Guides / Water · Updated
What water filter do I need?
From EPA, CDC and Virginia Cooperative Extension guidance, checked November 2026.
Short answer
Test first, then buy a filter certified for what's actually in your water. Look for the NSF/ANSI standard on the box: 42 for taste and chlorine, 53 for health contaminants like lead, 58 for reverse osmosis, 55 for UV, 44 for softeners. The certification has to name the contaminant, not just the standard.
By problem
| Problem | What to look for | Why |
|---|---|---|
| Chlorine taste or smell | A filter certified to NSF/ANSI 42 | EPA: "The NSF/ANSI 42 standard addresses products that improve taste and odor." |
| Lead | NSF/ANSI 53 with a lead reduction claim, plus NSF/ANSI 42 for particulates | CDC and EPA both point to 53 for lead and 42 (Class I) for particulate lead |
| PFAS (PFOA, PFOS) | NSF/ANSI 53 or 58 with a PFAS reduction claim | EPA, 2024: certifications cover PFOA and PFOS, and "do not yet indicate that a filter will remove PFAS down to the levels EPA has now set" |
| Nitrate, arsenic, fluoride, high TDS | Reverse osmosis, certified to NSF/ANSI 58, for the contaminant | Virginia Tech: RO reduces most dissolved inorganic impurities; carbon "does not remove bacteria or nitrate" |
| Bacteria (well water) | UV certified to NSF/ANSI 55, or continuous chlorination | EPA maps UV to NSF/ANSI 55; Virginia Tech lists chlorination and UV for household disinfection |
| Hard water | A softener certified to NSF/ANSI 44 (or IAPMO/ANSI Z601) | EPA WaterSense home water treatment guide |
| Iron, manganese, rotten-egg smell | An oxidizing filter | Virginia Tech: "Filters out oxidized iron, manganese, and hydrogen sulfide" |
| Low pH, blue-green stains | An acid neutralizing filter | Virginia Tech: "Raises pH, reducing water corrosiveness"; it can add hardness |
Certification bodies list each product and the exact contaminants it's certified to reduce. EPA's advice for PFAS and lead is the same: check the packaging for the standard and the specific claim.
Don't know what's in it?
City water comes with a yearly water quality report from your utility. A private well needs testing: EPA and CDC say every year for total coliform bacteria, nitrates, total dissolved solids and pH. Put your results into the well water test reader.
Whole-house filters: size for flow
A filter on the main line has to pass the whole house's peak flow. Nebraska Extension's minimums, adding 2 gpm per extra bedroom or bath:
| Home | Minimum flow |
|---|---|
| 2 bedrooms, 2 baths | 10 gpm |
| 3 bedrooms, 2 baths | 12 gpm |
| 4 bedrooms, 3 baths | 16 gpm |
| 5 bedrooms, 3 baths | 17 gpm |
Penn State suggests at least 6 gpm for most single-family homes. Compare that with the filter's rated service flow: Aquasana rates its Rhino at 7 gpm, 3M its Aqua-Pure AP1001 at 8 gpm. For hard water, see what size softener you need.
Where the numbers come from
- EPA WaterSense: home water treatment products guide (Nov 2024)
- EPA: reducing PFAS in your drinking water with a home filter
- EPA: PFAS water filter fact sheet (April 2024)
- CDC: lead in drinking water
- EPA: consumer tool for filters certified to reduce lead
- Virginia Cooperative Extension 442-670: household water treatment
- Nebraska Extension G2149 (flow by bedrooms and baths)
- Penn State Extension: estimating water needs
- Aquasana Rhino and 3M Aqua-Pure AP1001