The short answer: Period underwear works in layers. A thin top layer next to your skin pulls flow down and away quickly, a multi-layer absorbent core in the gusset soaks it up and holds it, and a leakproof barrier underneath stops it reaching your clothes. Because flow moves down into the core faster than it spreads across the surface, the layer touching your skin stays fairly dry. Fabpad's Mid Waist period panty has a 5-layer absorbent core that holds about 80 to 100 ml, and every Fabpad style has the same absorbency.
What is inside a period panty
From the outside, a period panty looks like ordinary underwear. The difference is in the gusset, the part that sits between your legs. Cleveland Clinic describes most period underwear as having an absorbent core that traps flow, a moisture barrier to keep you dry, and a leak-prevention layer. Here is how those parts fit together in a Fabpad period panty.
| Layer | Where it sits | What it does |
|---|---|---|
| Top layer | Against your skin | Lets flow pass straight through and moves it away from the surface |
| Absorbent core | Inside the gusset | Several layers that soak up flow and spread it out so it is held, not pooled |
| Leakproof barrier | Under the core | Stops flow from passing through to your clothes |
| Outer fabric | The rest of the panty | Soft, breathable cotton spandex that fits like everyday underwear |
Fabpad's Mid Waist period panty has a 5-layer absorbent core with a leakproof barrier, in cotton spandex.
Why you do not feel wet
Three simple ideas explain the dry feeling.
1. Wicking pulls liquid downwards
Think of dipping the corner of a tissue in water: the water climbs up the tissue on its own. This is called capillary action. Tiny gaps between fibres pull liquid along them. In a period panty, the top layer and core are arranged so liquid is drawn down into the core, away from your skin.
2. The core spreads and holds flow
Once flow reaches the core, it spreads out across several layers instead of sitting in one spot. Spread thinly, it is held within the fibres rather than as a puddle. That is why a period panty can hold flow without feeling soggy.
3. The barrier keeps it in
Underneath the core, the leakproof barrier stops flow going any further. It is what keeps your jeans, salwar or bedsheet clean.
Together, these layers mean the surface next to your skin feels fairly dry for most of the time you wear it. Cleveland Clinic notes there can be an adjustment period at first, especially if you are used to tampons, because you feel your flow differently.
How much a period panty holds
Fabpad rates its Mid Waist period panty at about 80 to 100 ml of absorbency. For context, the NHS says a typical period involves losing about 20 to 90 ml of blood over the whole period, spread across 2 to 7 days, with the heaviest flow in the first 2 days.
That does not mean you should wear one pair for your whole period. Absorbency is the most a pair can hold, not how long it stays fresh and comfortable. Blood starts to smell when it sits in fabric for a long time, and a fuller core feels heavier. That is why Fabpad recommends wearing each pair for up to 8 hours, depending on flow.
Every Fabpad period panty has the same absorbent core, so you do not need to choose a "light" or "heavy" pair. What changes from day to day is how quickly your flow fills it, and so how often you change. Our guide One Period Panty, Every Flow explains how to choose a style instead.
Why period panties can feel damp sometimes
If you do feel damp, one of these is usually the reason:
- The core is nearly full. Once the layers are holding a lot, there is less room to pull new flow away from the surface. Time to change.
- A gush or clot arrived quickly. A sudden heavy moment can reach the surface before it is fully drawn down.
- Fabric softener has coated the layers. Softener blocks the tiny gaps between fibres that make wicking work.
- The pair was not completely dry when you put it on, which is easy to miss in the monsoon.
- Sweat on a hot day. Heat adds moisture that has nothing to do with your flow.
Our guide to period panty rolling, bunching or leaking covers each of these with a fix.
Why fit is part of the science
The layers only work if flow actually lands on them. If the leg openings are loose, flow can escape at the sides before it reaches the core. If the panty slips down, the gusset moves away from where it needs to be. A snug fit at the legs, without digging in, keeps the gusset flat against your body so every layer can do its job. Our period panty size chart shows how to measure.
Coverage matters too. When you lie down or sit for hours, flow travels towards the back. A High Waist pair or Period Boy Shorts extend the covered area so the core has more room to work.
Why washing affects absorbency
The same tiny gaps between fibres that pull flow away from your skin can be blocked or damaged by the wrong wash.
- Fabric softener leaves a coating that stops fibres soaking up liquid.
- Too much detergent that is not rinsed out can do the same.
- Hot water and tumble drying can weaken the leakproof barrier and elastic.
Rinse in cold water until it runs clear, wash gently with a mild detergent, and air-dry in the shade. Our guide on how to wash period panties has the full routine, and How Long Do Period Panties Last? explains how care affects lifespan.
Period panties compared with pads and tampons
- Pads also use a top layer, absorbent core and backing, but they stick to your underwear and can shift or bunch. A period panty has the same idea built into the underwear itself.
- Tampons and cups collect flow inside the body. Period panties are worn outside the body.
- Reusable cloth pads work in a similar layered way to period panties, in a pad shape.
Many people combine them. A cup or tampon with a period panty as backup, or a pad on top for the heaviest hours, are both common setups.
Common myths about how period underwear works
"You sit in your blood all day." Flow is pulled down into the core and held there, so the surface next to your skin stays fairly dry.
"Thin means it cannot hold much." The core is made of several thin layers, which is why it can hold flow without looking bulky.
"Heavy flow needs a special heavy panty." Not with Fabpad. Every style has the same absorbency; on heavy days you change more often.
"If it holds 80 to 100 ml, you can wear it all period." Absorbency is the maximum. Change within 8 hours depending on flow, for freshness and comfort.
Frequently asked questions
How does period underwear absorb blood? A top layer pulls flow down, a multi-layer absorbent core soaks it up and holds it, and a leakproof barrier keeps it in.
Why don't period panties feel wet? Flow moves down into the core faster than it spreads across the surface, so the layer against your skin stays fairly dry.
How much can a Fabpad period panty hold? The Mid Waist period panty holds about 80 to 100 ml. Every Fabpad style has the same absorbency.
Do period panties smell? Not if you change on time and wash them properly. Blood only starts to smell when it sits in fabric for a long time.
Why does my period panty feel damp? It is usually nearly full, was not fully dry, or has been washed with fabric softener.
Related reading
- One Period Panty, Every Flow
- How Long Can You Wear a Period Panty?
- Are Reusable Period Panties Hygienic and Safe?
- First Time Using Period Panties: What to Expect
Sources: Cleveland Clinic: What Is Period Underwear and Does It Work?, NHS: Periods, NHS inform: Choosing period products