How do I know which water filter to buy?

Start with a water test, not a filter. You can't choose the right filter without knowing what you're filtering. Once you have test results, match your contaminant to the NSF standard that covers it: NSF 53 for lead and cysts, NSF 58 (reverse osmosis) for PFAS/arsenic/nitrates/fluoride, NSF 42 for chlorine taste and odor. Then choose a filter format that fits your situation: pitcher, under-sink, countertop, or whole-house.

Does a Brita filter remove lead?

Standard Brita pitcher filters are certified under NSF 42 for chlorine taste and odor. They are not certified under NSF 53 for lead reduction. To filter lead with a pitcher filter, you need one certified to NSF 53 for lead, Clearly Filtered and ZeroWater both carry that certification. For higher lead levels, a reverse osmosis system or under-sink carbon block filter certified to NSF 53 is more reliable.

What filter removes PFAS from water?

Reverse osmosis systems certified to NSF 58 remove 90, 99% of PFAS (PFOA and PFOS). Activated carbon reduces PFAS but inconsistently and incompletely, it's not a reliable primary treatment for PFAS. The Clearly Filtered pitcher is certified under NSF P473 for PFOA and PFOS reduction and performs well in independent testing, but RO is the more complete solution for high-PFAS water.

Do I need a whole-house filter or a point-of-use filter?

For drinking and cooking water, a point-of-use filter at the kitchen tap (under-sink or countertop) is usually sufficient and more cost-effective. A whole-house filter makes sense if you have a contaminant that affects all uses, iron or sulfur smell from a well, for example, or chlorine that bothers you in the shower. For lead: note that a point-of-use filter only treats the tap it's connected to. If you have lead service lines, that tap should be your primary drinking and cooking source.

What is the best water filter for well water?

There's no single best filter for well water because wells vary. The right filter depends on what your well contains. Common well water issues and their treatments: iron (oxidizing filter or air injection), bacteria (UV Class A disinfection), hydrogen sulfide (aeration or oxidizing filter), hardness (water softener), nitrates (RO certified to NSF 58), arsenic (RO or activated alumina). Test first, then treat.

What Water Filter Do I Need? A Decision Guide

The question sounds simple. The answer depends entirely on what’s in your water.

Most filter marketing works backward from the product to the problem. This guide works forward: start with your specific contaminant, find the right certification standard, then pick a filter format.

Step 1: Test Before You Buy

Buying a filter without testing is like buying medicine without a diagnosis. You might get lucky. More likely, you’ll spend money on the wrong thing.

What to do:

City water: Start with your Consumer Confidence Report (CCR), your utility sends one annually. Look for contaminants above EPA limits or with asterisks. Then consider a mail-in lab test for contaminants the CCR doesn’t cover (PFAS, lead at the tap, microplastics).
Well water: Test annually at minimum. Use a certified lab. A basic well water panel should cover bacteria, nitrates, pH, hardness, iron, and manganese. Add arsenic if you’re in the Northeast, Southwest, or Midwest. Add PFAS if you’re near a military base or industrial site.

A mail-in water test from a certified lab costs $30, $200 depending on the panel. That’s less than most filters.

Step 2: Match Your Contaminant to an NSF Standard

This is the key step most buyers skip.

Contaminant	NSF Standard	Filter Types That Qualify
Chlorine taste/odor	NSF 42	Pitcher, under-sink carbon, whole-house carbon
Lead	NSF 53	Solid carbon block, reverse osmosis
Cysts (Giardia, Crypto)	NSF 53	Solid carbon block, RO, UV Class A
PFAS (PFOA, PFOS)	NSF 58 / NSF P473	Reverse osmosis, some certified pitchers
Arsenic	NSF 58	Reverse osmosis, activated alumina
Nitrates	NSF 58	Reverse osmosis
Fluoride	NSF 58	Reverse osmosis, activated alumina, bone char
Bacteria/viruses	NSF 55 Class A	UV disinfection (paired with pre-filtration)
Iron (ferrous)	None federal	Oxidizing filter, air injection, water softener
Hardness	None federal	Water softener (ion exchange)
Emerging contaminants	NSF 401	Certain activated carbon filters

A filter certified under one standard isn’t certified under others. Verify the specific contaminant in NSF’s database: info.nsf.org/Certified/DWTU/

Step 3: Choose a Filter Format

The right format depends on your contaminant, your usage, and your budget.

Pitcher Filters

Best for: chlorine taste/odor, some PFAS and lead reduction (with NSF 53/P473 certified models) Not for: whole-house treatment, nitrates, arsenic, bacteria

Cost: $30, $90 upfront, $60, $200/year in replacement filters Note: Filter life matters. An overloaded pitcher filter can leach contaminants back. Track gallons, not just time.

Certified for lead + PFAS: Clearly Filtered. Certified for lead: ZeroWater. Taste/odor only: standard Brita.

Under-Sink Reverse Osmosis

Best for: PFAS, arsenic, nitrates, fluoride, lead, broad dissolved contaminant removal Not for: well water with bacteria or iron (those need treatment before the RO membrane)

Cost: $200, $600 upfront, $50, $150/year in filters NSF standard: NSF 58 Top performers: see Best Under-Sink RO Systems

Countertop Reverse Osmosis

Best for: renters or anyone who can’t install under-sink systems, same contaminant coverage as under-sink RO Not for: high-volume households (slower flow rate than under-sink)

Cost: $150, $400 upfront NSF standard: NSF 58 Top performers: see Best Countertop RO Systems

Under-Sink Carbon Block

Best for: lead reduction at a single tap, chlorine, VOCs Not for: PFAS (inconsistent), nitrates, arsenic, fluoride, bacteria

Cost: $100, $300 upfront, $30, $80/year in filters NSF standard: NSF 53 (verify lead specifically)

Whole-House Filter

Best for: iron, sediment, sulfur smell, chlorine throughout the whole house Not for: dissolved contaminants like nitrates or arsenic (whole-house RO exists but is expensive)

Cost: $300, $2,000 installed depending on type See: Best Whole-House Water Filters

UV Disinfection

Best for: bacterial and viral contamination in well water Not for: chemical contaminants (UV doesn’t remove dissolved chemicals)

Must be Class A (NSF 55 Class A) for any actual disinfection. UV works best after sediment and carbon pre-filtration, dirty water blocks UV light and reduces effectiveness. See: Best UV Water Purifiers

The Boiling Misconception

Boiling kills bacteria and viruses. That’s all it does.

Boiling does NOT remove lead, PFAS, nitrates, arsenic, fluoride, or any dissolved chemical contaminant. For nitrates specifically, boiling concentrates them, it evaporates water but leaves the dissolved nitrates behind at a higher concentration. Never boil water to treat nitrate contamination.

One Direct Recommendation

If you have city water and you’re not sure where to start: get a mail-in water test, then compare it against your CCR. If lead is the concern, an NSF 53-certified under-sink filter or RO system handles it. If PFAS is the concern, an NSF 58 RO system is the reliable solution.

If you have a private well and you’ve never tested: test first. A basic panel from a certified lab costs less than any whole-house system and tells you exactly what you’re dealing with.

See also: NSF Certification Standards Explained, a full breakdown of what each standard covers and how to verify certifications.