Absorbency figures are the most quoted and least comparable numbers in the hygiene industry. "Absorbs 300 ml" can mean three entirely different measurements, and two suppliers quoting the same figure may be describing products that perform nothing alike. This guide explains what each common test measures, what it misses, and how to ask for data that can actually be compared.
Why the same number means different things
Absorbency is not one property. It is a family of properties that trade off against each other: a material can take up a lot of fluid slowly and release much of it under pressure, or take up less fluid quickly and hold nearly all of it. Test methods isolate different parts of that behaviour, and marketing usually quotes whichever method produces the largest number.
The second source of confusion is the test liquid. Deionised water is absorbed far more readily than saline, and saline more readily than urine, because dissolved ions reduce the osmotic driving force that makes superabsorbent polymer swell. A figure measured in pure water can be substantially higher than the same product's figure in saline. Always ask which liquid was used.
The measurements that matter, in order
| Measurement | What it describes | Where it is useful |
|---|---|---|
| Free swell capacity | Total uptake of a material with no applied load | Material screening only. It overstates real-use performance. |
| Centrifuge retention capacity (CRC) | Fluid retained after the sample is spun to remove what is loosely held | A better indicator of what stays in the core |
| Absorbency under load (AUL) | Fluid retained while a defined pressure is applied | The most relevant figure for products worn while sitting or lying down |
| Rewet | Fluid that transfers back to a dry filter paper pressed on the surface | Directly related to the "wet feeling" complaint |
| Strikethrough time | Time for a dose to disappear from the surface | Perceived dryness in the first seconds of use |
| Wicking / distribution distance | How far fluid travels along the core | Explains leaks that occur while the core still has capacity |
Absorbency under load, explained simply
Put a dry sponge on a table and pour water on it: it swells and holds a lot. Now put a weight on it: much of that water comes back out. A wearer's body weight does exactly this to an absorbent core. AUL measures how much fluid the material keeps while compressed, which is why it predicts real performance far better than free swell.
If a supplier can only give you one number, ask for AUL at a stated pressure, in saline, with the method named. If they cannot supply it, that is itself informative.
The repeat-dose question
Almost every published figure describes the first dose. Real use involves several: period products are worn for hours and receive multiple insults, diapers are worn until changed. Performance typically degrades across doses because the core nearest the topsheet saturates first and later fluid has to travel further.
A product that looks strong on a single-dose test can fail on the third, and this is where cheap cores separate from good ones. Ask for results at dose one, two and three, and for strikethrough and rewet at each.
Test methods vary, so name them
Several national and industry standards describe absorbency testing, and they do not use identical sample sizes, loads, liquids or timing. A figure without a method reference is not comparable to anything. In a request for quotation, state the method you want used, the liquid, the load and the interval, and require the same basis from every supplier. Where a supplier uses an in-house method, ask for a description of it; an honest in-house method you can read is more useful than a standard number you cannot interpret.
What lab data cannot tell you
- Fit. The best core leaks if the leg cuff gaps. No absorbency figure predicts fit.
- Comfort. Softness, breathability and bulk are sensory and are not captured by absorbency tests.
- Skin response. Requires clinical or user studies, not bench testing.
- Consistency. A single sample result says nothing about run-to-run variation. Ask how many samples were tested and across how many production runs.
This is why a proper evaluation runs bench data and wear testing in parallel and treats disagreement between them as a finding rather than an inconvenience. The material background is in Absorbent Core Basics; the commercial side of turning a specification into a purchase is in Sourcing Checklist.