Methodology

Last reviewed July 2026.

We do not test filters

Let's be unambiguous about this, because a lot of sites in this category are vague about it and the vagueness is the point. We own no lab. We run no tests. We have never put water through any of these filters.

What we do is duller and, we'd argue, more useful. Accredited certifiers — NSF International, the WQA, and IAPMO R&T — already test these filters, and already publish the results in public listings. Those listings are free, authoritative, and almost nobody reads them, because they are organised for auditors rather than shoppers. We read them, put the numbers next to the price, and link the source so you can check us.

The metric: cost per certified gallon

Every certification includes a rated capacity — the number of gallons the claim is good for. So:

$ / certified gallon = (price ÷ pack count) ÷ certified capacity in gallons

That's it. No weighting, no proprietary score, no editorial thumb on the scale. It is arithmetic you can redo yourself from the two numbers in the row, both of which are sourced.

We use it because sticker price is actively misleading here. A $7.99 filter rated for 40 gallons costs more per gallon than a $19.99 filter rated for 120 — and in Brita's case, the cheap one isn't certified for lead and the expensive one is. The per-gallon number is the only way to see that.

Standards are shown, never summed

We do not roll certifications up into a single grade, because doing so would destroy the information that matters most. A filter's standards are listed individually — 42, 53, 401 — and we give lead (53) and PFAS/emerging (401) their own columns, because those are the two questions people actually have.

A filter certified only to NSF/ANSI 42 is certified. For taste. It carries no lead claim. Any scoring system that compresses that into "certified: ✓" is hiding the single most important fact on the page, and we won't build one.

Data rules

• Every number is sourced or it is null. If we cannot verify a price or a rated capacity from a primary source, the row shows "price pending" or "no rated capacity". We never estimate, interpolate, or infer a figure to fill a gap.
• Certification claims come from the certifier — NSF's, WQA's or IAPMO's own listing — never from the brand's marketing, and never from another comparison site.
• Prices come from the brand's own store or a major retailer, and each row records where and on what date. Prices drift; the linked page is always authoritative.
• "Tested to" is never rendered as "certified to." Brands with independent lab results but no certification appear in a separate section and are never ranked. This rule is enforced in the code that builds the table, not just in our good intentions.
• We link affiliate products only where we verified the genuine listing. Where we could not confirm a genuine seller for a model, we link the brand's own site and earn nothing.

How we're paid

Some product links are Amazon affiliate links: if you buy through one, we may earn a commission at no additional cost to you. It changes nothing about the ranking, which is produced by dividing two numbers. It cannot change the ranking — the ordering is computed from the dataset, and the dataset is public. The filter that ranks first is whichever one has the lowest cost per certified gallon, and several of the best-ranked rows earn us nothing at all.

What we don't claim

We make no health, medical, or treatment claims. We reproduce published certification data with attribution and claim no endorsement by any certifying body. We are not affiliated with NSF International, the WQA, or IAPMO R&T. A certification describes a tested model against a specific standard — it is not a promise about your water, your plumbing, or your health.

Corrections

Certifications get added, withdrawn, and re-scoped, and prices change weekly. Every row links its source. If a number here disagrees with the certifier's listing, the listing is right and we are wrong — and we want to know.