When all else fails, go to "Joy of Cooking" --see "equivalents and or substitutions, and I quote:
1 Tsp raw ginger = 1/8 tsp pwd. This info is usually on the spice container, but I
use a less expensive brand and it does not have the info. cmg
To substitute powdered ginger for fresh ginger, a general rule of thumb is to use about 1/4 teaspoon of powdered ginger for every 1 tablespoon of fresh ginger. This is because powdered ginger is more concentrated in flavor than fresh ginger. It's best to adjust to taste, as the strength can vary by brand and freshness.
Avoid doing this. The proportions unbalance a recipe when you substitute ground for freshly grated ginger.
It is a spice
A one inch piece of fresh, grated ginger (generally yielding 1 tablespoon) equals approximately 1/8 teaspoon ground (dried) ginger. Source: http://www.evitamins.com/healthnotes.asp?ContentID=3602003
1 inch of ginger grated will produce about 4 teaspoons.
Approximately 2.5 cm of fresh ginger is equivalent to about 1 teaspoon of dried ginger powder. Fresh ginger has a more intense flavor than dried, so you may want to adjust the amount based on your taste preferences in recipes. Generally, the conversion ratio is about 1 tablespoon of fresh ginger to 1 teaspoon of dried ginger.
Forty grams of fresh ginger is approximately equivalent to about 4 to 5 teaspoons, depending on how finely it is minced or grated. Typically, one teaspoon of minced ginger weighs around 8 to 10 grams. Thus, when measuring, it's best to adjust based on your desired texture and flavor intensity.
To substitute fresh ginger with ground ginger in a recipe, use 1/4 teaspoon of ground ginger for every 1 tablespoon of fresh ginger called for in the recipe. Adjust the amount to taste, as ground ginger is more concentrated than fresh ginger.
1 tablespoon fresh rosemary equals 1 teaspoon dried
Dry is always more potent, as long as your spices are not old. If you have "fresh" dry ginger, 1 teaspoon would be the right amount to substitute for 2 tablespoons of fresh ginger. Since ginger is something that loses its potency in dry form very quickly, the best possible answer to this question is "to taste."
No amount of dried ginger equates to any amount of fresh finger; they have two completely different flavors and they will never behave the same in any recipe. Powdered ginger has a deeper, smokier, spicier flavor we know from gingerbread, gingersnap cookies, etc... Fresh ginger is sharp, bright, slightly lemony and even biting. Don't attempt an exchange, no matter what anyone tells you. It won't work.
To make 1 tablespoon of grated fresh ginger, you typically need about 1 to 1.5 inches of ginger root, depending on its thickness and juiciness. After peeling, you can grate the ginger to achieve the desired amount. It’s always a good idea to have a little extra on hand, as the flavor can vary with freshness.