There are only four ingredients that you need for a good pie crust: flour, fat, salt, and water. It’s the ratio of these four ingredients that’s tricky to master and key to ensuring a great pie crust. Your recipe may call for just a few tablespoons of water — and potentially even less than that. The purpose of this liquid in a pie crust is merely to bind the other ingredients together. If you don’t add enough, the dough will be dry and break easily; but if you add too much, you may be dealing with a soupy dough that sticks to everything. I’ve found its easier to add too much water than to add too little.
Since the amount of water that your pie crust needs is often determined by how coarse your butter is, you only need to add enough so that your pie comes together in a ball and has no dry pockets of flour remaining. It’s important to note that the water also kick-starts the development of gluten, so if you add too much and overwork it, you could risk making your pie crust too tough. If you’re using a food processor, add a scant amount of the cold water, pulse, take a peek, and decide if you need to add more.
Moreover, when I first learned how to make pie crust with my mom, she made a clear point that the water should always be ice cold before adding it to the flour. This will prevent the fat from melting.