How to Choose Period Underwear for Your Flow

How to choose period underwear? Start with your flow, not the style. Light flow pairs are for spotting and the first or last days, medium flow pairs are for your regular days and heavy flow pairs are for your strongest days and nights. Pick a cut you feel good in, start with 2 to 3 pairs, and you’re good to go.

Your period on day one isn’t the same as on day five, so it makes sense that your underwear isn’t either. The catch is that most of us have never really thought about how heavy our flow actually is. We just grab whatever pad or tampon is in the drawer. So let’s figure yours out, match it to the right absorbency and put together a small set that gets you through the whole cycle.

Totally new to period underwear? Have a quick read of what period underwear is and how it works first, then come back.

How do you know if your flow is light, medium or heavy?

Look at what you use now and how often you change it.

• Light. A liner or a light tampon does the job and you barely think about changing it.
• Medium. Regular pads or tampons, changed a few times a day.
• Heavy. Super tampons, night pads, or that feeling that you’re changing all the time.

Most periods have a bit of all three. Flow often starts light, peaks for a day or two and then slowly fades out. So instead of asking “what’s my flow?”, ask “what’s my flow on which days?”. That one small switch makes choosing a lot easier.

If your flow suddenly becomes much heavier than usual or changes noticeably, it’s worth checking in with a healthcare professional.

What do light, medium and heavy absorbency mean?

Period underwear absorbency tells you how much menstrual flow a pair is designed to hold. At THATDAY it’s measured in regular tampons, because that’s something everyone can picture.

Whatever level you pick, the build is the same. Three layers, with a top one that pulls moisture away from your skin and dries fast, an absorbent core in the middle and a waterproof but breathable layer on the outside. The fabrics are OEKO-TEX certified and every pair is handmade in Latvia. The real difference between levels is how much flow the pair is made to take.

Which period underwear should you wear on light days?

On light days, go for light flow period underwear. At THATDAY that’s the period proof thong. It holds up to the equivalent of 1 regular tampon and feels much like your everyday thong, so it’s an easy pick under leggings or a fitted dress.

It’s also a lifesaver on those in-between days. Agija, one of our customers, wears hers right before her period starts, so she isn’t caught off guard, and again when it’s almost over. She admits she wears it on regular days too, simply because it makes her feel good.

See everything in light flow period underwear.

Which period underwear is right for regular days?

For your regular days, choose medium flow period underwear. These pairs hold up to the equivalent of 4 regular tampons, and this is where you’ll find the most choice.

Like a mid-rise that sits on your hips? Try Trina, or Alma if you’re in a lace mood. Prefer something that comes up higher over your tummy? Katrina, Una, Mia and the lacy Emma are all high-rise. Several come in a few colours too, so this is the fun part.

Buying your first pair and still not sure? Medium is a really good place to start. Have a look at medium flow period underwear.

What should you wear on heavy days and overnight?

On heavy days and at night, go for heavy flow period underwear. There are three styles to choose from. The Ella super absorbent period boxers hold up to the equivalent of 6 regular tampons and give you shorts-style coverage. If you’d rather wear briefs, Kima is high-rise and Lola is mid-rise, both in soft cotton and both made for heavy flow.

For nights, choose a heavy flow pair that works for you, put it on before bed and change it in the morning. On your very heaviest day it’s completely fine to double up with a tampon or menstrual cup. Just skip the pad on top, it only gets in the way of the comfort you’re wearing period underwear for. Browse heavy flow period underwear.

Can you wear one absorbency level for your whole period?

You can, if your flow stays pretty even. Most people end up happier with a mix though, because a heavy pair on a spotting day can feel like more than you need, while a light pair might not be enough on your strongest day.

Here’s what a simple mix could look like.

• 1 medium flow pair for your regular days
• 1 heavy flow pair for your strongest day and nights
• 1 light flow thong for the start and end, if you like

That’s 2 to 3 pairs, and you can add more once you know what you love.

How many pairs of period underwear do you need?

For most people, 2 to 3 pairs per cycle is a good start, depending on how long your period lasts and how often you wash them. That way one pair is on while the other one dries. It’s one of the things people ask us most, so we answered it along with the rest of our top 9 period underwear questions on Instagram.

Just curious if period underwear is for you at all? One pair is enough to find out. Our tip is to try it at night first and see how it feels, because everyone’s flow is different.

Want to see a full cycle in real life? In this short reel we tested one whole period with 4 pairs, including the heavy flow Lola and the medium flow Trina, and they covered it from start to finish. That’s one example though, not a rule. Your number depends on your own flow and how often you like to change.

Nervous about buying your very first pair? There’s a 40-day risk-free trial. Try a pair, and if it’s not for you, send it back within 40 days for a full refund. And since one pair is made to last up to five years, even a small set goes a long way.

Still weighing it up against what you use now? Here’s how period underwear compares to pads, tampons and menstrual cups.

Does the cut of period underwear matter?

Yes, but for comfort, not protection. The cut decides how a pair looks and feels, and the absorbency level decides how much it holds.

• Thong. Barely there under fitted clothes, made for light days.
• Mid-rise. Sits on your hips like your everyday briefs.
• High-rise. Comes up higher over the tummy, if you like that snug, held-in feeling.
• Boxers. Shorts-style coverage for lazy days at home or under a skirt.

Fit matters just as much. Period underwear works best when it sits close to your body without digging in. Most styles come in sizes XS to 3XL, so check the size chart on the product page, and if you’re stuck between two sizes, go one size up.

One more tip. Wash a new pair before you wear it for the first time. It softens the fabric and helps it absorb better.

How long can you wear one pair of period underwear?

You can wear one pair for up to 8 to 10 hours, depending on your flow and the absorbency you picked. Change it when it starts to feel full, the same way you’d swap a pad or tampon. On a heavy day that might be sooner.

Wondering if it still feels fresh after a full day? We cover that in is period underwear hygienic.

So, how do you choose period underwear for your flow?

Figure out your days first, then pick your cut. A light pair for spotting and the start and end, medium for the regular days, heavy for the big days and nights. Start with 2 to 3 pairs, try the first one at night, and build your set from there.

Ready? Browse the full THATDAY collection and find the pairs that fit your cycle.

Frequently asked questions