Imagine that you have a 40 liter bug out backpack. Yes, there are 60 liters and 80 liters too, but those are extremely heavy and unfair for the average city girl. 40 liters backpack is not too small, and not too big, and people from 12 years old to 60 years old can carry without much strain.
Now that you've imagined that, let's imagine the contents. If you have prior knowledge or foresight that shit is about to hit the fan and you anticipate having to run away carrying only what you have in your bag, what would you put inside?
Please don't say you're gonna be carrying a loli or similar inside it. You're gonna have to eat what you carry.
The setting is not clear enough. Is it a post-apocalyptic survival or crime escape setting?
For the first case, one doesn't need to worry about blending into society, so documents and money are useless. I would take: a few bottles of fresh-water; dry or canned food; medical supplies (anti-inflammation medicine, antibiotics, cotton and bandage, some surgical supplies like a scalpel, med needles, and thread); something to light a fire; two knives (one as a weapon and one as a working tool); a blanket; a rope and a piece of glass; a paper map and a compass (as the phone won't be useful for a long time, of course, if I have a solar charger I'd take the phone and would download a lot of survival apps - edible and poisonous plants, types of knots, etc.). I think the bag won't be able to fit much more. Anyway, it now contains initial supplies and tools for survival and food procurement outside of a city. The initial chaos in the cities will be nasty, so it isn't a bad idea to retreat to a mountain for a while. Of course, it depends on the cause of the apocalypse. Clothes are essential, but you can wear light clothing under a set of heavy/warm clothes. A pair of sunglasses will be useful if you expect a snowy environment to protect your eyes.
PS: A pair of spare boots and socks will be needed as well. You can expect at some point in time to get your feet wet, and keeping them dry and warm, is very important for the temp regulation of the whole organism.
PS 2: Seriously, I'd get some charcoal or a bottle of potassium permanganate as well. You can use it to disinfect wounds or water.
In the second case, I'd probably try to get money and valuables and something to hide my identity. The capital will be helpful for bribery or to get fake identity documentation. Travelling as light as possible is crucial as well. Going around with a heavy bag will be suspicious and will slow you down.
