Updated July 9, 2026
How it works
Your Medigap Open Enrollment Period covers the six months from the date your Medicare Part B begins. Enter the month and year your Part B started and the calculator marks the exact window, no guesswork required.
Things to know about your enrollment window
- The clock starts from your Part B effective date, not your birthday and not the date you enrolled in Part A.
- It runs for exactly six months and does not pause, extend, or reopen for any reason once it starts.
- It is separate from Medicare's fall Annual Enrollment Period (October 15 to December 7), which governs Advantage and Part D, not Medigap.
- A few states layer extra rights on top of the federal window; this tool reflects the federal baseline only.
How the window is calculated
The math is simple once you know the rule: open enrollment begins on the first day of the month you entered and runs for six calendar months, closing on the last day of the sixth month. If your Part B starts in March, the window opens March 1 and closes on the last day of August. Enter June instead and it opens June 1 and closes November 30. The calculator above does that same arithmetic instantly and adjusts automatically across year boundaries, so a Part B start late in the year correctly rolls the closing date into January of the following year.