Medicare Enrollment Window Calculator

Your 65th birthday → the 7-month Initial Enrollment Period mapped month by month, with effective dates, and the permanent Part B penalty if you miss it.

65th birthday (month you turn 65)

Day of month (sets month-1 vs day-1 start) The 1st-of-month quirk: born Jan 15 → coverage starts Jan 1 (month of); born Jan 1 → starts Jan 1? No, born on the 1st = month-of start; everyone else = month-before start

Health coverage at 65

Data: 42 CFR 406 / SSA POMS. The penalty is a 10%-per-12-months-uncovered multiplier on the Part B premium for life, and it never drops (it's indexed with premiums every year).

—your Initial Enrollment Period (7 months)

—coverage start if you enroll in month 1–3

—enroll month 4–7 → effective dates

—Part B late penalty if you go uncovered

—Part D penalty (lifetime, on 2026 premium base)

—your coverage situation → the right move

How the math works

IEP = 3 months before birthday month, birthday month, 3 months after = 7 months. Enroll month 1–3 → effective on your 65th birthday month (day-1 rule); enroll month 4 (birthday month) → effective 1 month after enrollment; months 5–7 → effective 1–3 months after enrollment (the "delayed effective" trap: enroll late, start late, gaps pay out of pocket).
Part B penalty = 10% × full 12-months-uncovered × current Part B premium, permanent. 18 months uncovered → 10% (only full 12-month blocks count). 42 months → 30%. At 2026 Part B ≈ $202.90/mo, a 3-year gap = +$60.87/mo forever ≈ $1,826/yr extra, indexed with premiums.
Part D penalty = 1% × months-uncovered × national base premium (2026 ≈ $38.60), permanent, on every plan you buy later. 24 months uncovered → +$0.77/mo; 60 months → +$1.93/mo. The IEP for Part D is the same 7-month window; employer creditable coverage (get the annual certificate!) defers both penalties.
The employer trap: COBRA, ACA, VA, and retiree plans do NOT defer the Part B penalty. Only active employer coverage 20+ employees (yours or spouse's) defers, and then you get a Special Enrollment Period = 8 months after the coverage ends, with its own clock. Missing the SEP = same penalty math.

2026 Part B base = $202.90 (indexed annually, verify current year, SSA publishes Dec each year). Birth-on-the-1st rule, month-before rule, and month-4 effective rules as codified. This is a planning tool; SSA/ship enrollment agents finalize (ssa.gov/medicare, 1-800-772-1213).

Frequently asked

When can I sign up for Medicare?

The 7-month window around turning 65: 3 months before your birthday month, your birthday month, 3 months after. Enroll in the first 3 months and Part A/B start the month you turn 65 (day-1 birthday rule).

What happens if I miss Medicare enrollment?

Part B penalty: +10% of the premium for every full 12 months you went without Part B and creditable coverage, permanent, and it grows with annual premium indexing. Part D: +1% per uncovered month on the national base, for life.

Does COBRA count as creditable coverage for Medicare?

No. COBRA, ACA marketplace plans, VA, and short-term plans do not defer Part B penalties. Enroll in Medicare on time while on them (Part A is usually premium-free), keep employer plan as secondary, save the Part D creditability certificate from your employer plan every year.