Seven months. Then the meter runs for life.

The facts, in one place.

Six quotable sentences on the enrollment calendar and the penalties, 2026.

1. The initial enrollment period is seven months: the 3 months before the month you turn 65, that month, and the 3 after. A birthday on the first of a month moves everything one month earlier.
2. Sign up in the three months before and coverage begins the first of your birthday month; sign up in the birthday month or the three after and it begins the month after you sign up.
3. Miss the window without employer coverage and the next chance is the general enrollment period, January 1 to March 31, with coverage the month after signing up: up to a year without Part B.
4. The Part B late penalty is 10 percent of the standard premium for each full 12-month period without it, for as long as you have Medicare. Two years late is 20 percent: $40.58 a month on the 2026 premium of $202.90, every month, for life.
5. The Part D penalty is 1 percent of the national base beneficiary premium ($38.99 for 2026) for each month without creditable drug coverage after a gap of 63 days or more, rounded to the nearest ten cents, and it is recomputed each year on that year's base premium: 14 months is $5.50 a month.
6. Still working with employer coverage from a firm of 20 or more? The special enrollment period runs 8 months after the coverage or the employment ends, whichever is first, with no penalty; COBRA and retiree coverage do not count as employer coverage for this purpose.

The enrollment periods.

Each period, who it is for, when coverage starts, and what lateness costs.

How the calendar works.

Medicare eligibility begins at 65, and the calendar runs from the month of the 65th birthday: three months before it, the month itself, and three months after make the seven-month initial enrollment period. The one wrinkle is a birthday on the first of a month, which Medicare treats as belonging to the month before, so everything moves one month earlier. When coverage starts depends on when in the window you sign up: before the birthday month, and it starts on the first of that month; in the birthday month or after, and it starts the month after you sign up.

Miss the window with no employer coverage and the next chance is the general enrollment period each January through March, with coverage the month after signing up, and the Part B penalty attaches for every full year missed, for life. Employer coverage from a firm of 20 or more (yours or a spouse's) changes the calendar: no penalty, and an eight-month special enrollment period after that coverage or the job ends. COBRA and retiree coverage do not count.

Methodology.

1. Inputs. Birth month and year, whether the birthday is on the first of a month, and, for the penalties, months late for Part B and months without creditable drug coverage.
2. The window. The eligibility month is the 65th birthday month, or the month before for a birthday on the first. The window is 3 months before it to 3 after; coverage starts in the eligibility month when signing up before it, and the month after signing up otherwise.
3. The fallback. The first general enrollment period (January 1 to March 31) after the window closes, with coverage as late as the month after its last month.
4. Part B penalty. 10 percent of the standard premium ($202.90 for 2026) per full 12-month period late, added to every month's premium for as long as Part B lasts; the tool also shows twenty years of it at today's premium.
5. Part D penalty. 1 percent of the national base beneficiary premium ($38.99 for 2026) per uncovered month after a gap of 63 days or more, rounded to the nearest ten cents, recomputed each year on that year's base premium.
6. Validation. A June birthday, a birthday on the first, and a December birthday that crosses the year, each pinned to its months; Medicare's own two-years-late example ($40.58); and a 14-month Part D case with the rounding. A transcription error fails the build.
7. Not modeled. Premium Part A and its penalty, Medicare before 65 on disability or ESRD, Medicare Advantage and Medigap windows (Medigap's six-month open enrollment matters), the income-related surcharge (the IRMAA tool), and the special enrollment periods for exceptional circumstances. Educational, not advice.

Sources.

1. 1. Medicare.gov, When does Medicare coverage start?. The seven-month initial enrollment period (three months before the birthday month to three months after); coverage from the birthday month when signing up before it, and from the month after signing up otherwise; the rule that a birthday on the first of a month moves everything one month earlier; the January 1 to March 31 general enrollment period with coverage the month after; the eight-month special enrollment period after employer coverage or employment ends. Retrieved September 5, 2026; verified September 5, 2026.
2. 2. Medicare.gov, Avoid late enrollment penalties. Part B: 10 percent more for each full 12-month period without Part B, for as long as you have it (the page's own example: two years late, 20 percent, $40.58 on the $202.90 premium). Part D: 1 percent of the national base beneficiary premium for each month without creditable coverage after a gap of 63 days or more, rounded to the nearest $0.10. Part A (if bought): 10 percent for twice the years missed. Retrieved September 5, 2026; verified September 5, 2026.
3. 3. Centers for Medicare & Medicaid Services, Annual release of Part D national average monthly bid amount and base beneficiary premium for 2026 (July 28, 2025). The 2026 Part D base beneficiary premium of $38.99, the figure the Part D penalty is computed on. Retrieved September 5, 2026; verified September 5, 2026.
4. 4. Centers for Medicare & Medicaid Services, Original Medicare (Part A and B) eligibility and enrollment. Eligibility at 65; the enrollment periods as CMS states them for plans and employers; that Part B coverage under the general enrollment period begins the month after enrollment. Retrieved September 5, 2026; verified September 5, 2026.
5. 5. Centers for Medicare & Medicaid Services, 2026 Medicare Parts A & B Premiums and Deductibles (fact sheet, November 14, 2025). The Part B standard premium and deductible, the Part A inpatient deductible, and every row of the Part B and Part D income-related adjustment tables. Retrieved September 4, 2026; verified September 4, 2026.

Revision history.

The record's history.

September 5, 2026

First release: the initial enrollment window dated to a birth month, the general and special enrollment periods, and the Part B and Part D late penalties on the 2026 premiums.

Canonical address: https://consideratecapital.com/tools/medicare-enrollment-window