If you must substitute, see below:
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)
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.
Dill seeds taste similar to dill weed, but they have a slightly bitter edge to them. They appear frequently in pickles, bread, salad dressing, and soup recipes. While you might be tempted to use dill weed as a substitute for dill seeds, you’ll get better results if you use caraway seeds or celery seeds in their place. Replace them measure for measure, and you should come close to the intended flavor.
Dill is incredibly easy to grow, so consider adding it to your garden. It’s an annual, but it reseeds readily. Just allow some of the flowers to go to seed at the end of the season, and it should come up on its own next year.
When fresh dill is being used to flavor a recipe (as it is in pickles, soups, and sauces), use fresh tarragon in its place. To make the proper substitution, use an equal amount of fresh tarragon for the fresh dill, or dried tarragon for the dried dill. You can also use dried tarragon as a stand-in for fresh dill weed, but you’ll need to adjust the quantities, as it has a more intense flavor. Use one teaspoon of dried tarragon for every tablespoon of fresh dill called for in a recipe. Tarragon works well as a substitute for dill in seafood dishes and in salad dressings.
Substitutes for Dill Seed
Substituting fresh dill for dried dill (or vice versa) is easy to do. Just stick to these proportions, and you’ll get great results:
Dill weed is sometimes also referred to as dill leaves. It’s the bright green, feathery fronds of the dill plant. It’s highly aromatic, and tastes of caraway or anise, with a bit of citrus thrown in.
If dill weed is being used as a garnish for a dish, use fennel fronds instead. They look very similar. Fresh parsley can also be used as a garnish. It looks a bit different, but will still add that pop of green. If you don’t have either, just leave the garnish off, or get creative with whatever you have on hand.
Working on a recipe that calls for dill weed or dill seed? If you don’t have any on hand, there are several things that you can use in its place, including other forms of dill, tarragon, celery seed or caraway seed. Here’s how to make a successful substitution, using what you have on hand.
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.
So, you need to substitute some dried dill for the fresh. Purists may say “perish the thought”, but here’s some tips on how:
Other canners will find that their store has a deal on cucumbers but there’s no fresh dill weed within miles to be found.
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.
For a 3-5″ (8 to 12 cm ) sprig of fresh dill, you can substitute ¼ teaspoon of dried 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.
(Note: an umbel is a whorl, a round circle spray of the plants buds or flowers.)