at least 99.9 percent of particles at or above the stated size, under a stated challenge. A NOMINAL rating is not standardized at all. It generally implies something in the neighborhood of 85 percent removal, but the threshold, and often the test method behind it, is chosen by the manufacturer rather than by any published standard. That gap is exactly why two cartridges marked "5 micron nominal" from two different makers can pass a noticeably different amount of fine sediment, and why a nominal rating cannot be compared between brands the way an absolute one can.
This matters most for one specific claim. An NSF/ANSI 53 cyst reduction claim requires a 1 micron ABSOLUTE rating, certified for that exact model. A cartridge printed "1 micron nominal" does not make that claim and must never be treated as though it does, however close the number looks on the box. A 1 micron carbon block is a real, useful stage for fine particulate and taste, but one micron alone is not a cyst claim on its own; the certification has to name the claim specifically for that model.
The micron scale used across residential filtration
A cyst reduction claim requires 1 micron absolute specifically, not simply "1 micron" printed on the box.
Published figure A published standard, regulatory limit, unit conversion or manufacturer specification. It does not change because somebody disagrees with it.
| Micron rating | Typical use |
|---|---|
| 0.5 | Sub-micron carbon block, often a final polishing stage ahead of an under sink faucet |
| 1 | The cyst reduction threshold. Only meaningful as a cyst claim when the cartridge is rated absolute and certified to NSF/ANSI 53 |
| 5 | The most common single stage sediment or carbon block rating for city water with no visible sediment |
| 10 | A mid stage on water carrying some fine particulate, often paired ahead of a 5 or 1 micron final stage |
| 20 | A first stage on city water with fine particulate or occasional rust flakes from aging pipe |
| 30 | A coarser mid stage, less common than 20 or 50 but used on moderate well sediment |
| 50 | The usual coarse first stage on well water carrying visible sand or grit |
Rated micron size is the size a rating claims to stop, not a guarantee of how long a cartridge lasts against real sediment. The rated gallons figure printed on any cartridge is measured against a defined test challenge and behaves as an upper bound on real, dirtier water rather than a promise.
What each micron size actually stops
Comparing the micron scale against real particle sizes shows why staging from coarse to fine works, and why nothing on this scale touches anything dissolved.
Bacteria run 0.2 to 2 microns, smaller than nearly every sediment cartridge on this chart, which is why sediment filtration is not a bacteria treatment at any micron rating.
Published figure A published standard, regulatory limit, unit conversion or manufacturer specification. It does not change because somebody disagrees with it.
| Particle | Typical size |
|---|---|
| Fine sand | 50 to 100 microns |
| Silt | 2 to 50 microns |
| Human hair | 50 to 70 microns |
| Giardia cyst | 8 to 12 microns |
| Cryptosporidium cyst | 4 to 6 microns |
| Bacteria | 0.2 to 2 microns |
| Dissolved minerals | No particle size; they pass through every micron rating on this chart |
Dissolved minerals, including hardness, dissolved iron, and dissolved solids generally, are not particles at all, so no micron rating, however fine, removes them. That is a job for ion exchange or reverse osmosis, not for a sediment cartridge.
Staging sediment coarse to fine, by water source
Loading a fine cartridge straight onto visibly dirty water is the single commonest cause of a new whole house filter killing water pressure within days. The trade stages coarse to fine so each cartridge only has to catch what the one ahead of it missed.
Well water with fine silt or turbidity is staged in four steps, from 50 microns down to 1, rather than loaded directly onto a fine cartridge.
Rule of thumb Water treatment trade convention rather than a published standard. Taught everywhere, useful for planning, and not a specification. No standards body publishes it.
| Water source | Recommended staging |
|---|---|
| City water, no visible sediment | 5 micron |
| City water, fine particulate or rust flakes | 20 micron, then 5 micron |
| Well water, some sand or grit | 50 micron, then 20, then 5 |
| Well water, fine silt or turbidity | 50 micron, then 20, then 5, then 1 |
Nobody publishes this staging as a formal standard; it is trade convention built from how quickly a fine cartridge plugs when it is asked to catch coarse sediment it was never intended to load. A spin down sediment filter rated around 50 microns ahead of a finer cartridge extends the life of everything downstream of it, and a washable spin down stage never needs replacing at all.
Reading two "5 micron" cartridges the same way is not safe
Because nominal is undefined, a household comparing a 5 micron sediment cartridge against another maker's "5 micron nominal" cartridge is not necessarily comparing the same performance, only the same printed number. The only rating that carries a defined, standardized meaning is absolute, and the only place to confirm a cartridge earned it is the specific model's certification listing, not the number on the front of the box.
Frequently asked questions
What is the difference between absolute and nominal micron ratings?
Absolute is a defined test: at least 99.9 percent removal of particles at or above the stated size. Nominal is not standardized at all, generally implying around 85 percent removal at a threshold each manufacturer picks independently. Two cartridges marked the same nominal size can perform very differently, while absolute ratings can genuinely be compared between brands.
What micron rating removes Cryptosporidium and Giardia cysts?
A 1 micron absolute rating, certified specifically to NSF/ANSI 53 for a cyst reduction claim on that exact model. Cryptosporidium runs 4 to 6 microns and Giardia 8 to 12 microns, both larger than 1 micron, but the standard requires the finer absolute rating as its margin. A cartridge printed "1 micron nominal" does not carry this claim.
Does a finer micron rating always mean better filtration?
Not automatically, and it often means less flow and a shorter cartridge life against the same sediment load. A cartridge finer than the water actually needs plugs faster without removing anything more, since it is straining particles that were never present. The sediment staging chart on this page matches the rating to what the source water actually carries.
Why does my new fine sediment filter keep clogging quickly on well water?
The usual cause is loading a fine cartridge, such as 5 or 1 micron, directly onto well water carrying visible sand or grit that a coarse stage should have caught first. Staging coarse to fine, starting around 50 microns and stepping down, lets each cartridge only catch what the previous one missed, extending the life of the fine final stage considerably.
Do dissolved minerals get removed by any micron filter?
No. Dissolved minerals, including hardness and dissolved iron, have no particle size and pass through every micron rating on this chart, however fine. Removing dissolved minerals needs ion exchange, such as a water softener for hardness, or reverse osmosis, not a sediment or carbon block cartridge rated by micron size.
Is a 20 micron or 5 micron cartridge better for a first sediment stage?
For most city water with only occasional fine particulate, a single 5 micron stage is usually enough. For water carrying visible sediment, especially well water, a coarser 20 or 50 micron first stage protects a finer cartridge downstream from plugging quickly. The right first stage micron size depends on how much visible sediment the source water actually carries.
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Researched, not professional advice. This page is compiled from published standards, regulatory limits, manufacturer specifications and owner-review consensus, not hands-on testing. Nothing here diagnoses a water problem or says any product makes water safe to drink. Figures described as a rule of thumb are water treatment trade convention rather than published standards, and they are labeled that way wherever they appear. Test your water before you buy equipment: on city water start with the Consumer Confidence Report your utility publishes each year, and on a private well only a certified laboratory test tells you what you have. Bacteria, nitrate, lead, arsenic and PFAS are not do it yourself topics, they are invisible and tasteless, and they need a laboratory and usually a licensed professional rather than a product. Remember that a certification covers a specific claim under a specific standard, so "NSF certified" with no number attached tells you very little.