Fresh ginger is definitely preferred, but most packages of ground ginger have some sort of comparison on the label, as with vanilla extract.

Well, ground ginger is never a good substitute for fresh ginger, especially outside of baking. But if you're in a pinch, use about 1 tsp ground ginger to a 1/2-inch piece of fresh. If this is a savory dish, cook it for a few seconds in your oil first before you mix it into everything else. This will help release the flavors and keep that powdery sort of taste out of your dish.