Your Medicare Initial Enrollment Period lasts seven months: it opens three months before the month you turn 65, includes your birthday month, and closes three months after it. Sign up in the first three months and Part B starts the month you turn 65; sign up later in the window and coverage starts the month after you apply. Miss the whole window without other qualifying coverage, and you'll wait for the next General Enrollment Period and pay a Part B penalty of 10% for each full year you could have signed up — usually for as long as you have Part B.
The calculator above gives your two key dates. Everything below explains them.
Window rules from medicare.gov, as of October 2026. If you already get Social Security benefits, you're enrolled automatically.
Are you enrolled automatically?
- 3 months beforeWindow opens. Sign up now and Part B starts the month you turn 65
- 2 and 1 months beforeSame: coverage from your birthday month
- Birthday monthPart B starts the following month
- 1–3 months afterStill open; coverage starts the month after you sign up
- After thatWindow closed: wait for January–March and risk the penalty
Premium-free Part A starts the month you turn 65 regardless. Source: medicare.gov, When does Medicare coverage start?
Many people don't have to do anything. If you've been getting Social Security (or Railroad Retirement Board) benefits for at least four months before you turn 65, Medicare enrolls you in Part A and Part B automatically and mails your card about three months before coverage begins.
If you haven't started Social Security yet — and with full retirement age now 66 or 67 for most people, many haven't — you need to sign up yourself, through Social Security, online at ssa.gov or by phone at 1-800-772-1213. Even if you're putting off retirement benefits, SSA's advice is to apply for Medicare within three months of your 65th birthday. The full retirement age chart explains why the two ages no longer line up.
When coverage actually starts
Medicare coverage always begins on the first of a month. The month depends on when you apply:
- Part A (premium-free for most people): starts the month you turn 65, whenever you sign up in your window. If your birthday is on the 1st, it starts the month before.
- Part B, if you sign up before your birthday month: starts the month you turn 65.
- Part B, if you sign up in your birthday month or the three months after: starts the month after you sign up.
So the practical deadline, if you want no gap at all, is the last day of the month before your birthday month. The calculator's first option, Sign up by, shows it.
If you're still working at 65
This is where people get caught out — in both directions. If you or your spouse are still working and covered by that employer's health plan, Medicare's own sign-up tool asks whether the employer has 20 or more employees — the answer changes its advice on whether to delay Part B.
If you delay because of job-based coverage, you get a Special Enrollment Period of 8 months that starts when you or your spouse stop working (or when the coverage ends, if that comes first). Sign up within it and there's no late penalty. To have Medicare start the day your job coverage ends, Medicare suggests signing up for Part B the month before you retire. You'll fill out an extra form showing you had job-based coverage.
Because Part A is free for most people, Medicare points out you may want it at 65 even while you're working. One exception to check first: if you contribute to a Health Savings Account, Medicare has specific guidance on that, linked from its sign-up pages.
If your coverage isn't from a current job — yours or your spouse's — don't assume the 8-month period applies; Medicare's sign-up tool walks through each type of insurance.
The penalties, in real money
- +10%Part B premium for each full year you could have signed upusually for as long as you have Part B
- $202.90standard Part B premium per month in 20262 years late: $243.50 with the penalty
- +1% / moPart D penalty per month without drug coverageof the $38.99 national base premium in 2026
- 8 monthsSpecial Enrollment Period after job coverage ends
Source: medicare.gov, Avoid late enrollment penalties; figures for 2026.
Part B: you pay an extra 10% for each full 12-month period you could have had Part B but didn't. Medicare's own example: wait two full years without a Special Enrollment Period and you pay a 20% penalty on the 2026 standard premium of $202.90, which makes $243.50 a month. For most people, that's a lifetime surcharge.
Part D (drug coverage): 1% of the national base beneficiary premium ($38.99 in 2026) for each month you went without drug coverage that's as good as Medicare's, after a gap of 63 days or more. Wait 14 months and the penalty is 14%.
Part A, only if you have to buy it: up to 10% more, for twice the number of years you didn't sign up.
Missed the window entirely? The General Enrollment Period runs January 1 to March 31 every year, with coverage starting the month after you sign up — and the penalty, unless you qualify for a special period.
Get the paperwork ready
The manual approach: put your 65th birthday month in your calendar a year ahead, with a reminder three months before it. Gather what Social Security will ask about — your Social Security number, your employer's group health plan details if you're still working, and any letters showing when job coverage ends. The important documents for seniors guide lists the rest of the papers worth having in one place.
Paperlock keeps those letters, cards and plan documents in a Face-ID-locked vault and finds them when you ask — when does my employer coverage end? — with the document attached.
Medicare rules here are as of October 2026 and simplified for the common case of turning 65. If you have a disability, ESRD, retiree coverage or an HSA, check medicare.gov or call Social Security at 1-800-772-1213 before choosing your dates.