Period underwear glossary
Plain-language definitions for the words on the box - including the ones that do not mean what they seem to. Every entry here is the same discipline as the rest of the site: capacity in millilitres, claims labeled as claims.
Absorbency
- Absorbency
- How much menstrual fluid a pair of period underwear can hold before it leaks. The only way to compare it across brands is an objective measure - millilitres - because every brand uses different words for the same amounts.
- Millilitre (mL)
- The objective unit of capacity this site normalizes every brand to. A teaspoon is about 5 mL. Using millilitres instead of tampon counts or tier names is what makes brands comparable, because a tampon count depends on which tampon a brand means.
- Absorbency tier
- A brand's named absorbency level, such as Light, Moderate, Heavy, or Super. Tiers are not comparable across brands: a tier labeled Heavy holds about 20 mL at some brands and 36 to 63 mL at others.
- Regular tampon
- A common unit in absorbency marketing, but not a fixed amount. The FDA defines a regular tampon as absorbing 6 to 9 grams (21 CFR 801.430); most period underwear marketing counts about 5 mL per regular tampon, while some brands imply 8 to 9 mL. That is why "X tampons' worth" is not a reliable cross-brand number.
- Super tampon
- A higher-capacity tampon used as a counting unit by some brands (notably Knix), typically treated as about 9 mL. A claim counted in super tampons will look smaller in number but larger in millilitres than the same claim counted in regular tampons.
- Tampon equivalent
- A capacity expressed as a number of tampons rather than millilitres. Because brands disagree on how many millilitres a tampon holds, this site derives a single tampon equivalent at a uniform 5 mL and always shows the millilitre figure alongside it.
- Confidence grade (A / B / C)
- How a capacity number was sourced. A means the brand publishes millilitres directly. B means it was converted from a stated tampon count. C means sources conflict or the basis is unstated. Every number on this site carries one of these grades and a verification date.
- Saturation claim
- A brand's stated maximum capacity, usually from a saline lab test at full saturation. It is a maximum, not a leak-free guarantee - real-world capacity is lower, especially with blood, which is more viscous than saline.
- Overnight
- Usually a use-case tag, not an absorbency level. At most brands the products tagged "overnight" are simply the brand's Heavy or Super tier in a higher-coverage cut. Check the verified capacity rather than the word.
Construction
- Gusset
- The panel in the crotch of the underwear where the absorbent layers are built in. In period underwear the gusset is where all the absorbency lives.
- Absorbent core
- The thick middle layer in the gusset that holds the liquid. Its size and material determine a pair's capacity.
- Wicking layer
- The top layer against the skin, designed to pull moisture down into the core so the surface feels dry. When a pair feels wet, the core is usually near capacity or the wicking has degraded.
- Leak-resistant barrier
- A thin back layer that stops absorbed liquid from passing through to clothing. Tumble-dryer heat and bleach degrade this layer, which is why air-drying matters.
- Leakproof
- A common marketing term for the leak-resistant construction. Treat it as a design goal within a pair's capacity, not an absolute - any pair leaks once it passes what it can hold.
Fit and sizing
- Hip measurement
- The measurement around the fullest part of the hips, and the universal axis this site sizes from. Because every brand's size chart maps to hip (and waist), one hip number gives your size in each brand.
- Runs small / runs large / true to size
- How a brand's sizing compares to expectation. "Runs small" means many people size up; "runs large" means many size down; "true to size" means the chart is reliable as written. This site notes the direction per brand.
- Final sale
- Most period underwear is non-returnable for hygiene reasons. This is why the standing advice is to buy a single pair and confirm the fit and absorbency before committing to a full rotation of six to nine.
Materials and safety
- PFAS
- Per- and polyfluoroalkyl substances, a class of "forever chemicals" some period underwear was found to contain. Lawsuits and settlements followed, and several US states have menstrual-product PFAS bans taking effect in 2026, so PFAS status is becoming sellability information, not just consumer information.
- OEKO-TEX
- A third-party textile certification (Standard 100) that tests for harmful substances. Brands cite it as evidence a fabric was screened; it is a screening certification, not a guarantee about any single chemical class.
- Organic cotton
- Cotton grown without synthetic pesticides, often chosen for the layer against the skin by people with sensitivities. It speaks to the face fabric, not by itself to a pair's absorbency or its PFAS status.
The category
- Period underwear
- Reusable underwear with absorbency built into the gusset, worn in place of or alongside pads and tampons. Also called period pants (mainly in the UK) or period panties.
- Bladder-leak (incontinence) underwear
- Reusable underwear built for urine leaks rather than menstrual flow. The construction is similar but it is tuned for a different fluid and labeled on a different scale, so a brand's incontinence "light" is not its period "Light."
- Lochia
- Postpartum bleeding in the weeks after birth - heavy at first, then tapering. It calls for high-capacity, high-coverage underwear; the disposable mesh underwear from hospitals is a separate product for the first days.
- Heavy menstrual bleeding (menorrhagia)
- Clinically, losing more than about 80 mL over a whole period, or soaking through protection hourly. It is a reason to compare capacity carefully and, if new or worsening, to talk to a provider - underwear manages flow, it does not diagnose it.
- The BMJ 2023 study
- DeLoughery et al., BMJ Sexual & Reproductive Health, 2023 - the only published test of menstrual products with real blood components. It measured the period underwear it tested holding roughly 1 to 3 mL before leaking, far below marketing maximums, which is why this site treats brand capacities as comparisons, not guarantees.
Now put the words to work
The table shows every brand's tiers in millilitres; the translator turns your flow into the covering tier.