Ah - OK.
You probably need a winter jacket then, although you might be able to pick up a cheap one in India or Pakistan.
And you only really need one pair of winter pants and one sweater, unless you think it will be so cold that you will need to wear both pairs of pants and both sweaters together. I assume you will have T-shirts for the hot places, so a sweater with 3 or 4 T-shirts underneath might be enough.
If you are not trekking...good gear is not so important.I'd just buy a cheap heavy sweater/jacket from a market when you get to India and wear/carry it untilò it gets warm enough to give it away......
I have made multiple, multi-season backpacking trips for months at a time and do it comfortably with a 35L backpack.
The important thing is to accept that you don't need to take everything you need from day one. I buy clothes along the way, I donate clothes along the way, I wash clothes along the way and as a result, while my wardrobe is always changing, I don't need to have that much stuff at any one time.
The advantages to having a smaller, lightweight backpack that always qualifies as carry-on and is easy on my back far outweighs the disadvantage of having to occasionally buy and/or donate some clothes along the way.
@joolz2, lucapal: i'm still figuring out how cold can I take. I'm from SEA so you see we don't have that much coldness available. I once came to Korea when it was 4-5oC and I put all the clothes I had on (pretty around 1 jacket 1 sweater 3 Tshirts), still cold when outside.
@laketraveller: I'm choosing between 35L and 40L, will dig around more. Btw, your signature is awesome ;).
There is a weight limit too. For Air Asia it is 7kg for carry on luggage and 10kg for Jetstar.
To get around the weight limit, I actually put on heavy jackets and load it up with almost 3kg of batteries, iPad, phone, camera to get pass check in staff and check points where budget airlines ccasionally weigh carry on luggage.
On Air Asia and Jetstar you might get away with a kg or so overweight by smiling boradly.
Like Ryanair, Tiger Airways is very strict with weight limit and airline staff seem to have no discretion or sense of humor.
Let me see if I have this right. You are deciding how big a pack you need based on whether you can avoid paying a fee for checked baggage. Is that right?
You have your priorities mixed up davataged. Criteria number one should be how much weight can you comfortably carry on your back for as long as you think you might have to. Generally, that means a maximum of 25% of your body weight if you are physically fit. Ideally, you want it to be less than that.
Packing too much is an extremely common error that travellers make. But getting it down to 7kg to meet carry-on is beyond most people. While a 45L bag might meet size restrictions, it is not likely you will meet the weight restriction.
So, before you buy any pack, first determine the max. weight you want to have to carry on your back. Compare that to the 7-10kg. usual carry-on restriction. Then get together everything you think you want to take and WEIGH it all. Don't forget to add the weight of the pack itself. When you see what weight you are looking at you can make some decisions. ie. if you find you want to take 20kg. you can forget trying to avoid luggage charges. If you find you want to take 30% of your body weight, you can forget walking very far with it on your back.
It is that 7-10kg. weight criteria that is the killer.