Period underwear vs. a menstrual cup: capacity, cost, comfort, and when to use both

What each one actually is

A menstrual cup is a small silicone cup you fold, insert, and empty. Period underwear has an absorbent panel sewn into the gusset, which you wear and then wash. The FDA draws the line in one sentence: tampons and menstrual cups collect menstrual flow inside the vagina, and menstrual pads and period underwear absorb the flow outside the vagina. Nearly every difference below follows from it.

It is also where the toxic shock question sits, and the TSS guide covers why the risk is associated with internal products. The FDA adds two notes about cups: ask your gynecologist about using one with an intrauterine device, and if you get itching, rash, pain, swelling, or discomfort, stop using it and contact your healthcare provider.

Capacity, side by side, in millilitres

Cup brands publish one figure per size, and the three we read agree closely. Saalt gives 30 ml for its Regular cup and 25 ml for the Small. Flex lists 22 mL for size 1 and 30 mL for size 2. Mooncup says up to 29.3 ml for size A and 28.8 ml for size B. So a cup sits in a band of roughly 22 to 30 mL, filled to the rim. Saalt sells period underwear too and is in our catalog, so it publishes on both sides.

Period underwear has a ladder instead. Tier ceilings in our table run from about 5 mL to about 108 mL: Knix Light at 5 to 15 mL, middle tiers mostly 15 to 30 mL, Modibodi Super at 50 mL, WUKA Super Heavy at 60 mL, and Knix Ultra at up to about 99 mL, the highest any brand states in millilitres. The 108 mL top is computed, not published: Thinx says its period line holds up to 12 regular tampons' worth and prints no millilitre number. A cup lands in the middle: above every light tier, above the 20 mL that Saalt and WUKA both call Heavy, and below the top tier at most brands. The tier decides, not the category.

One caveat sits over all of it: capacity figures are the makers' own, usually measured with liquids thinner than blood. The one published test using real blood filled 21 products with expired human packed red blood cells and reported menstrual discs holding the most at 61 mL on average, menstrual underwear the least at 2 mL, and tampons, heavy or ultra pads and cups at approximately 20 to 50 mL. Our reading: treat an underwear claim as a ceiling and pick a tier with room to spare.

Hours, and the learning curve

On hours the brands agree with each other and with the regulator: Saalt, Flex and Mooncup all say up to 12 hours, Saalt adding that the cup is then emptied, cleaned and reinserted. The FDA says to remove and empty the cup according to the instructions for use, typically every 8 to 12 hours. Period underwear has no equivalent figure, because hours there are capacity divided by your own flow rate, which the hours-of-wear guide works through.

The learning curves differ in kind. The FDA says it can take practice to insert and remove menstrual cups, and that a reusable cup should be washed according to the instructions before it is used again. That is the awkward part away from home: emptying in the stall, rinsing at a shared sink. Period underwear asks instead that you carry a used pair home in a wet bag; the away-from-home guide covers both.

What each one costs

A cup is one purchase. As of 2026-09-05, Saalt lists its cup at $32, Flex at $29.99, and Mooncup, a UK brand, at £23.50, and all three say a cup lasts up to 10 years. The FDA makes the point without a number: a menstrual cup can last for years, while disposable options must be bought for each cycle. Period underwear is a rotation instead. How many pairs a period takes is in the pairs-needed guide, how long a pair lasts in the lifespan guide, and today's prices are in the catalog rather than printed here, where they would go stale. The cost guide does the running math from live prices.

Using both, and who tends to prefer which

A cup with a pair underneath is the most common setup among people who own both. The cup collects, and the pair catches whatever gets past it when the cup shifts or fills early. That does not need a heavy tier. Overnight is the exception, where a higher tier and rise do the work, and the backup guide covers the combinations.

Preference follows the trade you mind least: insertion and a shared sink on one side, a wet bag and laundry on the other. Anything that hurts is a question for a clinician.

Sources

The facts above trace to:

• Saalt capacity (30 ml Regular, 25 ml Small), 12-hour wear, $32 and 10-year life: Saalt, "Soft, Flexible & Reusable Menstrual Cup." Read 2026-09-05.
• Flex capacity (22 mL size 1, 30 mL size 2), 12-hour wear, $29.99 and 10-year life: Flex, "Flex Cup." Read 2026-09-05.
• Mooncup capacity (29.3 ml size A, 28.8 ml size B), 12-hour wear, £23.50 and 10-year life: Mooncup, "The Mooncup Original Menstrual Cup." Read 2026-09-05.
• Emptying every 8 to 12 hours, insertion practice, washing before reuse, the intrauterine device and provider notes, the internal-versus-external split, and all menstrual products marketed in the United States being subject to FDA regulation as medical devices: US Food and Drug Administration, "Menstrual Cups" and "Menstrual Product Options, Facts, and Safe Use." Read 2026-09-05.
• Real-blood capacities (discs 61 mL, menstrual underwear 2 mL, tampons, pads and cups 20 to 50 mL): DeLoughery et al., "Red blood cell capacity of modern menstrual products," BMJ Sexual & Reproductive Health, 2023. Read 2026-09-05.
• Underwear tier ceilings in millilitres, including the computed Thinx figure and its grade: our absorbency table. Current pair prices: our live catalog.