I suppose everyone develops their own approach to this. For what it is worth I've never carried more than two 1.5 ltr bottles even in the hight of summer on the long stretches of the Via de la Plata. I would carry much less in winter. If you are walking the Camino Frances the guide books will give information on the frequency of fuentes/bars to top up on water.
On the Camino Frances I only took a 2 liter Platypus with hose and only filled it about 3/4 each morning at the albergue. Villages are frequent and water is plentiful. I suggest you try not to ever carry more than is necessary between sources and multiple sources are readily available during the day. I leave soon for the VdlP and I shall still only carry the 2 liter Platypus hoser: http://www.moosejaw.com/moosejaw/produc ... GoogleBase
The 2 L Platypus weighs 3.5 oz. or 100 g. The Camelback is heavier, which is also a consideration.
Whichever your choice, start out carrying less weight. Each liter of water is 1.01 kilograms and 4 of them weighs 4.04 kg or almost 9 pounds, plus the container weights. You would only carry that weight were you crossing a hot desert.
On the Camino I recall being more concerned with where I would find my next 'cafe con leche', than with water.
We varied the amount of water, a minimum of 1.5 liters but up to 3 liters, depnding on the day. More water for the climbs or any long strecthes wilhout little towns. I still remember the time another person doing the Camino on a mountain bike pulled alongside us and said we were not drinking enough water. Steve
I took a platypus and a aluminum (I think) water bottle (both 1 L), both weigh about nothing when empty. Most days it was ok just to fill one of the bottles, as it was possible to refill along the way. I'm a fan of having the platypus bag being the extra because it compresses down when empty, so that it took up less space.
As others have said, you can buy bottles of water quite easily, and you can do this on days where you feel like you might need more than 2 liters. Don't bring 3 water bottles, days where you will need this are too rare to justify bringing the extra!
A 2lt water bottle should be more than enough for most days. But remember to fill it every opportunity you get. And make sure the water source is working before you empty the remains of your bottle onto the dust. The water tap near Eunate wasn't working last year and I had poured the contents of my water bottle away before finding I was at a dry tap.
I take two 600ml plastic bottles, and find that sufficient. There are plenty of fountains along the way and, if you don't find one, just ask for a refill at any bar. Water in Spain is of good quality so there should be no need to buy water.
Well, I am going to take the opportunity to ask the question that I suppose all the women that are going to do the camino want to know: After drinking so much water, is it easy to find places to peee between albergues?? Thanks, Sumachado.
Regarding availability of places to pee along the way, well, there's behind the bushes and in the barren meseta, you either wait until all the other pilgrims pass you or you can use one of the female urination devices.
We used two 750ml bottles and that was plenty - you need to make sure you drink a lot, especially in hot weather. An "average" human should consume 2-3 litres a day - when exerting make that 4-5 - in hot weather make that 6 or more. Some of that should be rehydration drinks (even something like isostar) to replace salts but do NOT count sweet drinks like Coke.
The early signs of dehydration are not things like thirst it's things like losing energy, becoming very emotional, struggling with usually simple actions etc... and it's just irresponsible to ignore this.
Coke and similar drinks require your body to use water to process the sugar content so although they are liquids you dont really get a net fluid gain from them. However, if you're suffering the symptoms of dehydration or sunstroke a cold coke - or much better still a cold, flat lemonade - is a life saver (possibly literally!)
Contrary to popular belief coffee does give you a net fluid gain but too much will cause other problems....like realising how crap the coffee back home is