Do Thinx and Knix use the same absorbency labels?

No. Thinx publishes 3 current tiers (108 mL across the range) and Knix publishes 4 (15–99 mL). Even where the words overlap, the millilitres differ, which is exactly why this page compares them in millilitres with a data-quality grade on every number.

How do Thinx and Knix sizes compare?

Thinx runs true to size; Knix tends to run small (many people size up). Both size from your hip measurement, so the size finder gives your size in each brand directly rather than guessing from the other brand's number.

Is Thinx or Knix better?

Neither is better across the board - they differ on measurable things. Compare three numbers on this page: the millilitres at the tier you actually need (not the tier names), how each brand's sizing runs for your hip measurement, and current prices in the catalog. The right brand is the one whose numbers fit your flow and your body, and that answer differs by person.

Thinx vs. Knix

How Thinx and Knix period underwear compare on the only thing that travels across brands - capacity in millilitres - plus how each one's sizing runs. No brand's thumb on the scale.

Updated 2026-06-12 · Every capacity is the brand's own claim, graded and dated

Thinx and Knix top out at nearly the same claimed capacity - about 108 mL for Thinx and 99 mL for Knix at their highest current tiers. The bigger difference is how they label the steps below the top: compare the millilitres at each tier rather than the words, because a tier name like "Heavy" is not the same capacity at the two brands.

The verdict, in plain terms

Which holds more is the wrong question: the ceilings are close, and only one of them is a brand's own number. Thinx states "up to 12 regular tampons' worth" and publishes no millilitre figure for its period underwear; on the FDA regular-tampon ceiling that is about 108 mL, which is how our table carries it (computed, grade B). The "up to 100 mL" on Thinx's site is its bladder-leak line, not this one. Knix Ultra states up to about 99 mL, the highest brand-stated figure in our table. The real difference is how each brand makes its claim. Thinx publishes one blanket maximum - the same "up to 12 regular tampons' worth" figure on every current style, from All Day to All Night to Teens - with no per-tier ladder. Knix publishes a four-step ladder (Light through Ultra, about 5 to 99 mL), which is genuinely more useful for matching a tier to a day, except that its own blog and collection pages state different tampon counts for the top tiers, which is why those numbers carry a C grade in our table.

The other differences you can check rather than take on faith: sizing runs opposite ways - reviews consistently report Knix's leakproof styles run small even though the brand labels core styles true to size, while Thinx presents its fit as true to size keyed to your hip. Both brands settled PFAS marketing class actions without admitting wrongdoing - Thinx for up to $5 million (final court approval June 2023), Knix for $1.4 million - and both settlements resolved marketing claims, not a court finding that either product injured anyone. Both now sit inside conglomerates: Kimberly-Clark majority-owns Thinx, and Essity owns 80% of Knix. On price, the two brands' live prices overlap heavily and move weekly, so check the catalog for today's numbers; the stable arithmetic is that Thinx's blanket 12-tampon claim, about 108 mL computed, makes its pairs inexpensive per millilitre even when the sticker price is not the lower one.

Pick Thinx if

You want the highest ceiling in our table - about 108 mL, our computation from Thinx's "up to 12 regular tampons' worth" at the FDA regular-tampon ceiling (grade B); Thinx publishes no millilitre figure of its own.
You prefer one simple maximum over tier arithmetic: every current Thinx style carries the same blanket claim.
You want sizing the brand itself pegs true to size, read from your hip measurement.

Pick Knix if

You want to buy by tier: real Light and Moderate steps (about 5 to 36 mL) instead of paying for a 108 mL ceiling on a light day.
You want the deepest catalog we track - Knix is the biggest brand in our data by product count, across cuts and its Kt teen line.
You do not mind sizing up: reviews consistently report the leakproof styles run small.

Absorbency tiers, side by side

Thinx

See Thinx

Tier	Capacity	Grade
All Day	108 mL	Data quality grade B, converted from tampon counts
All Night	108 mL	Data quality grade B, converted from tampon counts
Overnight (Teens)	108 mL	Data quality grade B, converted from tampon counts

Verified against Thinx's own pages 2026-09-05

Knix

See Knix

Tier	Capacity	Grade
Light	5–15 mL	Data quality grade B, converted from tampon counts
Moderate	18–36 mL	Data quality grade C, conflicting / unconfirmed claim
Heavy	36–63 mL	Data quality grade C, conflicting / unconfirmed claim
Ultra	63–99 mL	Data quality grade C, conflicting / unconfirmed claim

Verified against Knix's own pages 2026-06-11

Claims, not lab measurements

Every capacity above is the brand's own maximum claim, typically from a saline saturation test. The only published study with real blood components (BMJ Sexual & Reproductive Health, 2023) measured period underwear holding under 3 mL each, 2 mL on average, before leaking - far below these maximums. Use the numbers to compare the two brands like for like, not as a leak-free guarantee.

Sizing

Thinx runs true to size; Knix tends to run small (many people size up). Both brands size from your hip measurement, so rather than converting one brand's size into the other's, the size finder gives you your size in each directly.

Thinx: Runs true to size. Thinx states styles are "completely true to size" and recommends following the HIP measurement (measured at widest point, derriere included) for the most accurat

Knix: Brand labels core styles "True to Size" on product pages (e.g. Essential High Rise). However, customer reviews consistently report the leakproof styles run small / recommend sizing

Quick answers

Is Thinx or Knix more absorbent?: Thinx and Knix top out at nearly the same claimed capacity - about 108 mL for Thinx and 99 mL for Knix at their highest current tiers. The bigger difference is how they label the steps below the top: compare the millilitres at each tier rather than the words, because a tier name like "Heavy" is not the same capacity at the two brands. And remember these are saline-lab maximums, not leak-free guarantees - the only blood-component study measured far lower real capacity.
Do Thinx and Knix use the same absorbency labels?: No. Thinx publishes 3 current tiers (108 mL across the range) and Knix publishes 4 (15–99 mL). Even where the words overlap, the millilitres differ, which is exactly why this page compares them in millilitres with a data-quality grade on every number.
How do Thinx and Knix sizes compare?: Thinx runs true to size; Knix tends to run small (many people size up). Both size from your hip measurement, so the size finder gives your size in each brand directly rather than guessing from the other brand's number.
Is Thinx or Knix better?: Neither is better across the board - they differ on measurable things. Compare three numbers on this page: the millilitres at the tier you actually need (not the tier names), how each brand's sizing runs for your hip measurement, and current prices in the catalog. The right brand is the one whose numbers fit your flow and your body, and that answer differs by person.

Skip the comparison - get your answer

Tell the translator your flow and it gives the covering tier at both Thinx and Knix. Tell the size finder your hip and it gives your size in each.

What absorbency do I need?Find my size