💡 Quick Answer: How does period underwear work?
Period underwear uses multiple layers that work together to manage menstrual flow. A layer next to the skin helps manage moisture, an absorbent core draws in and holds fluid, and a waterproof barrier helps stop it from reaching your clothes. Unlike a pad, these layers are built right into the underwear—so you can wash, wear, and use them again.
joni Leakproof Period Underwear takes that system further with a premium merino wool lining, an ultra-thin absorbent core that holds up to 40 mL, and a breathable, flexible TPU leakproof barrier that provides full front-to-back coverage.
Wait. You just… bleed into your underwear?
Yep.
If you've spent years—or even decades—using pads and tampons while trying to protect your underwear from menstrual blood, this may be a tough one to get your head around!
Even though period underwear is built differently, it still looks like normal underwear. But there's a lot more happening inside.
Period underwear builds the absorbency and leak protection directly into the garment. The layers work together to move menstrual fluid away from your skin, absorb and hold it, and create a barrier between that fluid and your clothes.
So yes, you're bleeding into your underwear. But you're definitely not bleeding into ordinary underwear.
How does period underwear absorb blood?
Think of period underwear as a system rather than a single absorbent layer.
Most designs have three basic jobs to accomplish:
1. Manage moisture next to your skin
The first layer comes into contact with menstrual fluid and helps move moisture away from your body.
2. Absorb and hold menstrual fluid
An absorbent core underneath draws in and holds the fluid.
3. Stop leaks
A waterproof or water-resistant barrier helps prevent the absorbed fluid from passing through the underwear and onto your clothes.
Period underwear can do a lot, but how it's designed makes all the difference. The materials. The absorbency. The leak coverage. The way it feels against your skin hours later. It all matters.
So when we designed ours, we didn’t just ask how to stop leaks. We asked how period underwear could feel better, wear better, and work better.
New to reusable period care? Our Complete Guide to Leakproof Period Underwear covers everything from absorbency and wear time to washing and choosing the right pair.
How does joni period underwear work?
We didn't want to take regular underwear and essentially sew a reusable pad into it.
We started with the whole wearing experience.
joni Leakproof Period Underwear uses several materials, each chosen to do a specific job.
From your body out, here's what's happening.
Layer 1: Merino wool helps manage the next-to-skin environment
First up: the part touching your body. joni uses a premium 100% merino wool lining next to the skin.
Why merino?
Because period underwear doesn't just need to manage menstrual fluid. It also needs to manage what happens when you're wearing an absorbent garment close to your body for hours: moisture, warmth and odour.
Merino is naturally breathable and helps manage moisture, odour and temperature. So while the layers underneath are doing the heavy lifting on absorbency and leaks, merino is looking after the part you actually feel.
There's a lot more to this material choice. Read Why Merino Wool Makes Better Period Underwear for the full story.
Layer 2: The absorbent core pulls in and holds fluid
Next up: absorbency. Because obviously, it needs to handle your period.
joni’s multi-layer absorbent core uses viscose from bamboo and cotton to quickly draw in and lock away menstrual fluid, holding up to 40 mL—about the equivalent of four regular tampons.
The impressive part? We packed all that absorbency into an ultra-thin core.
Because we’re all for high absorbency. The bulk? Not so much.
Layer 3: The breathable leakproof part
If period underwear absorbs blood, why doesn't it leak through? This is the clever bit.
Underneath the absorbent material sits a very thin waterproof barrier. Its job is to stop fluid from continuing through the underwear and reaching your clothes.
In joni Leakproof Underwear, that barrier is made with TPU, or thermoplastic polyurethane.
If you're thinking that doesn't sound very breathable, here's where TPU gets interesting.
What is TPU, and why do we use it?
TPU can create a waterproof barrier while remaining thin, flexible and breathable. And that combination matters.
We need this layer to stop liquid menstrual fluid from passing through. But we don't want to create a heavy, rigid layer that traps you inside your underwear.
The TPU film is designed to allow water vapour to pass through while creating a barrier against liquid water.
In other words: Liquid stays where we want it. Vapour has somewhere to go.
That breathability helps manage the environment inside the underwear, while TPU's flexibility allows the barrier to move with your body instead of feeling like a stiff sheet of plastic.
Because leakproof shouldn't mean airtight. And it definitely shouldn't mean uncomfortable.
Why does breathability matter in period underwear?
Because leak protection is only part of the job.
Period underwear is designed to manage fluid for hours at a time. Add body heat and limited airflow, and you can create a warm, damp environment—the exact opposite of what you want next to sensitive skin.
Breathable materials allow air and water vapour to move through and help manage moisture rather than simply trapping it against the body. That can help reduce prolonged dampness and the discomfort or irritation that may come with it.
That's why we think about period underwear from a menstrual-health perspective, not just a leak perspective.
Keeping blood off your jeans matters. So does considering the skin underneath.
With joni, the breathable merino lining and waterproof-yet-breathable TPU barrier play different roles in helping create a more comfortable environment while the absorbent core manages your flow.
Leak protection is the immediate concern. Long-wear comfort deserves attention, too.
How can a material be waterproof and breathable at the same time?
