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.
For a 3-5″ (8 to 12 cm ) sprig of fresh dill, you can substitute ¼ teaspoon of dried dill weed.
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. ]
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.
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.
(Note: an umbel is a whorl, a round circle spray of the plants buds or flowers.)
Other canners will find that their store has a deal on cucumbers but there’s no fresh dill weed within miles to be found.
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.