Decide how you’re using nigella seeds (topping for bread, inside dough, or mixed into seasoning) so the substitute will have similar texture.
Pick the closest flavor direction from the table: nutty-crunch (black sesame), oniony-anise (caraway), anise (fennel), or peppery-warm (ajwain).
Swap using the listed ratio (start with 1:1). If your substitute is milder (like fennel), slightly increase the amount only if your recipe can handle it.
If you want stronger flavor, lightly toast the seeds for 30–60 seconds in a dry pan, then cool before using.
Taste-check after baking/finishing: if it’s missing bite, add a pinch of black pepper or a tiny extra crushed ajwain/carumay next time.