Best Water Filter Water filtration, tested, scored and explained.

How do water filters work?

Water filters use six mechanisms: mechanical filtration strains out particles, activated carbon adsorbs chlorine and organic chemicals, ion exchange swaps dissolved ions such as hardness minerals, reverse osmosis forces water through a membrane fine enough to reject almost everything dissolved, UV sterilises organisms without removing anything, and distillation boils water away from its contaminants.

No single mechanism removes everything, which is why most systems combine several in a fixed order — and why the right filter depends entirely on what is actually in your water.

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The six mechanisms

Filters are usually sold by shape — pitcher, under-sink, whole-house — but shape tells you where it goes, not what it does. What a filter removes is decided by the mechanism inside it.

Mechanical filtration

A physical barrier with holes of a known size. Anything larger than the pore is caught; anything smaller passes.

Removes
Sand, silt, rust flakes, scale fragments, and — at fine enough ratings — cysts like Giardia and Cryptosporidium.
Misses
Everything dissolved. Chlorine, fluoride, lead, nitrates and hardness all pass straight through, because they are not particles.
Where you find it
The first stage of almost every multi-stage system, where its real job is protecting the more expensive stages behind it.

Activated carbon adsorption

Carbon is processed to open up a vast internal surface area — a single gram can carry over 1,000 square metres of it. Contaminant molecules stick to that surface as water passes. With chlorine there is also a chemical step: the carbon converts it to harmless chloride.

Removes
Chlorine, chloramine, VOCs, pesticides, herbicides, and the compounds behind bad taste and smell. Carbon block designs with lead-specific media are also certified for lead.
Misses
Dissolved minerals, salts, nitrates, fluoride, hardness. Small highly-soluble ions have nothing to grip.
Where you find it
The workhorse. Pitchers, faucet filters, refrigerator filters, whole-house systems, and both the pre- and post-stages of a reverse osmosis system.

Ion exchange

Water passes over resin beads loaded with one kind of ion, which trade places with the ions in the water.

Removes
Hardness — calcium and magnesium swapped for sodium or potassium — and, with the right resin, lead, nitrates or radium.
Misses
Anything not carrying a charge. Bacteria, most organic chemicals, sediment.
Where you find it
Water softeners, deionising cartridges, and the lead-reduction media inside some carbon blocks.

Reverse osmosis

Household water pressure pushes water through a semipermeable membrane with openings around 0.0001 microns. Water molecules pass; almost everything larger is rejected and flushed to the drain.

Removes
The broadest range of anything on this list — 90–99% of dissolved solids, including fluoride, nitrates, arsenic, lead, chromium, PFAS and salt.
Misses
Some dissolved gases, and volatile organics that pass through the membrane — which is why RO systems always finish with a carbon stage.
Where you find it
Under-sink drinking water systems, countertop units, and whole-house installations where the water quality justifies the cost.

UV disinfection

Water flows past an ultraviolet lamp. The UV damages the DNA of any organism in the water so it cannot reproduce.

Removes
Bacteria, viruses and protozoa — including Cryptosporidium, which resists chlorine.
Misses
Everything else, entirely. UV removes nothing. Chemicals, metals, sediment and taste all pass through unchanged, and cloudy water shields organisms from the lamp, so UV needs clear water to work at all.
Where you find it
The last stage on well water systems, after filtration has cleared the water enough for the light to reach through.

Distillation

Water is boiled to steam and the steam is condensed back to liquid. Anything that does not boil at the same temperature is left behind.

Removes
Essentially all dissolved solids, metals, fluoride, nitrates, salts, and organisms.
Misses
Volatile organic compounds that boil below water and travel with the steam, unless the unit includes a carbon stage.
Where you find it
Countertop distillers. Thorough, slow, electricity-hungry, and the water comes out flat because the minerals that give it taste are gone.

