Period underwear is one of those purchases where the marketing all sounds the same and the differences that matter are invisible on a product photo. This guide covers the seven things that actually decide whether a pair works for you – in the order you should think about them.
1. Absorbency: match the day, not the average
Absorbency is usually shown as drops, from 1 to 5. Here is what those levels correspond to in practice:
- 1–2 drops: discharge, spotting, the last days of your cycle, and days when you only suspect your period is coming.
- 3 drops: a normal daytime flow.
- 4 drops: heavy days. At this level our styles hold roughly what three to four tampons hold.
- 5 drops: your heaviest days and overnight. Our sleep short holds up to 40 ml.
The most common first-purchase mistake is buying one mid-absorbency pair and testing it on the heaviest day of the cycle. It usually fails – not because the product is bad, but because it was never built for that day. Start with the overnight style instead: it has the largest capacity, you are lying still, and you know by morning whether it holds.
2. Fit: measure, don't guess
Period underwear only works if it sits flush against the body. A pair one size too big leaves a gap at the leg opening, which is exactly where leaks happen. A pair too small cuts in and becomes unbearable after three hours. Your usual high-street size is not a reliable guide, because every brand cuts differently.
Measure two things with a tape measure, ideally in the morning: your waist at its narrowest point, and your hips at the widest point of your seat, keeping the tape horizontal. Compare both to the size chart on the product page.
If you fall between two sizes, go by the hip measurement and size up. A slightly loose waistband is livable; a leg opening that cuts in is not.
3. What is actually in the fabric
This is the part most buying guides skip. Independent testing has found real differences between brands. In a 2023 test of twelve washable period briefs run by the Swiss consumer magazine Gesundheitstipp and reported by Germany's Stiftung Warentest, six of the twelve contained substances of concern – five of them silver chloride, a biocide that Germany's Federal Institute for Risk Assessment advises against using in textiles.
Three questions worth asking any brand before you buy:
- Is there an antibacterial finish? If the page mentions "antibacterial", "silver ions" or "odour-treated", ask what with. Silver-based biocides can irritate skin and disturb the natural vaginal flora, and a significant share washes out into wastewater within a few washes. Good construction does not need them – odour comes from blood reacting with air over hours, and a well-sealed core prevents that on its own.
- Is the waterproof layer PFAS-free? PFAS are persistent chemicals used to repel water. A waterproof layer does not require them: a PUL membrane does the same job without.
- Is the full composition published? A brand that lists every layer has nothing to hide. If the page only says "cotton", the most important part is missing.
For reference, here is our own composition, layer by layer: inner layer 95% cotton and 5% elastane; absorbent core 88% polyester and 12% nylon; waterproof layer an Air PUL membrane, PFAS-free; outer layer 92% bamboo viscose and 8% elastane. All materials are certified to OEKO-TEX STANDARD 100.
4. Certification: useful, but not a free pass
OEKO-TEX STANDARD 100 tests textiles for a long list of harmful substances and is a sensible baseline filter. It does not, however, rule out every antibacterial finish. Treat it as a floor, not a ceiling – and still ask question one above.
5. How many pairs you actually need
Count by laundry cycle, not by cycle days. One pair is in use, one is in the wash, one is drying.
- Three pairs is the sensible minimum if you combine them with another product.
- Five to six pairs if period underwear is your only protection – mixed: two for heavy days, two for normal days, one or two for overnight.
6. Care, and what it costs you in effort
The routine is short: rinse in cold water after wearing until the water runs clear, wash at 30–40°C in a laundry bag, no fabric softener, air dry. Never tumble dry – heat damages the waterproof membrane, and that is the single most common way people ruin a pair. Full instructions are in How to Wash Period Underwear.
One honest note: the absorbent core is synthetic in essentially every brand on the market, so fibres are released in the wash. A laundry bag, a cold cycle and a gentle programme reduce that considerably.
7. The guarantee, and why it matters more than the price
You cannot tell whether a pair works for your body until you have worn it through a full cycle. That makes the return policy part of the product, not a footnote. Look for a brand that lets you test an opened pair, and read what happens if it does not fit – store credit and a refund are very different promises. Ours is a 90-day comfort guarantee; the exact terms are listed on every product page.
What it costs over time
A pair kept for two years replaces several hundred individually wrapped disposables. Whether that adds up in your case depends on your flow and how many pairs you buy – we ran the numbers in Which Reusable Period Products Are Actually Worth It?. If your flow is heavy, this guide covers what actually keeps you protected.
Ready to look? Our period underwear collection is filterable by absorbency, so you can sort by the level you need rather than by style.
Frequently Asked Questions
How do I choose the right absorbency for period underwear?
Match the absorbency to the day, not to your average flow. One to two drops suit discharge and the last days of a cycle, three drops suit a normal daytime flow, four drops suit heavy days and hold roughly what three to four tampons hold, and five drops are for the heaviest days and overnight.
What size period underwear should I buy if I'm between sizes?
Go by your hip measurement and size up. A slightly loose waistband is comfortable enough, whereas a leg opening that cuts in is both uncomfortable and more likely to leak, because the fabric no longer sits flush against the skin.
Is period underwear safe? What should I check?
Check three things: that there is no antibacterial or silver-based finish, that the waterproof layer is PFAS-free, and that the brand publishes the full composition of every layer. OEKO-TEX STANDARD 100 certification is a useful baseline but does not by itself rule out an antibacterial finish.
How many pairs of period underwear do I need?
Three pairs is the minimum if you combine them with another product. Five to six pairs are needed if period underwear is your only protection, mixed between heavy-day, normal-day and overnight styles, because one pair is always in the wash and one is drying.
How long can period underwear be worn at a time?
Around 8 to 12 hours when the absorbency matches your flow, and less on very heavy days. Most people have an accurate sense of what their own styles can handle after one or two cycles.
Can period underwear be tumble dried?
No. Heat damages the waterproof membrane, which is the most common way a pair is ruined. Rinse cold, wash at 30 to 40 degrees in a laundry bag without fabric softener, and air dry.
About the author
Mia Hartman writes for Floena about period care, sustainable living and feeling good in your own body. She believes periods deserve an open conversation without shame – and that the right products fit quietly into your life instead of interrupting it.
