The L stands for Liters. So when you see a 25L backpack, that means the capacity of the main pouch is 25 liters.
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If you need a bit more space than what a small backpack offers but don't want to lug around a large pack, go for a medium size. These have a capacity of 25 to 40 liters, which is just enough room for a change of clothes, a laptop, and these must-have travel accessories for every destination.
SMALL BACKPACKS (< 20L)Suitable for everyday carry, tech carry, day travel, tablets… While a capacity of <20L is considered a compact size for a backpack, don't let that fool you. If the backpack is built well and is fully utilized, you should still be able to fit lots of essentials into it.
In general, a backpack is considered a personal item if it fits under the seat in front of you. This means that the backpack must be no larger than 22 inches in length, 14 inches in width, and 9 inches in height. If the backpack is larger than this, it will be considered luggage and must be checked.
Most bags range from about 30L (1,800 cubic inches) all the way to 100L+ (6,000+ cubic inches). I recommend a backpack around 40L-50L. Personally, I wouldn't go over 65L, but some people like a bigger bag.
For middle schoolers, Backpacks designed for 11 – 14-year-olds should measure 18 to 20 inches in length with approximately 16-inch widths spanning up to 8-inch depths. Depending on which grade they attend and how many accessories they plan on taking along with them, having a couple of options between 25-35L is ideal.
Admittedly, I have many, many backpacks, and I often get more looking for an ideal backpack for the occasion. I got the XL (30L) version of this backpack.
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.
The Average Traveler: 30-40 LitersFor the average traveler who likes to have some clothing choices and prefers to bring a few extra items, a carry-on luggage with a capacity of 30-40 liters is usually sufficient. This allows you to comfortably pack clothing for multiple days, including shoes and toiletries.
The most common personal item size limit is 18 x 14 x 8 inches (46 x 36 x 20 cm). If your backpacks or handbags don't exceed this number, you're good. Weight limitations for personal belongings are also regulated by several airlines ranging from 10 to 25 pounds (around 4.5 to 11 kg).
Generally, most of the European airlines limit carry-on luggage to a maximum size of 55 x 40 x 20 cm ( 21.6 x 15.7 x 7.8 in); however, a few airlines do accept bags up to 25 cm wide (9.84 inc) or do not accept more than 35cm tall (13.7 in).
A backpack is a bag worn on your back and typically has two straps over the shoulders. A rucksack is a specific backpack most often used for hiking or camping. It is larger than a backpack and usually has a frame to support the bag's weight.
40-50 Liters (The Ultralight Thru-Hiker)Expect to fit everything you could fit in a smaller pack, plus: A food bag that can hold up to 5-6 days worth of food. A pot for cooking. A camp stove and fuel.