Rucksacks. Litre size confusion

[Paul]

I'm looking for a new backpack.
Looking at a surplus site I found a nice little 37 litre bag.
I buy my milk UHT in cardboard cases of 12 x 1 litre bottles so looking at 3 crates, I'm thunking not too bad a size.
BUT, being cautious I check the physical size and on emailing got told the bag was 52 X 30 X 30 cms.
I'm no maths genius but capacity is Height X Width X Depth which comes to 46800 cu cms or 46.8 litres. (Lovely having convertors online isn't it.)

Advertised size 37 litres, and by their measurements its 46.8 litres???????

So I email and say something on the lines of either they've measured the wrong bag or the 37 litres is wrong. He promises to email back AND when he does the supplier has checked another bag out. "You're right" they say.
Same maths, different answer to advertised size or in this case 100 litre bag, 135 litres by calculation.
Can't understand why, never noticed this before.

Luckily it's the right way round i.e. more space than advertised so I would'nt have been short changed but now were both confused.
So my question to you experts is:-

How big is a rucksack litre?

[Bucket]

A couple of years back this question cropped up in the lab, but instead of doing the maths we went by the old fashioned method, and filled a 16 litre backpack (according to the label) with fluid (blue agar gel).

And the bag took 16 litres of gel. Bit on the heavy side to pick up though.

[Paul]

Thanks bucket, but why the discrepancy?
In your case, 16 liters would have been 25x25x25cms. or some variation round that theme.
Was that the bag size?

The junk I carry only weighs in at 7 kgs but bag, tarp, clothing, etc, is bulky so that's why I need a bigger size. It currently fits into a knackered old 30 litre bag BUT even then the bag size by maths (HxWxD) is 41 litres.

[Scythe13]

The difference is the internal volume compared to the external area coverage.

Although some parts of the bag will be 0.5ch thick, the padding on the back will be at least 1cm. If you have 1cm thickness on a 52x30cm area, that alone is a fair volume difference.

Also, the bag is probably not a perfect rectangular cube shape. It'll have smaller parts and things like that. The area you measure will probably be largest points.

Hope that helps.