NSF water filter certifications explained: 42, 53, 58, 401 and more

Clear water filter cartridge packed with granular activated carbon on a white background
Photo: Public Domain Photos / Unsplash

Affiliate disclosure: Aquasana (through the Rakuten network) and iSpring are affiliate partners. Product links below use our affiliate tracking and may earn a commission at no extra cost to you. For iSpring, code REFISPRING is a referral discount; confirm terms at checkout. We have not lab-tested these products; certification claims and prices are summarized from the brands' live product pages as of November 8, 2026. See our affiliate disclosure.

Last reviewed: November 8, 2026.

Almost every water filter box carries a string of numbers: 42, 53, 58, 401. Those numbers are American National Standards written through NSF, and they are the closest thing homeowners have to an apples-to-apples check on filter claims. NSF points out that no federal regulations exist for residential filters, purifiers and reverse osmosis systems; the standards are voluntary. It also stresses that the numbers reflect the order the standards were written, not a ranking. A "401" filter is not better than a "53" filter—it was tested for different things.

This guide explains what each common standard covers, how to tell real certification from marketing language, and how to check a specific model before you buy. It applies nationwide to pitchers, faucet-mount, under-sink, reverse osmosis, shower and whole-house systems.

1. The standards homeowners run into most

2. Certified to a standard is not the same as certified for your contaminant

NSF's own warning is the most useful line on its standards page: certification to a standard does not mean a filter reduces every possible contaminant. Each product is certified for specific claims. A carbon filter certified to NSF/ANSI 53 for lead may not be certified for cysts, and one certified for cysts may not be certified for lead.

The fix is the performance data sheet. Reputable brands publish one for each model listing the exact contaminants, the reduction percentage, and the rated capacity in gallons. The EPA WaterSense guide to selecting treatment systems tells shoppers to review that sheet or check a third-party certification website to confirm the system is certified for the target contaminant.

A few practical matches:

3. Who certifies: NSF is not the only lab

The standards are often called "NSF standards," but several accredited bodies certify products to them. The EPA WaterSense guide names CSA Group, ICC Evaluation Services, IAPMO R&T, NSF, UL and the Water Quality Association (WQA). A filter that says "WQA tested and certified to NSF/ANSI 42, 53 and 401" is making the same kind of claim as one with the NSF mark; you check it in WQA's listings instead of NSF's.

4. Red flags in filter marketing

5. How to check a filter in five minutes

  1. Write down the exact model number of the system and the replacement cartridge.
  2. Search the NSF drinking water treatment unit listings or the WQA certified product listings, depending on whose mark the product carries.
  3. Confirm the listing names the contaminant you care about, under the right standard.
  4. Open the brand's performance data sheet and note the rated capacity and replacement interval.
  5. Price the replacement cartridges. A cheap system with expensive or short-lived cartridges can cost more over a few years.

If you do not yet know what is in your water, start with your utility's annual water quality report or a lab test. Our guide to reading a water quality report uses Texas examples but the same report format is required nationwide.

Curated certified picks (2)

Two partner products whose live product pages state specific third-party certifications. Prices were checked on the brands' live pages on November 8, 2026 and change often. As always, confirm the listing for the contaminant you care about.

  1. Aquasana Claryum 2-Stage Under Sink Water Filter, $249.99 on aquasana.com (the page showed a lower price for its Water for Life auto-ship plan). Aquasana says it is WQA tested and certified to NSF/ANSI 42, 53 (including P473) and 401 for the claims on its performance data sheet, listing reductions such as 99.6% of lead and microplastics, 99.7% of PFOA/PFOS, 97.3% of chlorine and 91% of chloramines. It is a carbon-based filter, not RO, so it keeps minerals; capacity is 500 gallons or about 6 months, with a 0.5 gpm rated flow and a dedicated faucet. Aquasana notes a UV filter must be added for bacteria and viruses.
  2. iSpring RCC7 5-Stage Under Sink Reverse Osmosis System (75 GPD), $229.99 on ispringfilter.com. iSpring states the RCC7 is certified to NSF/ANSI 58 and that the certification covers the whole system's performance rather than individual components. As an RO system it targets dissolved solids and hardness minerals at the kitchen tap, with the trade-offs the EPA notes: reject water and fewer minerals in the finished water. Yearly pre-filter sets are listed on our Products page. Use code REFISPRING at checkout.

Certification claims and prices come from the Aquasana and iSpring product pages and can change. Verify the model in the NSF or WQA listing for your target contaminant before you buy.

Where this fits in the house

Certification tells you whether a filter does what it says; it does not tell you which type of filter you need. For that, see countertop vs under-sink water filters, filter vs softener vs RO, and the whole-house sediment filter guide for well and city water prefiltration. More certified filters are on the Products page.

Bottom line

Treat NSF numbers as a map, not a grade: 42 for taste and odor, 53 for health contaminants, 58 for reverse osmosis, 401 for emerging contaminants, 44 for softeners, 55 for UV and 177 for shower chlorine. Then go one step further than the box—confirm the exact model and contaminant in the NSF or WQA listing, and read the data sheet for capacity so you replace cartridges before the claim runs out.

Sources

Information only; not medical or water-treatment engineering advice. For a private well or a known contamination problem, test your water through a certified lab and talk with your local health department.