That is a matter that has many answers as chicken keepers. It depends on several facts. How big is your coop, your flock, the climate, and what you prefer. The bigger the coop the less percentage of the coop the water takes the easier it is to have one in the coop. The bigger the flock the more you need two waters. How cold and what time of year may make having one outside difficult. If you are one to let your chickens out late, you may need one in the coop. With my larger flocks I keep both in and out. The brooder and bachelor pad generally have one. Although the boys can get rambunctious and knock even the heavier ones over.