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I am planning my first multi-day hike. I haven't selected a pack yet. I do not know which size pack I need. I see that packs are rated in liters. That got me thinking since some white garbage bags are rated in liters.

Can I simply use different size garbage bags and try to fit all my gear to approximate my required pack size? Say if it fits in a 40L bag, I need a pack around 40L, or are there other considerations in play?

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BTW: when trying on backpacks in a shop, have them fill it with weight so you have a correct feeling of how the bag works for you. –  Sdry Mar 8 '13 at 7:57
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3 Answers 3

up vote 6 down vote accepted

The garbage bags are soft and flexible, so they'll fit around stuff a lot easier than a rucksack. As well as being stiffer a rucksack will be divided into different compartments. The volume will include the side pockets and the lid space. Something like the sleeping bag will take up a large space and then other stuff will have to fit around it. Things may not fit very neatly. The volume is not used so efficiently.

So if it fits in a 40L bag then the rucksack will have to be bigger. You could take your largest items and make sure they fit in the pack and you'll see how much space is left for your other stuff. If it's your first trip you'll probably find that everything is too heavy. And you'll want to try and minimise the weight of things you are carrying.

I have see some people with small packs with lots of stuff tied to the outside. This is not a good idea. If it shakes about it's annoying and something could slip off even if strapped tightly. Also if you have something heavy tied to the back of the pack, the side furthest from the wearer's back, then the leverage effect means it will feel even heavier.

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While I agree with the above comments, one thing worth noting is that, like all "luggage", there is a certain element of "stuffability" possible in a backpack, which would not be possible in a bin liner, at least not easily.
You can really jam all the softer things into a backpack (like clothes, sleeping bag and liner, etc), and then cinch it tight afterwards (a good thing of course :)) - attempting the same in a bin liner might just tear it, or worse, stretch it, therefore not giving you any sort of useful indication.
So my gut feel is that unless you have any particularly large or weirdly-shaped items, it is probably an ok way to get a rough estimate. Once you have a backpack, you will always pack to that size anyway: I always manage to fill the darn thing up to capacity, whether its a 1-nighter or 3-nighter :(

As a side note, I tend to pack my clothing in a cheap bin liner first anyway, and then suck all the air out: its amazing how small and compact you can make it, and of course it is mostly waterproof afterwards too...

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Trying to stuff your gear into a plastic bag will only give you a lower bound on the pack size. As with many parameters, the backpack size in litres is more to compare different models, not to be taken as an absolute value. Also, different manufacturers tend to use different litres :)

A 60–65 litre backpack with an adjustable top is a reasonable first choice for a multiple-day hike with sleeping and cooking. (It’s the only outdoors backpack I have.) A larger one is overkill for smaller adventures and invites you to take too much. But YMMV.

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