PeriodFinder

Frequently asked questions

The short versions. The methodology page has the long ones.

Sizing and fit

Why is period underwear sizing so inconsistent between brands?
Each brand drafts its own chart against its own fit model, so the same hip measurement lands in different sizes - a 40-inch hip is an M at some brands and an XL at others. On top of that, some brands consistently run small or large relative to their own charts, which is why community advice like 'size up from your Knix size' exists. Our find-my-size tool reads every brand's published chart at once from one hip measurement.
How do I find my size across brands without trying them all on?
Give the find-my-size tool one hip measurement in inches. It reads all ten brands' published charts at once and shows the size you'd order at each, adjusted for whether that brand tends to run small or large. Hip is the axis every brand publishes, so it's the one measurement that translates cleanly across the whole shelf. The math runs in your own browser - nothing is sent to us.
Which brands run small or large?
It varies by brand and even by style. Knix labels its core styles true to size, but reviewers consistently report the leakproof cuts run small and recommend sizing up. Other brands publish a chart but skew one way in practice. The find-my-size tool folds each brand's known run-direction into the size it shows you, and we say where the signal comes from (published chart versus repeated reviewer reports) so you can weigh it yourself.
Should I size up in period underwear?
Sometimes - but not as a blanket rule. Sizing up helps on styles that are known to run small, and some people prefer a looser leakproof gusset. But too large and the gusset can gap and leak at the legs. Check your hip measurement against the specific brand's chart first (that's what the tool does), then size up only where the run-direction or the style calls for it.
What if I'm between two sizes?
Between sizes, the safer choice is usually the larger one for a leakproof gusset, because a snug-but-not-tight fit keeps the absorbent layer flat against you. But it depends on the cut and the brand's run-direction. Since most brands treat period underwear as a hygiene item with no returns once worn, order a single pair in your best-guess size to confirm the fit before buying a full rotation.

The absorbency numbers

What does "12 tampons' worth" actually mean?
Less than you'd think, and not the same thing twice. Brands count different tampons: at one brand "12 tampons' worth" means 60 mL (5 mL per tampon), at another it means 100 mL (about 8.3 mL per tampon). That 67% gap on identical words is why we normalize every claim to millilitres instead of tampon counts.
What do the A/B/C grades on absorbency numbers mean?
A means the brand publishes the capacity in millilitres directly - no conversion needed. B means the brand publishes tampon counts and we converted them at a stated basis. C means the brand's own sources conflict or the claim couldn't be confirmed first-party. The grade travels with the number everywhere it appears on the site.
What does "Heavy" mean across brands?
Not one thing. A tier labeled Heavy holds about 20 mL at Saalt and WUKA but 36 to 63 mL at Knix - roughly a threefold spread on the same word. Some brands also use "Heavy" as a use-case tag on a product whose real tier is something else. That's why every guide here ranks by the verified millilitre number, not by the label on the tag.
Are these numbers lab-tested?
No - and we say so everywhere it matters. The site normalizes each brand's own published claims so they can be compared fairly; it is not independent measurement. Published research that tested period products with real blood has measured substantially lower effective capacity than some marketing claims. Treat the tiers as honest comparisons of claims, size up when between tiers, and plan a backup on your heaviest days.
How many millilitres do I actually need?
It depends on how fast you bleed, not on a label. If you soak a super tampon (about 9 mL) every two hours, you're losing roughly 4 to 5 mL an hour, so a 40 to 60 mL pair buys you several hours. Tell the absorbency translator how fast you bleed in your own words and it shows the covering tier at each brand - in your browser, nothing stored.
What's the most absorbent period underwear?
By the brands' own published claims converted to millilitres, the highest-capacity styles come from Thinx (flagship styles claim up to 100 mL), Knix (its Ultra tier claims up to about 99 mL) and WUKA (its Super Heavy claims about 60 mL). Those are saline-test maximums; the only blood-component lab study measured 1 to 3 mL of real leak-free capacity, so treat the numbers as a fair comparison between brands, not a leak-free promise.

Privacy and your data

Is what I type into the tools private?
Yes, by architecture: the size and absorbency math runs as code in your own browser. Your measurement and flow description are never transmitted to us, never appear in a URL, and are saved only in your own browser so the tools can work together. A one-click control clears them. There are no ad pixels or session recording on any tool page.
Do you store my measurements or flow description on a server?
No. There is no account and no server-side profile for the tools. What you enter stays in your browser's local storage on your own device, which is what lets the size tool and the absorbency translator share your answers. Clear it any time with the reset control, or by clearing your browser data. We never see it.
Are there ad trackers or session recording on the site?
No advertising pixels and no session-replay recording, and none at all on the tool pages where you enter body data. We keep the pages that handle your measurement free of third-party trackers on purpose - a health-adjacent tool shouldn't leak what you typed to an ad network.
Do I need an account to use PeriodFinder?
No. Both tools work with no sign-up. The only thing we offer an email for is an optional restock or price-drop note, and that's a plain email address, never tied to your measurements or flow.

Buying, returns and care

Can I return period underwear if the size is wrong?
Usually not once it's been tried on - most brands treat it as a hygiene item and offer store credit at best, sometimes nothing. That no-returns reality is exactly why we built cross-brand sizing: the first pick has to be right. The community's standing advice is to order one or two pairs to confirm fit before buying a full cycle's worth.
How long do period underwear last?
Most brands quote about two years, or roughly 40 to 50 washes, before the absorbent layer starts to lose capacity. Air-drying instead of tumble-drying and skipping fabric softener (which coats the fibers) both extend that. When a pair starts to feel like it wets through faster than it used to, that's the layer wearing out, not you imagining it.
How do I wash period underwear?
The common care across brands: rinse in cold water until it runs clear, machine wash cold on a gentle cycle, and hang to dry. Skip fabric softener and bleach, and skip the dryer's high heat - all three shorten the absorbent layer's life. A mesh laundry bag protects the gusset. Always follow the specific brand's tag, but that pattern holds nearly everywhere.
How many pairs do I need?
Enough to cover the days you'll wear them between wash cycles. A common starting rotation is five to seven pairs if they're your main protection, or two or three if you're using them as backup to a cup or tampon. Because they take a day to air-dry, add a pair or two for the drying gap. Start with one pair to confirm the fit, then build the rotation.

How we grade and make money

How does the site make money?
Some links to brand stores are affiliate links - if you buy after clicking, the brand may pay a commission at no extra cost to you. The data is not for sale: sizes and millilitres come from published claims, are graded the same way for every brand, and don't change based on who pays us.
How do you decide the "best" picks in a guide?
Every best-of list is derived from the live catalog by a stated rule, and the rule is printed on the page. "Best for heavy flow," for example, is every in-stock product whose verified capacity clears a millilitre threshold, ranked by that capacity, capped at a few per brand so one brand can't dominate. There's no hand-picked editor's favorite and no sponsored slot - you can check the picks against the printed rule.
Do brands pay to rank higher?
No. Ranking is by the verified data (capacity in millilitres, or price, whichever the guide states), never by commission rate or any payment. A brand paying a higher affiliate rate does not move it up a list. If a claim can't be verified first-party, the product doesn't appear rather than getting a soft grade.

Something else? Ask us directly.