How to read the four lists
Start from your heaviest hours, not your average day. Most people bleed hardest on day one or two, and a pair that covers that stretch covers everything after it. Pick the band that matches those hours, buy one pair, wear it on a heavy day, and only then buy a rotation. Buying six pairs off an average is how people end up with a drawer that is fine on day three and useless on day one.
The band you need is a question about your flow, not about the label on the pair. The translator takes a plain description, something like "I soak a super tampon every two hours," and returns the covering tier at each brand in millilitres. Two bands, not one, is the common answer: a moderate pair for most of the period and a heavier pair for the first night.
- 17 mL and under: light days, spotting, the last day, and backup to a cup or tampon.
- 18 to 36 mL: the everyday middle, and the band with the most choice in the catalog as of 2026-09-05.
- 37 to 55 mL: a genuinely heavy day, or a long stretch with no chance to change.
- 56 mL and over: overnight, the heaviest hours, and postpartum weeks.
What "best" cannot mean on this page
These lists rank one property: the capacity the brand claims, converted to a common unit. They do not rank fit, comfort, how a pair feels when damp, whether it rides up, or how many washes it survives. We have no test data on any of those, so putting a number on them would be making it up. Where a sibling guide covers one, it says what the brands publish and what wearers report, and labels which is which.
The capacity figures themselves are ceilings, not promises, measured in the brands' own lab tests with liquids that behave like water. The one published study that tested menstrual products with human blood found period underwear held the least of every product type, under 3 mL each. That gap between a claim and a real wear is why we are running our own capacity screen, and the bench test page publishes the method before the results.
The same word, different capacity
The reason this page has four lists instead of one is easiest to see in two rows of our cross-brand table. Take the word Heavy. Saalt puts its Heavy tier at about 20 mL and Knix puts its Heavy at 36 to 63 mL. Same word, roughly three times the capacity from one end to the other.
Now take the word Light. Modibodi's Light tier is 20 mL in our absorbency table, the same number as Saalt's Heavy. A shopper buying by the word has no way to know which one arrived. The heavy-mean guide runs the full spread brand by brand, and the guide on what pairs actually hold covers how the claims are measured. If a number here matters to you, check its grade and date in the table before you spend anything.
Price, and the two-year question
No prices in this prose: they move, and a stale number is worse than none. The pick cards carry the current listed price, and the catalog sorts by it.
The question worth asking is not what a pair costs but what a rotation costs against what you already spend on disposables, over the years the pairs actually last. Two guides do that arithmetic properly: the worth-it guide runs the comparison against disposable spending, and the budget guide covers the cheapest way into a working rotation. Read those before you buy a set of eight. A useful cross-check on a single pair is price divided by claimed millilitres, which the claims report computes across the category.
When to leave this page
Capacity is the first filter, not the only one. If any of these describe you, a narrower guide gets you there faster, because it ranks on capacity and then on the thing that decides the purchase.
- A first period, or a teen: the starter guide and the school-day guide cover sizing, the routine and how many pairs.
- Postpartum, or after a caesarean: rise and waistband placement matter as much as capacity, and those guides handle both.
- Plus size: the plus-size guide only lists pairs that are buyable in extended sizes today, not pairs the chart says exist.
- Sensitive skin: the sensitive-skin and cotton guides sort on what touches you, not just on how much a pair holds.
- Bladder leaks rather than a period: a separate lane with different labels, and no comparable millilitre figures published.
Sources
The facts above trace to:
- Every capacity figure in the four lists, and the Heavy and Light examples above: our own cross-brand absorbency table, which records each brand's published claim in millilitres with an A, B or C grade for sourcing, last verified 2026-06-11.
- The category-wide spreads and the price-per-millilitre comparison: our own State of Period Underwear Claims report for 2026, computed from that same table.
- Thinx's current period-underwear claim, up to 12 regular tampons' worth, with no millilitre figure published for that line (the 100 mL on the same page is the answer for its light-bladder-leak line): Thinx, "How They Work." Read 2026-09-05.
- The 6 to 9 gram regular-tampon absorbency range our 72 to 108 mL computation of that claim rests on: Code of Federal Regulations, Title 21, Section 801.430, tampon absorbency labeling, on GovInfo. Read 2026-09-05.
- Real-blood capacity of period underwear (the least of any product type tested, under 3 mL each): "Red blood cell capacity of modern menstrual products: considerations for assessing heavy menstrual bleeding," BMJ Sexual and Reproductive Health. Read 2026-09-05.
- Catalog counts by band (in-stock, distinct product names): our own catalog feed, computed 2026-09-05.
Quick answers
- What is the best period underwear?
- There is no single best pair, because the brands do not measure the same thing. A tier labeled Heavy holds about 20 mL at Saalt and 36 to 63 mL at Knix, so a mixed ranking compares words rather than capacity. The useful answer is the best pair in your capacity band, which is why this page runs four lists. Work out your band from your heaviest hours, then read that list.
- Which period underwear holds the most?
- Thinx labels its current period styles with one blanket claim, up to 12 regular tampons' worth of flow, and publishes no millilitre figure for that line. Our table computes it as about 108 mL, taking the 9 gram ceiling of the regular-tampon absorbency range in the US labeling rule, and grades it B; at the 6 gram floor it would be 72 mL. The highest ladder figure after that is the Knix Ultra tier at up to about 99 mL, and WUKA publishes a straight 60 mL for Super Heavy. All of them are maximums, not leak-free promises, and the overnight list here ranks every pair claiming 56 mL or more.
- Is Knix or Thinx better?
- Neither is better across the board. Which one wins depends on the tier you need and how each brand fits you. Thinx puts one blanket claim on its current period styles, up to 12 regular tampons' worth, which our table computes as about 108 mL, while the Knix ladder runs from a 5 to 15 mL Light through a 36 to 63 mL Heavy to a 63 to 99 mL Ultra, so Knix tells you more about which tier you are buying. The brand comparison page puts them side by side in millilitres with the grade on each number, and fit is the part no capacity figure settles.
- What is the best period underwear for heavy periods?
- Look in the 37 to 55 mL band for a heavy day, and the 56 mL and over band for the heaviest hours or overnight. Both lists on this page rank by the verified capacity ceiling, so the label word on the pair does not decide its place. If a pair that should cover you fills within an hour or two, the hours guide sets out what the health authorities say about that pattern, and the next step there is a clinician, not a bigger pair.
- Are expensive period underwear better?
- Not reliably. Price tracks brand, fabric and cut as much as capacity, and a cheaper pair in the right band beats a costlier pair in the wrong one. A fairer test is price divided by claimed millilitres, which our claims report computes across the category. The budget guide covers the cheapest route to a rotation that works.
Related guides
How to choose period underwear: the five decisions, in order
Too many brands, too many tiers, too many cuts. Period underwear comes down to five decisions - capacity, size, cut, fabric, and price - and each one has a neutral answer. The order to make them in, and the tool for each.
What does "Heavy" mean in period underwear?
A tier labeled "Heavy" holds about 20 to 63 mL depending on the brand - a 3x spread for the same word. What it really means, brand by brand, in millilitres.
Best period underwear for heavy flow
The most absorbent period underwear for heavy flow, ranked by verified capacity in millilitres - not by the word "Heavy," which means anywhere from 20 to 63 mL depending on the brand.
Want the one-page version of the data behind these guides? The printable absorbency cheat-sheet puts every brand's tiers in millilitres on a single sheet - free, no email needed.
Find your band, then pick from it
Describe your flow in your own words and the translator returns the covering tier at every brand, in millilitres. The best-of hub carries the narrower lists, by cut, by body and by use.