Menstrual pads are designed to absorb and contain menstrual fluid, providing a solution for managing monthly flow. The capacity of these products is determined primarily by the advanced materials science within their layers, not just their physical dimensions. Understanding how much liquid a pad can hold involves looking beyond the packaging label to the internal mechanics and standardized testing methods that govern absorbency ratings.
The Internal Mechanics of Pad Absorbency
A menstrual pad’s ability to absorb and retain liquid relies on its multi-layered construction. The topsheet, the soft, non-woven fabric against the skin, is highly permeable, quickly allowing fluid to pass through and preventing rewetting, which maintains a feeling of dryness.
Beneath the topsheet is the acquisition distribution layer (ADL), which rapidly draws the fluid away and spreads it evenly across the core. This distribution prevents liquid from pooling and causing premature leakage.
The absorbent core is a blend of cellulose pulp and Super Absorbent Polymers (SAPs). SAPs, often sodium polyacrylate, are the primary materials responsible for high capacity, absorbing liquid up to 30 times their weight and transforming it into a contained, gel-like substance.
The cellulose pulp provides a structural matrix, wicking fluid toward the SAP particles and preventing “gel blocking.” The final layer is the backsheet, an impermeable film that acts as a barrier to prevent absorbed fluid from leaking onto clothing.
Industry Standards for Measuring Capacity
Manufacturers determine a pad’s absorbency rating through specific laboratory testing protocols. Standardized testing uses a saline solution—a mixture of water and salt—rather than actual menstrual fluid. This test liquid ensures consistency across different product batches and brands, despite differing significantly from the complex composition of menstrual fluid, which contains blood, mucus, and tissue.
Unlike tampons, which have FDA-mandated standardized labeling, pads lack a single, universally required testing standard. Industry guidelines specify a test method where fluid is introduced at a controlled rate, measuring the volume the pad holds before leakage.
Absorbency designations on packaging, such as “Regular,” “Super,” or “Maxi,” are relative indicators correlating to tested liquid volumes. For instance, technical specifications suggest a “Regular” pad absorbs 15 to 20 milliliters (mL) and an “Extra Large” pad absorbs 30 to 40 mL under test conditions. The lack of a uniform standard means the precise volume for a “Heavy” designation can vary between manufacturers.
How Pad Size and Type Influence Total Hold
The physical design of a pad, alongside its core materials, directly influences its total holding capacity. Pads are categorized by length, thickness, and coverage area to correspond with different flow levels. A light or regular pad is shorter and thinner, designed to handle smaller fluid volumes over a few hours.
Larger pads allow for a greater volume of absorbent core material, including more Super Absorbent Polymers. Overnight or maxi pads are significantly longer and wider, often featuring a flared back end for greater coverage while lying down.
This increased surface area and volume allow these pads to achieve higher capacity ratings of 30 to 40 mL or more. A fully saturated regular daytime pad might hold around 5 mL of fluid, while a fully soaked overnight pad could hold between 10 and 15 mL in real-world saturation, though their maximum tested capacity is higher.
Translating Absorbency Capacity to Real-World Flow
The maximum capacity stated by manufacturers is much higher than the amount of fluid a pad absorbs in typical use before being changed. Normal menstrual flow ranges between 10 and 80 mL of total fluid over the entire cycle, averaging around 35 mL. Since menstrual fluid is a mixture of blood, tissue, and secretions, only about half of the total volume is actual blood.
The measured capacity of a single maxi pad (30–40 mL) can exceed the entire average flow of a monthly cycle. However, a pad is rarely used to its maximum saturation point due to factors like comfort, hygiene, and fluid viscosity.
Most people change a pad based on a feeling of wetness, which indicates the fluid is pooling on the surface or has not been fully absorbed. The functional capacity—the amount absorbed before it is changed—is therefore a more practical measure than total capacity.
Menstrual fluid can contain clots, which are not easily absorbed and can sit on the surface, leading to a perceived failure even when the core is not full. Changing a pad every one to two hours is a clinical indicator of heavy menstrual bleeding, defined as losing over 80 mL of blood per cycle.