What Is Period Underwear and How Does It Work

So what is period underwear? It looks and feels like normal underwear, but it does something ordinary underwear cannot. It absorbs your menstrual flow on its own, without a pad, tampon or cup underneath. You wear it, and when your day is done you wash it and wear it again.

If you have been curious about it but were not quite sure how a pair of underwear can hold a period without feeling wet, this guide walks you through exactly what period underwear is, how it works and who it suits.

What is period underwear

Period underwear is reusable underwear with built-in absorbent layers sewn into the gusset. Those layers soak up menstrual blood and hold it away from your skin, so you stay dry and protected through the day.

It comes in the same shapes you already wear, from thongs to mid-rise and high-rise briefs, and even boxers. From the outside nobody can tell it is any different. The difference is on the inside, where a normal-looking fabric is doing the work that a pad or tampon would usually do.

You can use period underwear as your only protection, or as a backup alongside a tampon or a menstrual cup on heavier days. Because it is washable and reusable, one pair replaces a whole stack of disposable products over its lifetime.

How does period underwear work

The secret is in the layers. THATDAY period underwear is built from three special fabric layers that each have a job, and together they keep you dry and leak free. The top layer sits against your skin, pulls moisture away quickly and dries fast, so the surface stays comfortable instead of feeling damp. The middle layer is the absorbent core, made from fine fibres that soak up liquid and lock it inside the fabric, and this is the layer that stores your flow. The outer layer is waterproof yet breathable, so nothing passes through to your clothes, which is what prevents leaks and helps keep odours in.

All three layers are made from OEKO-TEX certified fabrics chosen to be safe for contact with your intimate area, and every pair is handmade in Latvia. You can see the layers up close in our fabric explainer on the THATDAY Instagram.

How much can period underwear hold

Not every day of your period is the same, so period underwear is made in different absorbency levels to match your flow. Lighter styles are designed for the start and end of your period or for spotting, while heavier styles are made to carry you through your strongest days.

At THATDAY you can shop by exactly this. There are pairs for light flow, medium flow and heavy flow, so you can pick the right level instead of guessing. Depending on the style, a pair can hold up to the equivalent of 6 regular tampons worth of flow.

If you are not sure which level is right for you, our separate guide on how to choose period underwear for your flow goes deeper into matching a style to your cycle.

How long can you wear period underwear

How long a single pair lasts through the day depends on your flow and on the absorbency level you choose. Many pairs can be worn for up to 8 to 10 hours, so on a lighter day a pair can carry you through most of the day, while on a heavier day you will want to change sooner. A simple rule is to change when the pair starts to feel full, the same way you would swap a pad or tampon. Many people keep at least two pairs in rotation, so there is always a fresh one ready while the other is drying.

Is period underwear reusable and how long does it last

Yes, this is the whole point. A THATDAY pair is designed to be washed and worn again for up to five years rather than thrown away after one use.

Caring for it is simple.

• Rinse the pair in cold water first, without soap.
• Machine wash at 40 degrees together with your other clothes.
• Skip fabric softener, since it can coat the fibres and reduce how well they absorb.
• Air dry the pair rather than putting it in the dryer.

Our full guide on how to wash period underwear covers every step. Because one pair replaces so many disposable products over its life, switching also cuts down the waste and the ongoing cost of pads and tampons.

Who is period underwear for

Period underwear suits almost anyone who menstruates, at any age. It is a comfortable choice for many everyday moments.

• Teens who want an easier first period without awkward surprises at school
• Everyday protection on its own, or a discreet backup with a tampon or cup
• Sleeping through the night without worrying about leaks
• The postpartum weeks, when many new parents reach for it
• Active days and travel, when you want fewer products to carry

It is a good fit if you want fewer things to carry, less waste, or simply a more comfortable feeling than a pad or tampon. And if you have ever worried it might feel wet or unhygienic, the layered design is built specifically to avoid that.

The takeaway

Period underwear is reusable, absorbent underwear that handles your period on its own thanks to a clever three layer design that keeps you dry and leak free. It comes in different absorbency levels so you can match it to your flow, it lasts for years, and it quietly cuts down both waste and cost along the way.

If you are ready to find your first pair, browse the full THATDAY collection and choose the absorbency that fits your cycle.

Frequently asked questions