Period Underwear, Honestly: How Much It Actually Holds

Nobody regulates how much period underwear holds, and the only measurement worth having comes from a study of 20 people. A 2023 review of menstrual products says it plainly: no regulations exist regarding period underwear content or capacity. So the word "super" on a waistband is not defined by anyone, and cannot be compared between one pair and another. The one hard figure comes from a 2025 randomised trial of 20 adults with heavy menstrual bleeding: period underwear held 22 plus or minus 18 g of menstrual fluid, against 48 plus or minus 30 g with disposable products, and participants changed underwear 3.5 times a day against 5.5 times for disposables. That is the whole measured picture. It is thinner than the marketing.

This page is for anyone deciding whether to buy a pair, and for anyone who already wears them and wants to know what the number on the label means. Short answer to that second one: nothing standard.

What period underwear is, and why it is hard to measure

The NHS describes period underwear as pants made from absorbent fabric that soak up blood in the same way as pads, designed to be washed and reused. That is the honest whole description: a pad's job, built into the garment, used again instead of thrown away. Where it sits against pads, tampons and cups is set out in period products explained.

The measuring problem follows from the design. A pad is a discrete object that can be weighed before and after. Underwear is a garment that gets worn, washed and worn again, and the fabric itself differs from pair to pair with nothing forcing anybody to publish what is inside it. That is not a conspiracy — it is a category that grew faster than the rules around it.

The capacity table, with the hole where the answer should be

Read the last row against the four above it. Every other product has a published figure somebody can hold a manufacturer to. Underwear has a research finding from twenty people and a marketing word. Both are worth having; only one of them is a specification.

What the trial actually measured

The 2025 study is the closest thing to an answer that exists, so it is worth describing exactly. Twenty adults with heavy menstrual bleeding were randomised across two cycles. The results: period underwear held 22 plus or minus 18 g of menstrual fluid, disposable products held 48 plus or minus 30 g. Participants changed disposables a mean of 5.5 times a day, underwear 3.5 times, and menstrual cups 3.3 times.

Look at the plus-or-minus figures rather than past them. Eighteen grams of spread around a mean of 22 is enormous — it means the pairs in that study behaved very differently from one another, or the same pair behaved differently on different days, or both. A single average from that data would be a false precision, which is why this page prints the spread every time it prints the mean.

One comparison this page will not make: the NHS says you lose about 20 to 90 ml of blood, roughly 1 to 5 tablespoons, over a whole period. It is tempting to set 22 g against 20 to 90 ml and produce a tidy conclusion. Grams of menstrual fluid weighed on scales and millilitres of blood are not the same quantity, and nobody in the sources here converts between them, so no conclusion is drawn from putting those two numbers next to each other.

The finding that matters most, and it is not about capacity

The authors' own conclusion is the interesting part, and it is a warning rather than a review. They found that the frequency of changing or emptying reusable products is not equivalent to the frequency of changing pads or tampons, and concluded that reusable products may therefore lead to missed diagnoses of heavy menstrual bleeding.

Here is why that lands on you personally. One of the NHS's tests for heavy periods is needing to change your pad or tampon every 1 to 2 hours. That test counts changes. Switch to underwear and your change count drops from something like 5.5 a day to something like 3.5, while your bleeding stays exactly what it was. The test now reads reassuring, and nothing about you has improved.

So use the NHS's other signs, which do not depend on counting anything: periods lasting more than 7 days; passing blood clots larger than about 2.5 cm, the size of a 10p coin; needing to use two types of period product together; bleeding through to your clothes or bedding; avoiding daily activities or taking time off work because of your periods; feeling tired or short of breath a lot. The NHS says many women have heavy periods and they may be normal for you — and also says to see a GP if heavy periods are affecting your life, if you have had them for some time, if you have severe pain during your periods, or if you bleed between periods or after sex.

If you have switched products, say so at the appointment. "I change three times a day" means something different in underwear than in pads, and a doctor cannot know which you meant unless you tell them. The same trap applies to cups, and it is taken apart in menstrual cups, honestly.

What twenty people cannot tell you

The trial's authors were straight about their own limits, and repeating them is not a way of dismissing the study — it is the study's own paperwork.

The sample size was chosen for feasibility, because no prior studies existed to power it. Twenty is what you run when you are first into a field. Every participant had heavy menstrual bleeding, which was the point of the trial and is also its narrowest edge: how underwear behaves for somebody whose periods are light or ordinary was not measured, and this page will not guess it from the heavy-bleeding numbers. And nothing in any source here says how many washes a pair survives, how absorbency changes with age, or whether one pair differs from another by fabric — because with no regulation of content or capacity there is nothing standard to test against.

What you are left with is the ordinary situation of a new product category: some real evidence, mostly narrow, and a lot of confident claims standing on nothing. That does not make underwear a bad choice. It makes the label a poor guide, so the practical advice is to find out how a particular pair behaves for you rather than trusting a word on a tag. Whether it leaks is the question everybody actually has, and what is known about leaks across every product is in period leaks. A pair is bought once and worn again, which is a different shape of spending from a box bought over and over — how much does a period cost is honest about how far that gets you without a price.

The two months of writing that settle it for you

Since no label will tell you what a pair holds, your own record is the only measurement available — and it is the one a doctor can use, which the label never was.

Two cycles is enough. The printable period tracker carries the dates and the length of each period, so the "more than 7 days" test answers itself rather than being reconstructed from memory. The printable symptom tracker takes the rest of it: how many changes, which product you were in, whether anything soaked through, whether you sat out anything you wanted to do. Both print on one sheet and store nothing anywhere.

The column that does the real work is the product one. It is the difference between "she changes three times a day" and "she changes three times a day in underwear, and was changing five and a half times a day in pads two months ago" — and on the trial's own reading, that is the sentence that stops heavy bleeding hiding in plain sight.