What a micron rating means

A micron is a thousandth of a millimetre. When a filter is rated at 5 microns, particles larger than that are caught and smaller ones pass. The number is meaningless on its own, so here is what it is next to:

Thing Size
Human hair 70 microns
Sand grain (fine) 50 microns
Giardia cyst 8–12 microns
Cryptosporidium cyst 4–6 microns
Typical sediment filter 5–50 microns
Bacteria 0.2–10 microns
Carbon block filter 0.5–10 microns
Ultrafiltration membrane 0.01–0.1 microns
Virus 0.004–0.1 microns
Reverse osmosis membrane 0.0001 microns

Two things follow from that table. A 5-micron sediment filter stops sand and Giardia but not most bacteria. And nothing on the mechanical side gets anywhere near dissolved contaminants — that is the gap reverse osmosis exists to fill, four orders of magnitude further down.

One caveat on the numbers. Nominal means the filter catches most particles at that size; absolute means it catches essentially all of them. For taste and grit the difference does not matter. For cysts it decides whether the filter works.

Why systems have stages

A three-stage under-sink system is not three filters doing the same job harder. Each stage covers what the previous one misses, and the order is engineering, not marketing:

  1. Sediment first. Particles would clog the carbon within weeks. A cheap cartridge protects an expensive one.
  2. Carbon second. It removes chlorine and organics — and chlorine is what destroys a reverse osmosis membrane, so on an RO system this stage is what keeps the membrane alive.
  3. The membrane third, now receiving water that is clear and chlorine-free, which is the only condition it can work in for long.
  4. Carbon again last. A polishing stage on the way to the tap, removing anything picked up in the storage tank.

This is also why stage count is a weak signal of quality on its own. Three stages is normal for a whole-house system and thin for reverse osmosis, which is why it carries only 10% of the score in how we rate.

How to tell whether a filter does what it claims

Any manufacturer can print "removes 99% of contaminants" on a box. The claim means nothing without naming which contaminants and who verified it.

Independent certification is the only part of a filtration claim someone else has checked. The standards to look for are NSF/ANSI 42 (taste, odour, chlorine, particulates), 53 (health contaminants — lead, cysts, VOCs), 58 (reverse osmosis systems) and 401 (emerging contaminants such as pharmaceuticals). Certification bodies include NSF International, the Water Quality Association and IAPMO.

The detail that catches people out: certification is granted contaminant by contaminant. A filter certified to NSF/ANSI 53 for lead has been tested for lead — not for cysts, not for VOCs, and not for fluoride. Read which claims the certificate names, not just the standard number.

Common questions

What is the difference between a water filter and a water purifier?

A filter reduces contaminants. A purifier is held to a microbiological standard — under NSF/ANSI P231 it must remove 99.9999% of bacteria, 99.99% of viruses and 99.9% of protozoan cysts. Every purifier is a filter; most filters are not purifiers.

Do water filters remove minerals?

It depends on the method. Carbon filters leave minerals alone. Reverse osmosis and distillation remove nearly all of them, which is why some systems add a remineralisation stage — mostly for taste, since drinking water is a minor source of dietary minerals compared with food.

Why do water filters have multiple stages?

Because no single mechanism removes everything, and because the order protects the expensive parts. Sediment goes first so particles do not clog the carbon. Carbon goes next, partly to remove chlorine, which would destroy a reverse osmosis membrane. The membrane goes third, and a final carbon stage polishes the taste.

How often do water filters need replacing?

By volume, not by appearance. A carbon filter looks the same when exhausted as when new, and once its surface is saturated it stops adsorbing — it can even release what it has captured. Typical intervals: pitcher filters every 40 gallons or two months, under-sink carbon every 6–12 months, RO membranes every 2–3 years, sediment pre-filters every 3–6 months.

What do the NSF numbers mean?

They are the standard the filter was tested against. NSF/ANSI 42 covers aesthetic effects — chlorine, taste, odour, particulates. 53 covers health contaminants such as lead, cysts and VOCs. 58 covers reverse osmosis systems. 401 covers emerging contaminants like pharmaceuticals. Certification is granted per contaminant, so always check which claims a certificate actually names.

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