Identify the role of fennel seed in your recipe: sweet licorice flavor, warm aromatic depth, or crunchy seed texture.
Pick the closest match from the list above, then use the listed ratio to swap for the fennel seed amount.
Bloom the flavor when possible: toast dry spices 30–60 seconds (stir constantly) or briefly warm in fat before adding.
If your substitute is more concentrated (like star anise, licorice root, or extract), start slightly low and add more after tasting.
For baking: grind whole seeds/pods (or use ground) so the flavor distributes evenly without bitter specks.