I have noticed in some reviews that "the rucksack feels more like 30l rather than being 35l, as stated" or something similar. I also heard that one company says the rucksack is larger than it really is, hence the weight:volume ratio looks better. Meanwhile, I always thought that's just some feeling-based argument against/for a company.
Now I found almost identical rucksacks, Oxford Aqua 25R (
http://oxford-products.com/us/products/Aqua-25R-Back-Pack.html ) and Lomo 30l (
http://www.ewetsuits.com/acatalog/drybag-daypack.html ). Given Aqua 25R has an additional mesh pocket on the side, no matter how you measure the capacity, you can't get less than the obviously same rucksack!
Okay. It does not matter which volume is the right one (I believe it's over 25l as I manage to squeeze more things into Lomo 30l bag than into other 25l rucksack). But how do companies measure them?
1. Does the rucksack have to be closed in order to measure the volume (very actual for roll top design)? It's quite clear when there's a floating lid involved and then company provides two numbers.
2. Do external pouches count?
3. Do hip belt pockets count?
4. What does not count?
5. If there are different sizes for the rucksack, which size to use? Some manufacturers provide information for each size, but still claim the same volume in the name of the sack.
6. Can the number be rounded or more realistically, by how many litres? There are so many 40l, 60l or 70l rucksacks and I'm certain they are not exactly that.
Of course, PPE stuff must be regulated and tested. It's not enough to claim that the sling is rated to 22kN - you have to prove it. Clearly, volume of the rucksack is not so life-dependant and no one will find out whether it's 32.3l or 37.6l.