In general, spring-blooming flower bulbs should be planted at three times the height of the bulb. For instance, if the bulb is one-inch tall, plant it three inches deep.
Here are general measurements for how deep to plant common bulbs:Crocus: 2 to 3 inches deep
Grape hyacinth (Muscari): 3 to 4 inches deep
Dutch iris: 5 inches deep
Dutch hyacinth: 6 inches deep
Fritillaria imperialis: 8 inches deep
Tulips, daffodils: 8 inches deep; if the bulbs are small, as shallow as 5 inches will suffice
Many bulbs have contractile roots, by which they will pull themselves deeper if they are planted too shallowly—hyacinths, daffodils, some alliums, lilies, crocus and gladiolas among them.
Image courtesy Bulb.com