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Adult diaper absorbency is mainly tested with ISO 11948-1, known as the Rothwell method. The whole product is weighed dry, soaked in saline solution, drained, and weighed again. The difference is its total absorption capacity in grams. Buyers should also check absorption speed and rewet, because real-world leakage depends on design and consistency, not just capacity.
Key Takeaways
- ISO 11948-1 measures how much saline a whole product can hold. It is the standard way to compare absorption capacity in grams.
- A clinical study of 138 products found Rothwell capacity predicts leakage well enough to guide purchasing decisions.
- At the same lab capacity, all-in-one diapers leaked far less than inserts, so product type matters as much as the number.
- Products with inconsistent capacity between pieces leaked more often. Ask suppliers for batch variation, not just an average.
- Droplet scales on packaging are brand-specific. Ask for the gram value and the test method behind it.
What Is the Rothwell Method (ISO 11948-1)?
The Rothwell method, standardized as ISO 11948-1, is a whole-product test that measures the total absorption capacity of a body-worn incontinence product. It is the most widely used lab method for comparing how much liquid adult diapers and incontinence pads can hold. The standard remains in national use; Sweden, for example, publishes it as SS-ISO 11948-1:2022, identical to ISO 11948-1:1996.
The test works in four basic steps:
- Weigh the dry product.
- Immerse it in saline solution so the core is fully soaked.
- Let it drain under gravity for a set time.
- Weigh it again. The weight gained is the absorption capacity.
The method is simple and repeatable, which makes it useful for comparing products and checking batches. Its limit is what it leaves out. It measures the capacity of the absorbent materials, but it does not measure how fast liquid is absorbed, how dry the surface stays under pressure, or how well leg cuffs and fit prevent leaks.
How Well Does Lab Capacity Predict Leaks?
Lab capacity predicts leakage reasonably well, but not perfectly. Researchers at University College London compared Rothwell capacity with clinical leakage results for 138 disposable diapers and inserts for heavy urinary incontinence. They published the results in Medical Engineering & Physics (Cottenden et al., 2003). The correlation reached r ≤ 0.87, which the authors judged “still strong enough to help with purchasing choices”.
The most useful finding for buyers is that product type changes what a gram of capacity is worth. For the same Rothwell capacity, all-in-one diapers performed far better than inserts:
| Diaper capacity | Performed as well as an insert holding |
|---|---|
| 450 g of urine | 300 g |
| 300 g of urine | 100 g |
In practice, compare capacity numbers only within the same product type. A 300 g insert is not equivalent to a 300 g brief.
Why Does Product Consistency Matter?
Product consistency matters because diapers whose capacity varies a lot from piece to piece leak more often. The same 2003 study found that the coefficient of variation in Rothwell capacity, a measure of consistency, had a significant effect on leakage for diapers. The least consistent diapers were about 10 percentage points more likely to leak than the most consistent ones.
The study also found that diapers were about 10 percentage points more likely to leak at night than during the day. This is one reason many care settings use a higher-capacity product for overnight wear. For practical night-time advice, see our guide to preventing nighttime leaks.
For a buyer, this means an average capacity figure is not enough. Ask the supplier for results across several pieces and batches, and for the spread between them.
What Other Absorbency Tests Should Buyers Ask For?
Buyers should ask for tests that cover speed, dryness, and leakage, because capacity alone does not show how a product performs during wear. The international standard ISO 15621:2026 gives general guidelines for evaluating absorbent incontinence products and combines user needs with laboratory test methods. Useful tests include:
| Test | What it shows | Why it matters |
|---|---|---|
| Total absorption capacity (ISO 11948-1) | How much liquid the product can hold | Matching capacity to incontinence level |
| Absorption speed (acquisition time) | How fast liquid moves into the core | Slow uptake causes leaks during heavy voids |
| Rewet under pressure | How much liquid returns to the surface when pressed | Skin dryness and comfort when sitting or lying |
| Retention under pressure | How much liquid stays locked in the core under load | Real use involves body weight on the product |
| Leakage or mannequin testing | Whether liquid escapes at the edges | Captures the effect of cuffs and fit, which capacity tests ignore |
Lab tests should be confirmed with a product trial on real users. Our nursing home procurement guide explains how to run one.
How Should You Read Absorbency Claims on Packaging?
Absorbency claims on packaging, such as droplet scales or “maximum” and “super” labels, are set by each brand and are not standardized between brands. Four droplets from one brand may hold more or less than four droplets from another.
To compare products fairly:
- Ask for the absorption capacity in grams, together with the test method used.
- Compare only results produced by the same method.
- Compare within the same product type: briefs with briefs, pull-ups with pull-ups.
- Check that the size tested matches the size you will buy. Capacity usually rises with size.
If you are building your own range, use one consistent absorbency scale across all products so customers and care staff can trade up or down easily. Our guide to adult diaper sizing and absorbency shows how end users read those labels.
How Do You Compare Two Suppliers’ Absorbency Data?
Compare suppliers by asking both for the same tests, on the same product type and size, from recent production batches. This checklist keeps the comparison fair:
| Ask for | Why |
|---|---|
| Rothwell (ISO 11948-1) capacity in grams, per size | A common baseline for capacity |
| Results for at least several pieces, with the variation shown | Consistency predicts leakage |
| Absorption speed and rewet results | Covers speed and dryness, which capacity misses |
| Test dates and batch numbers | Shows the data reflects current production |
| Who ran the test: in-house lab or third party | Third-party results are easier to trust when comparing |
| Samples from the same batch | Lets you repeat the test yourself |
If the numbers look similar on paper, a short user trial is the best tiebreaker.
How Does TianZheng Test Adult Diaper Absorbency?
TianZheng’s adult diapers use a dual-layer core with a 100% SAP upper layer, designed to lock in fluid quickly and reduce rewet. Random batch samples are lab-tested for:
- liquid absorption capacity
- wicking speed
- rewet values
- seal elasticity under pressure
Raw materials are also tested before they enter production. For private label and wholesale projects, test reports for the specific product and size can be shared with buyers so they can compare suppliers on equal terms.
Related reading
Frequently Asked Questions
What is the Rothwell method?
The Rothwell method, standardized as ISO 11948-1, measures the total absorption capacity of an incontinence product. The product is weighed dry, soaked in saline, drained, and weighed again; the gain in weight is its capacity.
Does a higher absorption capacity mean fewer leaks?
Generally yes, within the same product type, but not always. Fit, leg cuffs, absorption speed, and product consistency also affect leaks. Research found all-in-one diapers leaked far less than inserts with the same capacity.
How many grams should an adult diaper absorb?
It depends on the user’s incontinence level and the product type. Heavy-incontinence briefs hold much more than light pads. Compare capacity in grams within the same product type and size, using the same test method.
Are droplet absorbency ratings standardized?
No. Droplet scales and labels like “maximum” are set by each brand. Ask for capacity in grams and the test method to compare products fairly.
What is rewet in adult diapers?
Rewet is the amount of liquid that comes back to the surface when the product is pressed after absorbing. Low rewet keeps skin drier and reduces the risk of irritation.