Calculation for amount of water needed for your dog:
http://www.mycockerspaniel.com/h2o.htm

What is covered on the above link:
–   Provides a formula for actually calculating how much your individual dog needs.
–   Gives an example of how the calculation works.
–   Know that the amount you see after calculating may seem very high. Remember, since many of us use dry food in some form or another, it only has a low percentage of water. Therefore, take that into account.