Period Underwear Absorbency: Levels, ML Capacity and Testing Guide
Absorbency is one of the most important specifications when developing a period underwear collection. It affects product performance, customer comfort, product positioning, and return rates.
However, terms such as “light flow” and “heavy flow” can mean different things to different manufacturers. For brands, wholesalers, and private-label buyers, the solution is to define absorbency with measurable specifications rather than relying on words alone.
In this guide, we explain how period underwear absorbency is measured in millilitres (ml), the differences between Light, Medium, Heavy, and Super Heavy levels, how absorbency testing works, and what to include in your OEM tech pack.
Table of Contents
- What Does Absorbency in ML Mean?
- Four Period Underwear Absorbency Levels
- How Style and Cut Affect Absorbency
- How Period Underwear Absorbency Is Tested
- Understanding the FEMTECHMAS Testing Protocol
- How to Specify Absorbency in an OEM Tech Pack
- How Shanhao Develops Absorbent Underwear
- Frequently Asked Questions
1. What Does Absorbency in ML Mean?
Absorbency in millilitres (ml) refers to the volume of fluid a pair of period underwear can hold before leakage occurs under specified laboratory testing conditions.
One millilitre is equal to one cubic centimetre. However, an absorbency figure is not a guarantee that someone can wear the underwear for a fixed number of hours without changing.
Real-world performance depends on several factors:
- Individual menstrual flow and flow rate
- Garment size and body fit
- Gusset length and width
- Absorbent core construction
- Physical activity and wearing conditions
- Washing and drying methods
There is another important point for buyers: two manufacturers may advertise the same ml capacity but use different test fluids or testing methods.
Therefore, when comparing suppliers, ask for the test protocol, test fluid, and whether the result represents maximum capacity before leakage or a comfortable-use target.
2. Four Period Underwear Absorbency Levels
Shanhao’s Flow Guide uses four absorbency levels: Light, Medium, Heavy, and Super Heavy. Each level is described using typical menstrual product equivalents over approximately 12 hours.
| Absorbency Level | Approximate 12-Hour Equivalent | Typical Use |
|---|---|---|
| Light | 1 pad, 1 tampon, or 1/2 menstrual cup | Spotting, light flow, beginning or end of a period |
| Medium | 3 pads, 4 tampons, or 1 menstrual cup | Regular everyday flow |
| Heavy | 4 pads, 5 tampons, or 2 menstrual cups | Heavy flow days and longer wear windows |
| Super Heavy | 5 pads, 7 tampons, or 2.5 menstrual cups | Heaviest days and overnight use |
Important: These are Shanhao’s usage-based categories, not laboratory ml measurements. Shanhao does not currently publish a confirmed ml capacity for each level. Buyers should request the confirmed figure for their exact product before printing absorbency claims on packaging or product pages.
Which Absorbency Levels Should Your Brand Offer?
You do not necessarily need all four levels in every product style. A practical starting collection could include:
- Light: For spotting and the beginning or end of a menstrual cycle.
- Medium: As the main everyday product for regular flow.
- Heavy: For heavier flow days and extended protection requirements.
- Super Heavy: For the highest-capacity designs and overnight use.
For overnight products, absorbency is only part of the design. A longer gusset and extended back coverage are also important because fluid moves differently when the wearer is lying down.
3. How Style and Cut Affect Absorbency
The underwear style has a direct effect on how much fluid the garment can hold. Gusset dimensions, core construction, and coverage all matter.
The following are general style averages provided in the source material. They are not guaranteed Shanhao specifications or substitutes for laboratory testing of the exact product.
| Underwear Style | Indicative Capacity | Typical Application |
|---|---|---|
| Thong | About 10 ml | Light flow or backup protection |
| Bikini and Brief | About 20–30 ml | Moderate flow protection |
| Boyshort | Up to 40–50 ml | Heavy flow or extended coverage designs |
A thong cannot necessarily hold the same volume as a boyshort because it has less gusset area. Even a dense absorbent core cannot completely compensate for limited coverage.
