Do Thinx and Knix use the same absorbency labels?

No. Thinx gives one figure for all its current styles (about 108 mL) and Knix publishes 4 current tiers (15–99 mL across the range). 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 is presented as true to size by the brand; Knix is one many people size up in. 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.

Verified against the brands' own pages 2026-09-05 · 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.

Not quite like for like: Thinx converts from a tampon count (grade B), Knix has pages that disagree with each other, so the range is shown (grade C). Cheapest in-stock pair today: Thinx $19, Knix $15.

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 gives one figure for 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: Thinx presents its fit as true to size keyed to your hip, while Knix labels its core styles true to size and reviews on its own product pages often suggest one size up in the leakproof styles. Both brands settled PFAS marketing class actions in 2023 without admitting wrongdoing; what those settlements did and did not establish is in our PFAS guide. On price, the two brands' live prices overlap heavily and move weekly, so check the catalog for today's numbers.

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 figure.
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 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 want a real four-step ladder to match a tier to each day, and you do not mind that reviews often suggest one size up in the leakproof styles.

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, the brand's own pages differ; range shown
Heavy	36–63 mL	Data quality grade C, the brand's own pages differ; range shown
Ultra	63–99 mL	Data quality grade C, the brand's own pages differ; range shown

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) poured blood slowly onto three pairs of one unnamed brand's super-absorbency underwear until it pooled: under 3 mL each, 2 mL on average. That is a different measurement, not a ranking of these claims. Use the numbers to compare the two brands like for like, not as a leak-free guarantee.

In stock now, from each brand

The pairs behind the ladders above, with today's price and a buy link. Every card also opens our graded data page for that pair.

Thinx

Everyday Comfort Hi-Waist

All Day· 108 mLData quality grade B, converted from tampon counts

Sizes XS–4X

$20

The full dataBuy at Thinx Search Amazon

Thinx

Everyday Comfort Lace Brief

All Day· 108 mLData quality grade B, converted from tampon counts

Sizes XS–4X

$20

The full dataBuy at Thinx Search Amazon

Thinx

Everyday Comfort Bikini

All Day· 108 mLData quality grade B, converted from tampon counts

Sizes XS–4X

$20

The full dataBuy at Thinx Search Amazon

Knix

40% off

Knix

Ultra Leakproof Zones+® Bikini - Last Call

Ultra· 63–99 mLData quality grade C, the brand's own pages differ; range shown

Sizes XS–XXXXL

$21.60$36

The full dataBuy at Knix Search Amazon

Knix

SuperPlus Leakproof® No-Show Bikini

Ultra· 63–99 mLData quality grade C, the brand's own pages differ; range shown

Sizes XS–XXL

$36

The full dataBuy at Knix Search Amazon

Knix

SuperPlus Leakproof® No-Show Boybrief

Ultra· 63–99 mLData quality grade C, the brand's own pages differ; range shown

Sizes XS–XXXXL

$39

The full dataBuy at Knix Search Amazon

How these picks are chosen

The 3 in-stock pairs from each brand with the highest verified capacity claim, from the live catalog, highest first. Where a brand publishes no per-product capacity we can verify (Ruby Love pulled its claims), its 3 cheapest in-stock pairs instead, and the cards say so by showing no millilitre figure. Prices and stock are from the weekly refresh; a brand with nothing in stock shows no cards.

Sizing

Thinx is presented as true to size by the brand; Knix is one many people size up in. 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: Thinx says its styles are completely true to size and to follow your hip measurement, measured at the widest point. By design they fit a little more snugly than regular underwear for leak protection, not undersized. The Teens line fits true to size and is narrower at the thigh.

Knix: Knix labels its core styles true to size and says its cotton pairs fit a little smaller. Reviews often suggest one size up in the leakproof styles, especially the bikini cut. The fabric is designed to feel snug at first and relax with wear.

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. These are saline-lab maximums, not leak-free guarantees; the one test with real blood (BMJ 2023, three pairs of one brand) measured a few millilitres before pooling, a different measurement from a claimed maximum.
Do Thinx and Knix use the same absorbency labels?: No. Thinx gives one figure for all its current styles (about 108 mL) and Knix publishes 4 current tiers (15–99 mL across the range). 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 is presented as true to size by the brand; Knix is one many people size up in. 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