A 40L backpack is your perfect weekend companion. With clever storage, you'll have plenty of room for a sleeping bag, solo tent, camping mat, food, and extra clothing and layers to keep you comfy. You can take 40-45L backpacks as carry-on for flights.
A 40 liter backpack is the best size for traveling in Europe as it's small enough to fly carry-on only. If you aren't up to the challenge of travelling with a 40 liter pack, you just need to make sure that you choose a backpack you will be comfortable carrying for long periods of time.
A 40L backpack sits in the sweet spot between lightweight daysacks and heavy-duty 80L rucksacks. They're extremely flexible – working for average day hikes and overnight wilderness camping. They also give you the extra room needed for climbing gear or winter hikes.
This is why most new backpackers think they need a larger backpack to travel, but with some clever organising and good packing cubes, a 40L backpack could just be the perfect size. You'll be incredibly surprised at just how small of a space you can fit everything you need for 3, 6 or even 12 months.
In most cases, the compartment size may range from about 35 to 45 liters. In some cases, the bag may also have an expandable pull string storage for more space.
Trips of 5 days or more usually call for packs of 70 liters or more. These are also usually the preferred choice for winter treks lasting more than 1 night. (Larger packs can more comfortably accommodate extra clothing, a warmer sleeping bag and a 4-season tent, which typically includes extra poles.)
It is the perfect size for travel, just don't pack it full. A 40L backpack half empty weighs essentially the same as a 20L backpack stuffed full. I did 4 months abroad with a GORUCK GR2 34L a few years ago.
Trips of 5 days or more usually call for packs of 70 liters or more. These are also usually the preferred choice for winter treks lasting more than 1 night.