For OEM orders, specify both the underwear style and the absorbency level. Then confirm the tested capacity for that exact combination.
Explore our period underwear collection to discuss different styles and custom product development.
4. How Period Underwear Absorbency Is Tested
A useful absorbency test should evaluate more than the total amount of liquid a garment can hold. Three measurements are particularly important.
1. Wicking Speed
Wicking speed measures how quickly fluid moves away from the fabric surface and into the absorbent core.
Under the FEMTECHMAS protocol described below, a defined amount of fluid, 1.5 ml, is applied to the gusset and the absorption time is recorded.
Faster wicking can help the wearer feel drier. A product may have high total capacity but still feel uncomfortable if fluid remains near the surface.
2. Maximum Absorption Capacity Before Leakage
This test measures how much fluid the garment can hold before leakage is detected.
Fluid is added in controlled amounts until leakage occurs at the gusset edge. The result is recorded in ml and can be averaged across multiple test specimens.
This is the measurement commonly used as a headline absorbency figure. However, it should always be accompanied by the testing method and fluid used.
3. Rewet
Rewet measures how much absorbed fluid transfers back toward the skin side when pressure is applied.
A blotting material is pressed against the wet garment under defined conditions and the transferred fluid is measured.
Lower rewet can contribute to a drier wearing experience. This is why capacity alone does not tell the complete story of product performance.
Buyers should also ask whether samples were washed before testing, which garment size was tested, the gusset dimensions, and where leakage occurred.
5. Understanding the FEMTECHMAS Testing Protocol
FEMTECHMAS-6513-1:2025 is a testing protocol for reusable absorbent underwear. It covers products designed for menstruation and urinary incontinence.
The protocol described in the source material standardises three measurements:
- Wicking speed, measured in seconds.
- Maximum absorption capacity before leakage, measured in ml.
- Rewet, measured as fluid transferred back toward the skin side.
It uses different test fluids for different applications:
- Synthetic blood simulant: For menstrual underwear testing.
- 0.9% saline solution: For urinary incontinence underwear testing.
The described method uses controlled fluid additions, a 1 kg stamp, and three test specimens. Results are averaged to support comparison between products.
There are three important limitations buyers should understand:
- It is a testing protocol, not an ISO standard.
- It does not define universal ml thresholds for Light, Medium, Heavy, or Super Heavy.
- It is a test method, not a product certification.
Shanhao has not publicly stated that it tests to FEMTECHMAS-6513-1:2025. Buyers who require this protocol should specify it in their order and request the relevant test report. If a factory uses another method, ask for its name and testing details before comparing results.
6. How to Specify Absorbency in an OEM Tech Pack
One of the most common sourcing mistakes is writing only “heavy absorbency” in a product specification.
This description is not precise enough for production. Instead, buyers should define measurable requirements for every style.
OEM Absorbency Specification Checklist
| Specification | What to Confirm |
|---|---|
| Style / SKU | High-waist brief, bikini, boyshort, seamless, etc. |
| Absorbency level | Light, Medium, Heavy, or Super Heavy |
| Target capacity | ___ ml, confirmed by the factory |
| Test protocol | FEMTECHMAS-6513-1:2025 or another agreed method |
| Test fluid | Synthetic blood simulant or 0.9% saline, depending on use |
| Wicking time | Maximum agreed seconds per 1.5 ml |
| Rewet | Maximum agreed percentage or value |
| Gusset dimensions | Length, width, and front-to-back coverage |
| Core construction | Layer count and material specifications |
| Wash durability | Required wash cycles and washing method |
| Acceptance criteria | Agreed capacity, leakage, and rewet limits |
Also specify the intended use: daytime, overnight, sport, teen, postpartum, or incontinence. The same capacity figure may perform differently depending on the garment cut and intended wearing conditions.
Finally, request samples and test them in the correct size after the recommended first wash. A laboratory result is important, but it does not replace a real-world wear evaluation.
