The claim: a saturation maximum
Brands rate capacity with saline lab tests run to full saturation - the absolute most the fabric can hold under ideal conditions. Those are the 20 to 108 mL figures you see. The top of that range is Thinx, which states "up to 12 regular tampons' worth" and no millilitre figure, so we compute it at the FDA regular-tampon ceiling; the highest figure a brand states itself is Knix Ultra at up to about 99 mL. They are useful for ONE thing: comparing brands on a like-for-like basis, because every brand is measured roughly the same way. That is what the cross-brand table does, with a data-quality grade on each number.
The measurement: real blood, much lower
In 2023, researchers (DeLoughery et al., BMJ Sexual & Reproductive Health, Oregon Health & Science University) tested 21 menstrual products with packed red blood cells - the closest published proxy for real use. The period underwear they tested absorbed under 3 mL each, 2 mL on average, before leaking, far below the marketing maximums. Blood is more viscous than saline, and real wear is not a lab. So the practical takeaway: use the claims to compare brands, but buy with margin and change on time - never wear a pair to its claimed limit.
Quick answers
- Do period underwear really hold as much as they claim?
- The claimed 20 to 108 mL are saline-saturation maximums - the most the fabric can hold in a lab. The one study with real blood components (BMJ 2023) measured under 3 mL per pair, 2 mL on average, before leaking. Use the claims to compare brands like for like, but treat them as maximums, not as how long a pair keeps you dry. Buy with margin and change on time.
- Why did my period underwear leak if it claims 50 ml?
- Because a claimed 50 mL is a saline-saturation maximum, and real leak-free capacity with blood is lower - the only blood-component study measured under 3 mL per pair, 2 mL on average, for the products it tested. A leak usually means the pair was at or past its real capacity, or its absorbency had dropped from fabric softener or age. Match a higher tier with margin and check care.
- How much blood does a pair of period underwear hold?
- In blood, far less than the label number. The 20 to 108 mL ratings come from saline lab tests run to full saturation. The one published test with real blood components (BMJ 2023) measured under 3 mL per pair, 2 mL on average, before the tested pairs leaked, because blood is thicker and real wear is not a lab. Use the mL claims to compare brands against each other, and change pairs well before the claimed number.
Related guides
Why "12 tampons' worth" means 60 mL at one brand and up to 108 mL at another
Two brands use the identical phrase for capacities 80% apart, and one of them publishes no millilitre figure at all. The receipts, the missing standard behind them, and how to compare honestly.
What AI gets wrong about period underwear absorbency
Five measurable failure modes - invented tampon math, marketing tags read as tiers, stale brand ladders, same-word-different-number tiers, claims quoted as lab facts - and the 5-question audit that catches them.
Do period underwear actually work?
Yes, within their capacity - and the honest version of that answer is the useful one. What the independent lab data shows, where they work best, and the trick to getting them right the first time.
Want the one-page version of the data behind these guides? The printable absorbency cheat-sheet puts every brand's tiers in millilitres on a single sheet - free, no email needed.
Compare the claims honestly
Every number on the table is graded for how it was sourced and dated - and the lab-reality note sits right alongside it.