Specifying absorbency — what 60 to 400 ml actually means

Absorbency figures are the most quoted and least comparable numbers in the category. One catalogue says 120 ml, another says “super”, a third says “absorbs 8 times its weight”. They can all be true, because none of them share a test method. Before a number goes into your specification, three questions decide whether it means anything.

What the millilitre figure actually measures

Almost every factory tests absorbency the same way in principle: place the product on a flat surface, dose it with a fixed volume of liquid, and record either how much it holds before leaking or how quickly the surface returns to dry. The differences that break comparability are all in the details.

• The liquid. Distilled water absorbs faster than 0.9 per cent saline, and both differ from a viscous test fluid designed to mimic menstrual fluid. Water-based figures are typically the highest and the least useful.
• Loading position. A centre pour gives a different result from a pour at the rear third, which is where overnight products are actually challenged.
• Rate and interval. One 10 ml dose gives a different number from five 2 ml doses at 30-second intervals, because the core needs time to distribute.
• Endpoint. “Capacity before leakage” and “surface dryness after 60 seconds” are two different products with two different numbers.

There is no single harmonised method for menstrual pads in the way there is for some absorbent medical devices, so the honest position is this: an absorbency figure is only comparable within one supplier’s method. Ask for the method in writing, and when you switch suppliers, re-baseline rather than assuming the numbers still mean the same thing.

The working ranges behind rating systems

Most factories map an internal millilitre range onto a rating scale. The mapping below is the pattern seen across the category, not a standard, and it should be treated as a starting point for a conversation rather than a specification:

Rating	Typical saline range	Where it is used	Length band
1 / Regular	60 to 90 ml	Light day, panty liner territory	up to 245 mm
2 / Super	90 to 140 ml	Standard day use	245 to 285 mm
3 / Super plus	140 to 200 ml	Heavier day, long wear	285 to 320 mm
4 / Overnight	200 to 280 ml	Night use, extended wear	320 mm and above
5 / Maternity	280 ml and above	Postpartum, clinical use	350 mm and above

The overlap is real. A well-built 285 mm product with a dense core can beat a badly built 320 mm product, which is why length alone is a poor proxy for absorbency and why the length band in the right-hand column is guidance rather than a rule.

Where the capacity actually comes from

Three components share the job, and their proportions explain most of the price difference between two pads of the same length.

Fluff pulp

Wood pulp fluff provides the void volume that takes liquid in quickly. It is cheap per gram and bulky. More pulp means faster uptake and a thicker product; it also means the pad collapses under pressure and can feel wet on the surface.

SAP

Superabsorbent polymer locks liquid away under pressure, which is what keeps a surface feeling dry after the pad is loaded. It costs many times more per gram than pulp. A core with 0.3 g of SAP and one with 1.2 g can look identical on the shelf and behave completely differently at hour four.

The distribution layer

The layer between top sheet and core spreads a concentrated dose along the length of the pad instead of letting it sit in one spot. It is the component most often removed in cost engineering, and its absence is what produces the “it leaked at the side even though it was not full” complaint.

A pad that holds 200 ml but releases it under sitting pressure is worse than a pad that holds 120 ml and keeps it.

What to write in the specification

• Test liquid and concentration, stated explicitly.
• Dose volume, number of doses, and interval between them.
• Loading position (centre, or rear third for overnight).
• Endpoint definition, and whether rewet under pressure is measured.
• Pulp weight, SAP weight, and whether SAP is distributed or patched.

Four lines of method plus three lines of core construction give you a specification that can be re-quoted, re-tested and held against a production batch. A bare millilitre figure gives you a marketing claim.