Measure your hip, not your memory
Take a soft tape around the fullest part of your hips and seat, level with the floor. That one number maps to a size on each brand's own chart. It is the right measurement for period underwear because the fit that matters for leaks is at the hip and seat, not the waist. If you are between sizes, size up: a snug-but-not-tight fit seals better, and a pair that digs in can wick and leak at the edges.
Brands run differently, so check each one
Because every brand sets its own chart, a hip that is a Medium at one brand can be a Large at another, and some brands run small or large against their own labels. A size that fits you in one brand is a starting point, not a guarantee, in the next. The size finder takes one hip measurement and returns your size in every charted brand at once, with a note on any brand that runs small or large, so you are not guessing brand by brand.
Quick answers
- How do I know my period underwear size?
- Measure around the fullest part of your hips and seat with a soft tape, then match that number to each brand's own size chart. Period underwear is sized on the hip, not your usual dress or bra size. If you are between sizes, size up for a better seal.
- Do period underwear run small?
- Some brands do and some run large; there is no single answer, because every brand draws its own chart. The same hip can be a Medium at one brand and a Large at another, so check each brand rather than assuming your usual size carries over.
- Should I size up in period underwear?
- If you are between sizes, yes. A snug but not tight fit seals against leaks; a pair that is too small can dig in and wick at the edges. Size up when in doubt.
Related guides
How does period underwear work?
Period underwear works through layers: a wicking top, an absorbent core, and often a leak-resistant barrier. What each does and why capacity is a ceiling.
Do period underwear actually work?
Yes, within their capacity, and that phrase is the useful part. What the independent lab data shows, where they work best, and the first-pair trick.
How many pairs of period underwear do you need?
A full cycle usually takes six to nine pairs, depending on your flow, how often you change and your laundry. How to work out your own number.
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.
Get your size in every brand at once
One hip measurement returns your size across every charted brand, with runs-small and runs-large notes, so there is no guessing brand by brand.