Backpack Volumes - Marketing or Mathematics?

archieduh on 01/05/2022 - 20:23
Last edited 01/05/2022 - 20:41

So… I am looking for a spacious backpack..
And I am left confused by how capacity is labelled…
Isn't it simple L x W x D in cm each = [Vol] in cc = ([Vol in cc]/1000) in litres?

Take thie Kmart backpack for e.g.
https://www.kmart.com.au/product/45l-medium-hiking-pack-4306…

It is advertised as a 45L backpack.
The dimensions are :
53cm (H) x 32cm (W) x 20cm (D) = 33,920 cc ~ 34 Litres.
How is this a 45 Litres backpack ? Do I have something wrong in my mathematic fundamentals or it's just marketing (that 10 Litres for the straps and what not?)

Travel Backpack

Comments

  • aoeueoa on 01/05/2022 - 20:37
    +2

    It's meant to be measured by what can actually be fit inside, which includes a bit of bulge space inevitably. But often it'll be under the quoted spec.

  • elgrande on 01/05/2022 - 20:39
    +1

    Pockets

  • whatwasherproblem on 01/05/2022 - 20:42
    +1

    That LxWxD calculation assumes that it's a perfect rectangular prism, which is isn't. Even in the photo they've got on the website you can tell that the bag stretches out in the middle/ends to accommodate more stuff.

    But even if that was a useful first approximation, I'd argue that you're assuming a lot of the salespeople that they're not just flat-out exaggerating the numbers. It's always in their interests to slightly overestimate the capacity ("because what, you reckon anyone is literally going to fill it with 45L of liquid and complain when they find that they actually can't?") and slightly underestimate the dimensions ("My laptop is 32cm wide and it should absolutely be able to fit along the side of this bag, so make sure the width is 32cm (with a bit of leeway) and we're good").

  • star-ggg on 01/05/2022 - 21:30
    +1

    With hiking backpacks you can fit more by expanding the top out and put stuff between the hood and the main backpack. 34L + 10L sounds about right.

    A tip for hiking backpacks is to select one based off your height. It's a good indicator of whether you are packing too much or too little for your size.

  • archieduh on 01/05/2022 - 21:43

    Thanks for taking the time and replying all.
    I did think of the above but thought overstuffing may be a valid reason for 10-15% more rated volume but 33% seemed a bit overkill.
    But maybe it is the combination of all factors - expanding the top, pockets etc. that justify that rating.

    Just makes comparing different stores and brands a nighmare..

  • KaTst3R on 01/05/2022 - 22:25
    +1

    Bags are generally somewhat stretchable…if not, you will be carrying a box on your back and it will give the exact volume to spec.

    • archieduh on 01/05/2022 - 23:15

      Thanks. Get that but the difference in this is 1/3rd of theoretical rectangular volume which was a bit too much. And looked at 100s of bags today roaming around, few bags did stick out having a huge difference but it's not the norm and makes me wonder what makes these bags so special that they stretch out whereas more expensive and better reviewed ones don't..

      • ATTS on 02/05/2022 - 20:43

        Did you end up getting that kmart hiking bag? or got a different brand?

        • archieduh on 02/05/2022 - 22:06

          No haven't bought. I am looking at bags for city travel so still confused.

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