7. How Shanhao Develops Absorbent Underwear
Shantou Shanhao Knitting Co., Ltd. is a factory-direct functional underwear manufacturer based in Shantou, China. Established in 2015, the factory supports OEM and ODM product development for international brands, wholesalers, and distributors.
Patented 4-Layer Leak-Proof Core
Shanhao’s standard construction uses a patented four-layer leak-proof core. The multi-layer structure combines fluid-wicking and absorbent materials with a barrier layer designed to help prevent leakage. Confirm the exact layer construction for your selected style before ordering.
Absorbency Customisation
Buyers can discuss Light, Medium, Heavy, and Super Heavy product options, together with fabric selection, colours, logos, and retail packaging.
You can provide a tech pack or reference sample, or start with an existing design and adapt it for your brand.
Production Quality Control
Absorbency testing on the laminated core is one of Shanhao’s five quality-control gates:
- Fabric inspection at intake.
- In-line production monitoring.
- Absorbency testing on the laminated core.
- Stitching and sizing verification.
- Final packaging audit.
Ask the team for the test report covering your selected style and absorbency level.
Material and Certification Options
Material options include organic cotton, bamboo fibre, modal, and recycled nylon, depending on the product specification. Shanhao’s stated certifications include ISO 9001, OEKO-TEX STANDARD 100, GOTS, OCS, GRS, and BSCI. Buyers should request current certificates and confirm their scope and validity.
PFAS-Free Product Development
Shanhao states that PFAS is screened at fabric intake and that its leak barrier uses a breathable membrane without a fluorochemical coating. Finished goods are also screened for specified substances. Buyers can request relevant testing evidence for their exact product.
Read our related guide: PFAS-Free Period Underwear: How to Verify Your Supplier.
Shanhao’s monthly production capacity is approximately 500,000 units. Samples are available in approximately 5–7 business days, subject to product and material availability.
Frequently Asked Questions
1. What does absorbency in ml mean for period underwear?
It refers to the volume of fluid a garment can hold before leakage under specified laboratory conditions. It is not a guaranteed number of wearing hours.
2. What absorbency levels does Shanhao offer?
Shanhao offers four levels: Light, Medium, Heavy, and Super Heavy. They are currently defined through typical menstrual product equivalents over approximately 12 hours. Confirm the ml capacity for your exact style with the factory.
3. How much fluid can period underwear hold?
Capacity depends on the style, gusset dimensions, absorbent core, and testing method. General style averages in the supplied guide are about 10 ml for thongs, 20–30 ml for bikini and brief styles, and up to 40–50 ml for boyshorts. These are not guaranteed Shanhao specifications.
4. What is the difference between absorbency and wicking?
Absorbency describes how much fluid the product can hold, while wicking measures how quickly fluid moves away from the surface into the core.
5. What is rewet in period underwear testing?
Rewet measures how much absorbed fluid transfers back toward the skin side when pressure is applied. It is an important comfort measurement alongside capacity and wicking speed.
6. Does Shanhao use the FEMTECHMAS testing protocol?
Shanhao has not publicly stated that it tests to FEMTECHMAS-6513-1:2025. Buyers who require this method should specify it in their order and confirm the testing arrangement with the factory.
7. How should I specify absorbency for an OEM order?
Include the absorbency level, target ml, testing protocol, test fluid, wicking time, rewet, gusset dimensions, core construction, wash durability, and acceptance criteria. Confirm the target ml with the factory rather than copying another brand’s specifications.
Develop Your Custom Period Underwear Collection with Shanhao
Choosing the correct absorbency level is essential when developing a reliable period underwear collection. A clear specification helps your manufacturer understand the product, test the right performance requirements, and produce samples that match your expectations.
Send Shanhao your tech pack or reference sample and tell us whether you need Light, Medium, Heavy, or Super Heavy absorbency.
Our team can discuss the product construction, confirm the applicable ml target and testing method, and prepare a quotation for your OEM or ODM project.
Or explore our full OEM underwear range and wholesale information.
Shanhao Undies — Functional Underwear Manufacturing for Your Brand.