Some feature extra pockets for staying organized. Their capacity lets you carry an extra layer, food and gear for day trips. 21–35 liters: This is the sweet spot for most hiking and travel daypacks. There's enough capacity to hold food, clothing and some extras, like a camera and a book.
Multiday (3-5 nights; 50-80 liters)Packs in the 50- to 80-liter range are also great for shorter trips where you pack a little more luxuriously or multisport activities like backcountry skiing.
Normal backpacks or daily backpacks have volumes where you can easily fit small items that may be needed at work or school during the day. Their volumes vary between 15 liters and 30 liters.
Their capacity lets you carry an extra layer, food and gear for day trips. 21–35 liters: This is the sweet spot for most hiking and travel daypacks. There's enough capacity to hold food, clothing and some extras, like a camera and a book.
MEDIUM BACKPACKS (20 – 35L)This capacity range is considered a medium-size backpack and is probably the most popular bag capacity out there. It's great for taking to school or work, depending on the styling, and can also be a great travel pack for taking on a plane.
40-50 Liters (The Ultralight Thru-Hiker)A pack this size can be used for multi-day backpacking trips and even Continental Trail thru-hikes provided you keep a lightweight and minimalist philosophy.
For a day hike, a pack between 10 and 25 liters should be enough. You could fit your water bottle and your picnic in this pack, as well as a jacket and sunscreen, to cater for all weather conditions! For more than a day hike, you should think big! You will need a pack between 20 and 35 liters.
While personal item size restrictions vary across airlines, most airlines consider a small backpack, purse, briefcase, or laptop bag to be a personal item. These must always fit under the seat in front of you. Many airlines restrict passengers to one personal item and one carry on bag.