Period underwear is a popular, reusable alternative to traditional menstrual products, offering comfort and sustainability. The ability of this specialized underwear to manage menstrual flow depends entirely on its capacity to absorb and contain fluid without leaking. Understanding how much fluid these garments can hold is necessary for consumers to choose the right protection for their individual needs. This capacity is measured through standardized metrics that translate laboratory results into practical, real-world flow management.
Standard Measures of Absorbency
The capacity of period underwear is primarily quantified using two related metrics: milliliters (mL) and the equivalent number of tampons or pads. Manufacturers use these measurements to provide a clear comparison with familiar disposable products. For example, a light-absorbency pair might hold the equivalent of one regular tampon, while a heavy-flow or overnight style may claim the capacity of up to 12 regular tampons.
The actual fluid volume absorbed ranges significantly, typically from around 5 mL for very light styles up to 71 mL or more for maximum absorbency designs. To establish these figures, products undergo laboratory testing, often using a synthetic fluid. The testing protocols aim to determine the maximum amount of liquid the absorbent core can hold before leakage occurs under simulated pressure, such as the FEMTECHMAS-6513-1 standard.
The volume in milliliters is the most precise measure, with one regular tampon generally holding about 5 mL of fluid. The “tampon equivalent” is often the most accessible metric for consumers, communicating the product’s function in familiar terms. It is important to note that testing with saline or synthetic fluids may not perfectly replicate the absorption of actual menstrual blood, which has a different viscosity.
Matching Capacity to Flow Level
Translating the measured capacity into practical use requires matching the underwear’s absorbency to the user’s specific flow level. Period underwear is generally categorized into classifications such as light, moderate, and heavy, each corresponding to a distinct volume range. Light absorbency garments manage spotting or the final days of a period, typically holding less than 5 mL of fluid, which is similar to one tampon or less.
Moderate absorbency styles are suited for regular flow days and usually absorb between 10 mL and 15 mL, offering protection equivalent to two to three tampons. These are often suitable for all-day use for many individuals, with a recommended wear time of up to 12 hours before saturation.
Heavy or overnight underwear provides the highest capacity, starting at about 20 mL and extending up to 71 mL for specialized designs. These high-capacity options are intended for the heaviest days of the cycle or for overnight wear when the user may not change the product for an extended period. For those with a very heavy flow, the wear time for any absorbency level will be less than the maximum 12-hour window and may require changing the garment more frequently. Understanding the milliliters a product holds allows for a more personalized flow management strategy.
Design Elements That Determine Absorbency
The capacity of period underwear is achieved through a sophisticated, multi-layered construction, distinct from ordinary undergarments. The layer closest to the skin is a wicking fabric, often made of technical microfibers or specialized cotton blends, which quickly draws fluid away from the body. This wicking action is necessary to maintain a dry sensation and prevent skin irritation.
Beneath the wicking layer lies the absorbent core, which is the powerhouse for fluid retention. This core is typically made from highly absorbent materials like specialized polymers, bamboo, or dense microfibers designed to trap and lock the menstrual fluid within the fabric structure. The thickness and composition of this core directly correlate with the total volume the underwear can hold.
The final layer is a leak-proof barrier, often a thin film of waterproof material such as polyurethane laminate (TPU), which acts as a safeguard against leaks soaking through to clothing. The overall garment design also impacts capacity; styles with extended coverage, such as sleep shorts or high-waisted briefs, incorporate the absorbent core further up the back and sometimes the front, which increases the total available area for absorption.