Unfortunately, there can be no dependable rule of thumb because herbs differ so much from one to another.
But the following considerations may give you a few clues. Remember that it’s not the amounts of fresh or dried vegetable matter that count in flavoring, but the amounts of essential oils they contain, because that’s where the flavor is.
The leaves of herbaceous plants are 80 percent to 90 percent water by weight. At 80 percent water, 100 grams of leaf should contain 20 grams of thy matter, so the dried herb is five times more potent.
In using this dried herb, then, you would use one-fifth the weight of the fresh herb. At 90 percent water, the fresh-to-dry factor is ten, so for that dried herb you would use one-tenth the weight of the fresh. All of this assumes, however, that the only thing lost in the drying process is water, and no volatile oils, and that’s a rather shaky assumption.
The problem is that in the kitchen we generally measure herbs not by weight but by volume (teaspoons or tablespoons). Volume depends on the physical forms of the fresh and dried herbs, whole leaves, withered leaves, minced pieces, powder, and so on, and the volume ratios are therefore pretty unpredictable.
Thus, short of taking your herbs to a laboratory, having them analyzed for their percentages of essential oils, and then weighing them out, there can be no rule of thumb for how much volume of dried herb to use instead of the fresh form.
Nevertheless, if you have to substitute dried herb for fresh and can’t spare the lab fees or the thirty-day wait for the essential oil analysis report to come back, try using somewhere between one-fourth and one-half as much of the dried herb as of the fresh kind.
Of course, if you’re substituting in the other direction, use between twice as much and four times as much of the fresh herb as of the dried one. In most cases, you won’t be straying disastrously far from the Yellow Brick Road.
Okay, so I lied. That’s a rule of thumb.