Millie can be a nickname for various formal names, including:

• Millicent: This is the most common formal name for Millie.
• Amelia: Amelia is a popular name that can be shortened to Millie.
• Matilda: Matilda is another formal name that can be shortened to Millie.
• Camille/Camilla: Both Camille and Camilla can be shortened to Millie.
• Emilia/Emily: Emilia and Emily are formal names that can be shortened to Millie.
• Mildred: Millie is most commonly considered the English pet form of Mildred.

Other formal names that can be shortened to Millie include Wilhelmina, Melissa, and Milena.