It sounds contradictory, right? But "waterproof" and "breathable" are describing two different things.
→ A waterproof barrier is designed to prevent liquid water from passing through.
→ A breathable barrier allows much smaller water-vapour molecules to move through the material.
It's the same basic idea behind breathable waterproof materials used in performance clothing: keep liquid water on one side while still allowing moisture vapour to escape.
For period underwear, that gives us a pretty great combination.
Leakproof + flexible + breathable.
Exactly what we want this layer to be.
Why does joni period underwear have front-to-back leak protection?
Leaks don't always happen where you'd expect them to.
Periods move when you do. You sleep, sit, bend, stretch, roll onto your side—and your flow doesn’t necessarily stay neatly in the middle of your underwear.
So we designed our leak protection with real life in mind. Instead of limiting the leakproof barrier to the central absorbent area, we extended it all the way from front to back for 360° leak protection.
That extra coverage can make a big difference overnight, on heavier days, or anytime you want to get on with your day without wondering what’s happening back there.
Because great leak protection should work wherever your period goes.
Does period underwear feel wet?
This is probably the question people who use pads and tampons want answered most.
And it's so fair.
The goal of well-designed period underwear is not to have menstrual fluid sitting against your skin. No one wants wet underwear.
That's why the layers matter.
The next-to-skin layer helps manage moisture while the absorbent core underneath draws in and holds fluid. With joni, the merino lining adds another level of moisture management because of merino's natural ability to absorb moisture vapour while helping the skin-contact surface feel comfortable.
Of course, period underwear has a capacity just like a pad or tampon does. So, once you're approaching that capacity, it's time for a fresh pair.
But until then, your underwear is doing a lot more than simply catching blood.
How much blood can joni period underwear hold?
Up to 40 mL, or about the equivalent of four regular tampons.
For some people, that's enough to wear joni Leakproof Underwear on its own for much of the day. For others—particularly on very heavy days—it might become backup protection alongside a tampon, cup or another period product.
Your flow can also change throughout your cycle.
So rather than thinking of period underwear as something that has to replace every other period product you use, think of it as another option.
Wear it alone.
Wear it as backup.
Wear it overnight.
Wear it on I know my period is coming but apparently it enjoys suspense days.
Your period. Your call.
Do I have to choose between reusable and disposable period care?
Not at all. In fact, for a lot of people, the best period routine is a mix.
At joni, we make both reusable and disposable period care because your needs can change—from person to person, day to day, and even cycle to cycle. Period underwear doesn't have to replace your pads or tampons to be useful. It can simply give you another option.
If you're a tampon user, period underwear can be a great backup on heavier days. You get the protection you're used to, plus an extra layer of security for leaks. It's a combination that just makes sense.
For someone newer to periods, period underwear can also make things feel a little simpler. You put it on just like regular underwear—no positioning a pad or learning how to use an internal product. That ease can also be helpful for neurodivergent people who prefer a familiar routine or find certain period products uncomfortable or difficult to manage.
Then there are the when exactly is my period coming? periods. Cycles aren't always predictable. Perimenopause, PCOS, and other conditions can make it harder to know when your period will arrive, and sometimes cycles are simply irregular.
That's where period underwear can become less of a period-day product and more of an everyday staple. Put on a pair in the morning and you've already got protection built in if your period decides to make an appearance—no emergency tampon, backup pad, or please don't let that be my period bathroom check required.
Because when your period is unpredictable, your period care can still be ready.
And sometimes you just want period underwear because it's comfortable. That's also why we designed joni Leakproof Underwear with a high-rise fit for full coverage and support. It should feel like a great pair of underwear first—one that just happens to be ready for your period, too.
Wear it on its own. Pair it with a tampon. Use it overnight. Keep it for lighter days. Wear it when your period is unpredictable. Build a whole rotation.
Period care doesn't have to be either/or. It should meet you where you are.
Okay, but how do you wash blood out of period underwear?
Another completely reasonable question.
After wearing joni Leakproof Underwear, you can rinse or hand wash it if you're not ready to do laundry right away. For a full wash, simply wash it in cold water without bleach and hang it to dry.
No tumble dryer. No dry cleaning.
Then wear it again.
joni Leakproof Underwear has been validated through wash-cycle testing for 100+ washes (that's at least double standard period underwear) because reusable period care should actually be built for reuse.
Wash. Wear. Repeat.
It's underwear. It just works a lot harder.
The first time you hear about period underwear, bleeding directly into it can sound like a pretty big leap. Once you understand what's inside, it makes a lot more sense!
With joni leakproof period underwear, this means:
- a premium merino wool lining that helps manage the environment next to your skin,
- an ultra-thin absorbent core that pulls in and holds menstrual fluid,
- a flexible, breathable TPU barrier that helps stop leaks, and
- a full front-to-back coverage that helps support you when your period doesn't stay exactly where you'd like it to.
All built into a pair of underwear.
No complicated routine. No special technique to master. Just another way to manage your period—and a pretty smart one, if you ask us.
Ready to see what period underwear can do?
Meet joni Leakproof Period Underwear →
Want to learn more?
Explore our Complete Guide to Leakproof Period Underwear
or discover Why Merino Wool Makes Better Period Underwear.