The layers
Most period underwear uses three working layers built into the gusset. A top layer wicks moisture away from the skin so it feels dry. An absorbent core - the thickest layer - holds the liquid. And a leak-resistant back layer stops it from passing through to your clothes. A comfortable outer fabric finishes the pair so it looks and feels like ordinary underwear.
Capacity is a lab maximum, not a guarantee
Every brand states a capacity, usually from a saline lab test at full saturation. The only published study that tested menstrual products with real blood components (BMJ Sexual & Reproductive Health, 2023) measured the period underwear it tested holding under 3 mL each, 2 mL on average, before leaking - far below the marketing maximums, because blood is more viscous than saline. So a claimed capacity is best read as a way to compare brands on a like-for-like basis, not as a promise of how long a pair will keep you leak-free. Match a tier with margin, and the comparison still works because every brand is measured the same way.
Quick answers
- How does period underwear hold so much liquid?
- Through layers built into the gusset: a wicking top layer pulls moisture off the skin, a thick absorbent core holds it, and a leak-resistant back layer stops it passing through. The whole system is sewn into what otherwise looks like normal underwear.
- Do you wear anything with period underwear?
- On a light or moderate flow, no - the underwear absorbs on its own. On a heavy day, some people pair it with a tampon or cup for extra hours, using the underwear as backup. Which approach you need depends on your flow rate; the translator turns that into a covering tier.
- Does period underwear feel wet?
- The wicking top layer is designed to keep the surface feeling dry even as the core absorbs. A pair that feels wet usually means the core is near capacity (time to change) or has lost absorbency from fabric softener or age.
Related guides
Do period underwear actually work?
Yes, within their capacity, and that phrase is the useful part. What the independent lab data shows, where they work best, and the first-pair trick.
Period underwear vs. pads and tampons
How period underwear compares to pads and tampons on absorbency, comfort, cost over time and convenience, and where each still wins. No thumb on the scale.
How to wash period underwear
Rinse cold, wash cold on gentle, skip the fabric softener, air dry. The routine that keeps the absorbent layer working, and the two mistakes that wreck it.
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.
See what the layers actually hold
Capacity is the whole game. Compare every brand's claim, normalized to millilitres and graded for how it was sourced.