How we measure period underwear absorbency

1. The problem: a “tampon” is not a unit

Nearly every period-underwear brand states absorbency as “holds N tampons' worth.” But the word “tampon” spans a wide official range. Under the FDA's tampon-labeling standard (21 CFR §801.430, measured with the syngyna saline test), absorbency classes are:

So when one brand counts a “regular tampon” as ~5 mL (below the FDA floor of the regular range) and another counts it as 9 mL (the top of the range), a “10 tampons' worth” claim at the first brand means roughly half the capacity of the same claim at the second. Tampon counts are marketing shorthand, not a unit. Millilitres are a unit.

A tier LABELED 'Heavy' ranges from ~20 mL (Saalt, Proof, WUKA) to ~36-63 mL (Knix) across brands - roughly a 3x spread for the same word. A shopper buying 'Heavy' has no way to know which she is getting.

2. Our normalization rules

1. mL is the anchor. When a brand publishes millilitres, we use them unchanged.
2. Counts convert at the brand's own stated basis. Thinx states a 9 mL regular tampon, so Thinx counts convert at 9 mL. Where no basis is stated we use the industry-common ~5 mL per regular tampon and 9 mL per super tampon, and say so in the source note.
3. Unstated basis = honest range, not invented precision. When a claim has no stated basis and the plausible bases disagree (e.g. a “12 regular tampons” claim that could mean 60 mL at 5 mL/tampon or 108 mL at 9 mL/tampon), we publish the full range and grade it C.
4. The brand's verbatim claim is always preserved in the source note next to every converted number, so you can check our arithmetic.
5. Tampon-equivalents are derived once, uniformly. The friendly “tampon-equivalent” column is always mL ÷ 5, the common basis, so every brand is on the same footing. It will not match a brand's own count when that brand uses a different basis - that mismatch is the point.
6. Scope is standalone period underwear only. Period swimwear, reusable pads, removable boosters/inserts, and bladder-leak (incontinence) lines are excluded and noted per brand.

3. Data-quality grades: every number tells you how much to trust it

No other source grades period-underwear absorbency data at all. We grade every tier:

4. Brand claims vs. independent testing

All figures are the brands' OWN maximum-capacity claims (typically saline lab tests at full saturation). The only independent test with actual blood components (DeLoughery et al., BMJ Sexual & Reproductive Health, Aug 2023, Oregon Health & Science University; 21 products tested with packed red blood cells) measured period underwear absorbing on the order of 1-3 mL before saturation - far below claimed capacity. Blood is more viscous than saline and real-world leak-free capacity is lower than lab maximums. Use this table to compare brands on a like-for-like basis; do not read any mL figure as a leak-free guarantee.

Reference: DeLoughery E. et al., “Red blood cell capacity of modern menstrual products: considerations for assessing heavy menstrual bleeding,” BMJ Sexual & Reproductive Health (2023). The community-run capacity tests you may have seen on Reddit (shoppers pouring measured water into pairs at home) exist precisely because no comparable public data does.

5. Per-brand basis, verification date, and known conflicts

Proof's DTC store (shopproof.com) states it fulfills orders only until 2026-01-30; the brand continues at retail as 'Proof. by Always' (P&G). Tier names overlap confusingly across the two lines - see the Proof entry. Cora exited period underwear entirely (cora.com/products/period-underwear 404s; underwear absent from nav as of 2026-06-10). Not included. Thinx's current flagship claim ('up to 12 regular tampons / up to 100 mL' on how-they-work, mirrored by the retail/Amazon listings) is not reconcilable with the brand's classic per-tier ladder (Super = 45 mL) - both are encoded under Thinx, with the contradiction flagged. The identical '12 regular tampons' phrase means 60 mL at WUKA and 100 mL at Thinx - the single clearest proof that tampon counts are not a unit. Merchandising labels are NOT tiers (v3 finding): 'overnight' is a use-case tag at Saalt/Knix/The Period Company (the products underneath are Heavy or Super tiers); Thinx 'Ultra Soft' is a FABRIC line, not an absorbency level; Hanky Panky's 'medium' product tag contradicts its own PDPs (thong = Light, other cuts = Regular); Saalt's 'Light to Moderate' is flow guidance on Regular-tier products - Saalt publishes no Moderate tier. Catalog labels therefore come from per-brand canonical sources: Knix's own absorbency collections, Saalt Absorbency_ tags, Modibodi PIM- tags, Thinx product_type + handle prefixes, TPC 'For X Flows' titles, HP per-cut PDP claims.

6. Updates and corrections

Brands change tier systems without notice - Knix's own blog and live collection pages already disagree with each other, which is why per-brand verification dates matter. We re-verify rows on a rolling basis and date-stamp every change. Spot an error? Tell us via the contact page and we will re-check the brand's live pages and correct the table. We have no affiliation that affects the numbers: when we earn an affiliate commission it never changes a mL figure, a grade, or a tier mapping (see the affiliate disclosure).