This article covers testing methodology and product selection. It is not medical advice. If you have concerns about your bladder, speak to your GP.
If you have ever bought a pad rated at 1600ml and watched it leak inside an hour, you were not misreading the box. The number is accurate. It just does not mean what you assumed.
Where the number comes from
That headline figure comes from one laboratory test: ISO 11948-1, universally known as the Rothwell test. The pad is weighed dry. Features like the leak-guard elastics, the very things that stop leaks in real life, are cut off first. The pad is laid absorbing-side down on a drainage tray and fully submerged in a saline solution for thirty minutes, lifted out, drained for five, then folded into a beaker and weighed again. Wet weight minus dry weight is the total absorption capacity. That is the number on the pack.
The industry body that represents the manufacturers, the AHPMA, is refreshingly blunt about its limits. The test, in their words, "does not reflect the way in which urine is passed into the product in use," and "when used to assist in purchasing decisions it should not be used as the main criteria for measuring product performance." Even on its own terms it is not perfectly tidy: validation work has found meaningful differences between testing laboratories, of the order of eight to thirteen percent. So the headline figure is a consistent lab benchmark, not a promise about your day.
The gap between the lab and your body
In real life urine does not arrive as a calm half-hour soak. It arrives in bursts, fast, under pressure, and more than once. The pad is held against you by clothing or its own strips, and it is dealing with movement, body heat and your particular shape. The Rothwell test captures none of that, because it was never meant to. It exists to give manufacturers a reproducible figure and procurement teams a common unit. It does that job. It is just a different job from predicting whether a pad survives your commute.
The upshot is that two pads with identical Rothwell figures can behave completely differently worn. Fit decides how well it conforms. The backsheet decides breathability and whether you sweat. The acquisition layer decides how fast the surface goes dry again after a void. None of those show up in the number, and the leak guards that matter most were snipped off before testing even began.
The bigger hole: the evidence barely exists
Step back from any single product and the picture is worse. The International Continence Society, the global expert body, reported through its 7th Consultation on Incontinence (2023) that across thirteen categories of continence product reviewed, only three (products for preventing incontinence-associated dermatitis, urinary catheters, and female mechanical devices) had so much as a single new randomised controlled trial since the previous consultation in 2017. The companion paper on research priorities, by Fader and colleagues (2024), makes the same point about how thin the comparative evidence is.
Read that again, because it is the heart of it. This is not manufacturers behaving badly. It is that the research needed to say "this product genuinely outperforms that one for this kind of user" mostly has not been done. Expert opinion and individual preference are filling a gap that clinical trials have not closed, and that gap quietly shapes what the NHS stocks and what you buy.
What this means when you choose a pad
The absorbency figure is a starting point and nothing more. It tells you the ceiling under ideal lab conditions. It does not tell you how long you can wear the thing before it leaks, because that depends on how much you void at once, how often, how well it fits, how much you move and what you wear over it. No standard test tells you whether a pad stays put on a two-hour train, whether the backsheet makes you sweat in July, whether the adhesive irritates by evening, or whether the surface still feels dry after the third void. Those are the things that decide whether a pad actually works for you, and they are exactly what the testing system is not built to measure.
The AHPMA itself recommends that proper evaluation combine laboratory testing, real usability testing and clinical outcomes. That is, more or less, the case for testing products the way they are actually used, on real bodies, across real days. Which is what we do, and why the number on the box is where we start, not where we stop.
Not medical advice. General information only.