Form factor is not just a convenience choice. It sets a ceiling on what the filter can do, because contaminant reduction depends on contact time between water and medium — and the amount of medium a pitcher cartridge can hold is a fraction of what an under-sink housing holds.
That is why the claim lists get longer as you move down this page, and why a pitcher certified for lead is a more impressive piece of engineering than a under-sink unit certified for the same thing.
The media, because that is what is actually doing the work
Before the form factor, know what is inside. Four media cover almost everything in domestic filtration.
Granular activated carbon (GAC). Loose carbon granules. Water finds paths of least resistance between them, so contact is uneven, but flow is fast and pressure drop is low. Good for chlorine, taste and odor. Less reliable for contaminants needing consistent contact.
Carbon block. Carbon powder compressed with a binder into a solid block, so all water must pass through the matrix rather than around granules. Contact is far more uniform, and the block also acts as a mechanical filter with a rated pore size — which is how carbon filters achieve cyst reduction claims. Higher pressure drop, slower flow, and the medium behind most lead and PFAS claims in carbon filters.
Anion exchange resin. Swaps unwanted anions in the water for ones held on the resin. One of the two carbon-free routes to a PFAS claim under NSF/ANSI 53, and used for specific inorganics.
Semi-permeable membrane (reverse osmosis). Pressure forces water through a membrane that rejects dissolved ions. This is the only domestic technology that removes dissolved inorganics at scale — nitrate, arsenic, fluoride, total dissolved solids — and it is certified under NSF/ANSI 58.
A general rule that saves a lot of confusion: carbon removes things that stick to carbon; membranes remove things that are dissolved. Chlorine, VOCs, many organics, and with the right block, lead and PFAS, are carbon work. Nitrate, arsenic and fluoride are membrane work. Neither does the other’s job.
Pitcher and dispenser
How it works: gravity pulls water through a small cartridge, typically GAC with an ion-exchange component.
The constraint: a cartridge the size of a drinks can, and gravity rather than mains pressure. Contact time is short, so claim lists are usually the shortest in the category — chlorine taste and odor under NSF/ANSI 42, sometimes with a few NSF/ANSI 53 claims on better models.
Choose it if: you rent, you only treat drinking water, your water quality report shows nothing serious, and taste is the complaint.
Watch for: cartridge cost per gallon, which is the highest of any form. A pitcher is cheap to buy and expensive to run.
Faucet-mounted
How it works: screws onto the tap outlet, with a diverter to switch between filtered and unfiltered. Usually a small carbon block.
The advantage over a pitcher: mains pressure through a carbon block rather than gravity through granules, so claim lists are often noticeably longer for a similar price — lead and cyst claims are common.
Choose it if: you want NSF/ANSI 53 claims without plumbing work, and you are filtering one tap.
Watch for: thread compatibility — pull-out and sprayer faucets usually will not take one. Flow is modest, and the unit is bulky in a small sink.
Countertop gravity and countertop connected
How it works: either a large gravity vessel with substantial ceramic or carbon elements, or a housing connected to the tap by a hose.
The advantage: physical space. A gravity countertop unit can hold far more medium than a pitcher, and long contact times are achieved by letting water take its time.
Choose it if: you want capability without plumbing, and you have counter space.
Watch for: certification specifically. This category contains both properly certified systems and uncertified products making broad claims. Check the element’s listing, not the marketing.
Under-sink carbon
How it works: a cartridge in a housing plumbed into the cold supply, feeding either a dedicated tap or the existing one.
The advantage: a large carbon block at mains pressure. Long claim lists, good flow, and a cartridge life measured in hundreds or thousands of gallons rather than tens — which makes the cost per gallon the lowest of the carbon options.
Choose it if: your contaminants are carbon-addressable and you can fit it. For most households on a treated municipal supply, this is the best capability-per-dollar point in the category.
Watch for: rated flow. Plumbed to a full-pressure tap, a filter can be pushed past its rated flow, which reduces contact time and therefore reduces reduction below the certified figure. Check the rated flow against your tap.
Under-sink reverse osmosis
How it works: sediment and carbon pre-filters, an RO membrane, a storage tank, and usually a carbon post-filter, on a dedicated tap.
The advantage: the broadest reduction available domestically, and the only practical route to nitrate, arsenic, fluoride and high TDS.
Choose it if: your water quality report or well test shows dissolved inorganics. If it does not, RO is usually more system than the problem requires.
Watch for: wastewater, slow production, a tank under the sink, a drain connection, and more cartridges to replace on different schedules. The full trade-off is here.
Whole-house (point of entry)
How it works: large housings on the incoming main, typically a sediment stage then a carbon stage.
The advantage: every tap, shower and appliance gets treated water. Sediment and chlorine removal for the whole building, which protects fixtures and appliances as much as it improves the water.
The limitation: whole-house flow rates are high, so contact time per gallon is low. These systems are specified for coarse work, not for fine drinking-water claims. A point-of-entry carbon filter is not a substitute for a certified point-of-use filter at the kitchen tap.
Choose it if: you have visible sediment, strong chlorine throughout the house, or appliances suffering. Then add a point-of-use filter for drinking water.
Ultraviolet and distillation
Ultraviolet inactivates organisms and does nothing chemical. Certified under NSF/ANSI 55, where Class A inactivates bacteria, viruses and cysts in contaminated water and Class B reduces non-disease-causing bacteria in water that is already disinfected. It needs clear water to work, so it sits downstream of a sediment filter, and it belongs on an untreated source such as a well rather than on a chlorinated municipal supply.
Distillation, under NSF/ANSI 62, boils and condenses. Excellent on heavy metals — and with an explicit caveat in the standard’s own scope: some volatile organic chemicals can pass through with the vapor. Slow, energy-hungry, and the opposite of the common assumption that distilled water is free of everything.
Side by side
| Form | Typical medium | Claim breadth | Cost per gallon | Dissolved inorganics? |
|---|---|---|---|---|
| Pitcher | GAC + ion exchange | Narrow | Highest | No |
| Faucet-mounted | Small carbon block | Moderate | High | No |
| Countertop | Large carbon or ceramic | Moderate to broad | Moderate | No |
| Under-sink carbon | Large carbon block | Broad | Low | No |
| Under-sink RO | Membrane + carbon stages | Broadest | Moderate | Yes |
| Whole-house | Sediment + bulk carbon | Coarse only | Lowest | No |
| UV | None — irradiation | Microbiological only | Low | No |
| Distiller | None — phase change | Metals; some VOCs pass | High (energy) | Yes |
The arrangement that suits most households
For a treated municipal supply with no dissolved inorganic problem: an under-sink carbon block at the kitchen tap, certified under NSF/ANSI 53 for the contaminants your water quality report and your own lead test identified.
Add a whole-house sediment and carbon stage if the water arrives visibly dirty or strongly chlorinated. Step up to RO only if nitrate, arsenic, fluoride or TDS appears in your results — those are the cases carbon cannot address, and they are the only reason to accept RO’s wastewater and complexity.
Related guides
- How to choose a water filter, starting from your own water
- NSF/ANSI 42 vs 53 vs 401
- Reverse osmosis: what NSF/ANSI 58 covers
Sources: NSF, Standards for Water Treatment Systems, for the scope of NSF/ANSI 42, 44, 53, 55, 58, 62, 177, 244 and 401 including the UV classes and the distillation VOC caveat; NSF, NSF/ANSI 42, 53 and 401; NSF, Forever Chemicals and the Advancement of Filtration Standards, for activated carbon, anion exchange and RO as the common PFAS technologies. Cost-per-gallon and claim-breadth columns are relative characterizations of each form factor, not measured figures.