Dill seed is not a good substitute for fresh dill weed because of the difference in flavor strength but it does depend on the recipe. The seed has a camphorous, slightly bitter flavor, and the weed has a delicate flavor. The differences are like night and day.
Do you know how much dill seed is equivalent to one head of fresh dill?? I am going to make pickles, and fresh dill is not available. Thanks in advance for your help. – Judith Cartwright (8/22/01)
If you must substitute, see below:
3 heads dill = 1 tablespoon dill seed
1/2 ounce dill seed = 1/2 cup fresh dill
3- to 5-inch sprig of fresh dill = 1/4 teaspoon of dried dill weed.
When making substitutions, you should also consider the difference in appearance between the seeds and the leaves. Some people find the appearance of dill weed in pickle brine to be unappetizing. If you are using dill weed instead dill seeds to flavor your pickles, you may want to chop it finely to make it less noticeable.
The dill plant is versatile in that you can use both the leaves and the seeds to provide flavor. “Dill weed” is the term used for the leaves; you can use them as an herb and use the seeds as a spice. Both forms of dill are essential for your spice collection as they are both popular ingredients in a number of different cuisines from all over the world. If you have encountered one or both forms of dill in your local supermarket, you may have wondered if there are any differences between the two. Do they have the same flavor? Can you use one in place of the other? Our Spiceography Showdown will provide you with answers.
Like many herbs, the seeds and the leaves do have some similarities but they are not identical. The flavor of dill leaves is similar to that of parsley and anise with notes of lemon. While dill seeds do have the same notes of anise, they also have notes of caraway. The seeds’ flavor is more pungent and some cooks even consider it slightly bitter and reminiscent of camphor; on the other hand, the leaves’ flavor is more delicate. In addition to all that, dill seeds have a characteristic not found in dill weed: their flavor tends to become stronger when heated.
Is dill weed an effective substitute for dill seed or vice versa?
In the United States, the most well known use of dill seeds is as the main flavoring in dill pickles; however, they are widely used in Indian, Eastern European and Scandinavian cuisines. Dill seeds are excellent when used in acidic dishes including pickled beets, carrots and even pickled fish. You can also add them to your lentil dal or use them with any other legume to aid digestion.
Fresh dill weed is a popular complement to fish but can also be a pleasant addition to potato salad. Like the dill seed, dill weed works well with legumes but it is also enjoyable in coleslaw and is useful for flavoring dips. You can even use the seeds and the leaves of the dill plant together in some salad dressings and vinegars.
Because of the flavor differences, the seeds and leaves of the dill plant are not ideal replacements for each other; however, it is possible in a pinch. Keep in mind that you will need to use different amounts when substituting one for the other. Three heads of dill weed is roughly equivalent to a single tablespoon of the seeds. In addition, bear in mind that the seeds stand up to longer cooking times better than the leaves. This means that if you are using dill weed in place of the seeds, it is best to add them towards the end of the cooking time rather than at the beginning.
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The National Center for Home Food Preservation says, “For each quart, try 3 heads of fresh dill or 1 to 2 tablespoons dill seed (dill weed = 2 tablespoons).”  National Center for Home Food Preservation. Frequently Asked Pickle Questions. Accessed March 2015. [Ed: It’s not clear what they mean by the dill weed = addition at the end there: perhaps it means OR 2 tbsp dill weed. ]
Linda Ziedrich says that if a recipe calls for a fresh dill umbel and you don’t have one, use one teaspoon of dried dill seed instead.  Ziedrich, Linda. The Joy of Pickling. Boston, Massachusetts: The Harvard Common Press. 2009. Page 14.
Or it’s the dead of winter, you are doing winter canning with imported cucumbers (despite the very remote odds of getting a crisp pickle with them), and the balcony where you grow your herbs in the summer has howling winds from Siberia whistling through it off of Lake Michigan.
In real life, outside of coffee-table beautiful home canning books, some gardeners may find that their cucumbers are ready before their dill weed is.
If you do try to grow some dill herb, in a small patch of garden or in a window box, don’t plant the seed all at once. In her recipes in The Joy of Pickling, Linda Ziedrich is a big believer in using fresh herbs, even suggesting that it’s worth trying to grow some dill yourself, staggering the planting every few weeks until midsummer so that you have a constant supply as it matures. She says that fresh dill seeds taste very different; that they “taste fresh and mild” before they dry out.  Ziedrich, Linda. The Joy of Pickling. Page 14.
(Note: an umbel is a whorl, a round circle spray of the plants buds or flowers.)
So, you need to substitute some dried dill for the fresh. Purists may say “perish the thought”, but here’s some tips on how: