When replacing a fresh or leaf herb in a recipe with the dried or powdered kind, what is the ratio I should normally use for the substitution?
E.g. if the recipe calls for 2 teaspoons finely chopped rosemary, how much dried rosemary should I use?
The Cook's Thesaurus is a great resource for these types of conversion and substitutes. There conversions differ for each herb, but for rosemary, they list
1 tablespoon fresh = 1 teaspoon dried
Rosemary is a VERY intense herb. Depending on the freshness of the dried, 1/2 tsp dried per 1 TBSP fresh is a more accurate conversion. For other herbs, such as sage or thyme, 1 tsp per 1 TBSP works well.
I live in the mountains, where fresh herbs are neither plentiful nor inexpensive, so dried is pretty much all I cook with.
(HINT): During the holiday season from November through December, you can sometimes find small rosemary 'Christmas Trees' in pots for indoor decorations. These are great to provide you with uber fresh rosemary for a few months, longer if you don't have a 'black thumb' with houseplants...
my rule of thumb is about half dried for whatever it says for fresh, but Ray's answer is much more precise. : )
Here is a handy chart that I have come to love and use! If they are off tasting to you or odd or too strong go lighter :D