55L backpack allowed as carry-on?

Impossible to say without knowing the dimensions, and the weight of what you're putting into it.

It's 3,400 cubic inches. It's about 2 feet tall, I think. All I'm putting inside are clothes and a few toiletries, as I will have a small shoulder-bag.

The European size for carryon, 55cm x 40cm x 20cm (21-5/8" x 15-3/4" x 7-7/8") is described as 45 l (actually, it's 44 l), so I really doubt it.

55L is a pretty large frame pack. You might look at a 35 or 45 Liter.

My son and I traveled to Germany last year, I used a 35L, it was plenty large for a week,

Carry on size is max 21" high....I'm pretty sure your 55Liter will be too large, especially if you load the hood (it does have a hood- right?)

Other posters have mentioned, it will depend on the airline, and mostly on the gate agent if they will allow the bag on if it's close to carry on size.

Thanks everyone. Yes, Ellen, it's got a hood. I can't really use anything smaller, as I will be in Europe an entire month, only in Germany 8 days; will
be in the Czech Republic 7 days, and in Austria 5 days, and will finish up in the low countries & Paris
so need the space. I'll just have to check the bag.

I spent two months in the UK with a backpack that I was able to carry on. It's very doable.

It's not that you can't do it, it's that you don't want to do it. :)

currently on a month long trip--brought a 3/4 full kelty redwing 2650. i have more than enough. and i'm a woman, so i understand the need for clothing. :-) i didn't bring anything nearly warm enough for the climate (i thought 'how could it be cold in july?'--well, you could be in england, for example. or holland...or PARIS even, this week, anyway) but i bought a 15 euro black shrug in bath and i've been fine with just that ever since. you need WAY less stuff than you think. i promise. we just got back from two days in amsterdam--we left our backpacks in our apartment in paris and took a fold-up daypack with changes of underwear, toothbrushes, and our e-readers (for the train ride) i've been wearing the same black dress for two days, but hey, it matches the shrug!

I agree that you don't need a lot of clothes - I spent a month once in Italy and Greece and all I took was a carry-on and a personal bag. Just about every town in Europe has laundromats.

I don't think my backpack's all that big. I'll check with the airline and if they allow it, great. I'm only taking a week's worth of clothes for 4 wks.
The hostels I'll be staying at have cheap laundry service and there's always Febreeze for emergencies!

"I'm only taking a week's worth of clothes

Then what, pray tell, are you taking that takes up 55 liter. I take 3 days worth of clothes (and wash every night), and it fits into a lot less.

Folks on this forum tend to prefer to pack light and to go carry-on only when possible. That's why you're getting these responses here--many of us have gone for month long trips and carried maybe half of what you're planning to carry. So, if you want to travel the way most folks here travel, know that it is very possible to lighten your load considerably.

On the other hand, if you're happy to carry the things you think are desirable to have and you don't mind checking the backpack, that will work too.

You haven't mentioned what airline you are planning to take. Your backpack almost certainly exceeds all airlines carry-on limits, but European carriers tend to enforce their rules more vigorously than American carriers. So, though the odds are against you no matter what, there's a better, though still small, chance that an American airline will wave your backpack on-board. In any event, have a small personal item with anything delicate, valuable, or important to you so that in the likely event that they make you check your backpack, you won't be in a panic looking for the items you'd rather not check.