A properly packed backpack should ideally not weigh more than 13 kg. It is also important to note that if you are taking a domestic flight to the start of your trek, many smaller aircraft have weight restrictions. If you are flying from Kathmandu to Pokhara, you are only allowed 15 kg of checked luggage, and 5 kg of hand baggage. Thus, in case of a small flight, you should ideally pack a 15 kg backpack for checked luggage, and then add 5 kg in your hand baggage. You can then re-shuffle the weight once you start your trek at the trailhead.