Medicare Initial Enrollment Period calculator.

The window in one line

Medicare.gov describes the Initial Enrollment Period as lasting 7 months: it starts 3 months before the month you turn 65, includes that month, and ends 3 months after it. Born June 15, 1962? You turn 65 on June 15, 2027, and your window runs March 2027 through September 2027.

When coverage starts

Coverage always starts on the first of a month. For premium-free Part A it starts the month you turn 65 (the month before, if your birthday is on the first). For Part B and premium Part A it depends on when you sign up:

The calculator's table applies that rule to each of the seven months. Signing up early is how you avoid a gap; signing up late delays the start.

Example

Born June 15, 1962: sign up in March, April or May 2027 and Part B starts June 1, 2027. Sign up in June and it starts July 1; in September 2027, the last month, it starts October 1.

If you miss the window

Between January 1 and March 31 each year you can sign up in the General Enrollment Period, and coverage starts the month after you sign up. You may pay a monthly late enrollment penalty if you do not qualify for a Special Enrollment Period. Compare the numbers with the late enrollment penalty calculator.

Working past 65

If you or your spouse have coverage through current work, you may qualify for an 8-month Special Enrollment Period to sign up for Part B. Medicare.gov says it starts when you stop working and runs 8 months. Some situations depend on employer size: its questionnaire asks whether the employer has 20 or more employees, or 100 or more when the coverage is through a family member and you have a disability. Medicare.gov says the 8 months start when you stop working even if you choose COBRA or other coverage that is not Medicare.

Signing up for Part A later

You can sign up for premium-free Part A any time after you turn 65. Its coverage starts 6 months back from when you sign up or apply for Social Security benefits, but not earlier than the month you turned 65.

Health savings accounts

If you have an HSA, Medicare.gov flags that special rules apply when Part A starts. Check your HSA contribution limit with the HSA calculator before you enroll, and ask a tax adviser about the months involved.

What this does not cover

• People under 65 who qualify through disability, ESRD or ALS.
• Medicare drug plan (Part D) and Medicare Advantage enrollment, which have their own periods (see the open enrollment checklist).
• Eligibility for premium-free Part A, which depends on your work record.

Sources

Retrieved 2026-11-01. Each page was opened and read against the claim next to it.

• Medicare.gov, When does Medicare coverage start?: the 7-month window, coverage start by sign-up month, the General Enrollment Period, retroactive Part A and the first-of-the-month birthday note.
• Medicare.gov, When can I sign up for Medicare?: the 8-month Special Enrollment Period for people with job-based coverage.
• Medicare.gov, Joining a plan: the Initial Enrollment Period and joining a plan.