This is Goldicq, glad to see you here !
The backpack is counted by weight in Allods Online, at the first, you will have a either 6 by 6 or something else of room, and everything including equipments takes ONE space.
Not only does the room affect how much you can carry, the carrying weight also is a big factor. Sorry I have no clue about how do you calculate the carry weight, but every time you add one more stat to those that increases your carrying weight, you increases the carrying weight by a random number from 1~5. I believe so is the decreasing... but more chances of getting minus 5.
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