Can any one advise me which is the size ruck sack in liters do most people take walking the Camino?
I have developed some rules of thumb, but my first observation is that pack weight and capacity are loosely related. The bigger you are, the larger pack size you will need. My rule of thumb is that a suitable start point for looking at packs is to take your walking weight in kg, halve that and that is roughly the size of pack you will need in litres for a late spring to early autumn camino. For the average American male, that works out at around 45 li, for Britain, about 40 li. Averages for women are slightly less, so you could think about 40li and 35 li respectively.
Be honest though. Half of us are going to be over the average, and are likely to need something larger.
There are other factors that influence pack size, and you will find many resources on the web.
The Complete Walker IV is my goto reference, but there are now many web-based resources. Remember these are providing advice for multi-day bushwalkers - hikers that will need things that you would normally be unlikely to need on the camino. But the principles are still valid. Some people will be more cold or heat tolerant, might be able to afford the high prices of really lightweighct gear, etc, and will have more or less weight to carry.
Further, not everyone walks in summer. The earlier you start or the later you plan to finish needs to be considered. Early spring and late autumn might require anything from 25% to 50% more as your weight allowance, and pack volume.
Once you have an idea about the size, confirm this by getting your gear together and weighing it. My rule of thumb is that you need about a litre of pack size for every 160-170 gm of gear for a summer camino load. If you need much more than you calculated with my first rule of thumb, you might want to rethink your packing list. There are plenty of people here who will willingly help you with that. If you are prepared to buy a really light pack, you can add a little, depending on how much you are prepared to spend on really light gear. Think about adding up to 15 gm/li, ie up to 185 gm/li for a summer load.
Now you are ready to visit an outdoors retailer with your gear in hand (or a big plastic bag!) and actually try out some packs. Don't be tempted to stuff everything in until it is so tight that you cannot easily get the next item into the pack. Amongst other things you will want to carry some food and water in addition to your bare load, which is what we have been discussing so far, so some space will be needed for that. I personally count on those items occupying around 3-4 litres of pack space at the beginning of the day if I am carrying two litres of water, and a food bag with snacks, bread, cheese, chorizo, etc. You can hang some of this on the outside of your pack, but I try and avoid that.
Good luck with your pack hunting.
Postscript: Remember that the items that you are walking in don't need to be packed. So if you follow
@FSP's advice earlier, only one set of clothing needs to be packed. There is a trap here. I once met a pilgrim at the start of the CI with the most wonderfully small pack that he had carried on as cabin baggage. It looked as tight as a drum. But to achieve that he was wearing all his warm and wet-weather gear. Pretty clearly he had no room left inside the pack, and would have had to drape or hang these items outside the pack when he didn't need to wear them. I avoid that.
remember you could be carrying it 10 hrs a day
The hardest part is knowing and having the security to take only the bare minimum and knowing that you can buy whatever you need as you walk. On my last Camino (a winter one) my pack with a liter of water weighed 7.9 kilos. (I am 6'2'' 1.89 and weigh about 205. I think that is about 94 kilos) That included a sleeping bag, sometimes and a little throw pillow, always. I have learned that my little pillow is essential for me as the pillows in most hotels and albergues would leave me contorted in pain. Stuffing clothes in sacks are not any better. So I need lots of room and my pack looks full but that is because I have very bulky but very light items. But you know what you never will know what will be right for you until you do it. I think the best advice for new walkers is to check out what you are taking about 5 times and keep asking yourself what is absolutely essential. Then remove something. Once you start walking if you have too much weight get rid of things and if necessary add them later. But the odds are the things you leave behind you never really needed.
