Hi! I did the WHW with a 25ltr berghaus freeflow! Depends how you are doing it, backpacking needs a bigger sac, B&Bs etc need a smaller sac. The trick is, be ruthless with your gear! get it all together in 3 piles, absolutely necessary, maybe you need it, luxury's. Chuck out the luxury's, decide if the maybe's COULD go in the absolutely necessary's, then chuck them out! My Pack weighed in at 24lbs to include 1 days food (Youth Hosteling) The above mentioned sac is to be recomended. It comes in bigger sizes than 25ltr & the have brought out a newer model recently so the older ones maybe had for reduced price. Check out the sales online! Also think about lighter gear, heavy waterproofs can weigh up to 5/6lbs & thats dry! I use a Paramo Fuera smock & pants for all but winter conditions, very light & pack very small, but I won't go into that discussion, no doubt others have & will! Enjoy the walk, it's fabulous!