The short answer
Your initial enrollment period runs for seven months around your 65th birthday. If you are still working and covered by an employer plan you can usually wait without penalty, and you get eight months to sign up after that job or that coverage ends. Miss both windows and Part B costs 10% more for every full year you could have enrolled and did not.
The seven-month window around your 65th birthday
Your initial enrollment period starts three months before the month you turn 65, includes your birthday month, and runs for three months after it. That is seven months in total, and it is the window in which nearly everyone should act. Signing up in the three months before your birthday month is the only way to have coverage in place on the day you become eligible.
Some people never have to do anything. If you are already receiving Social Security or Railroad Retirement benefits when you turn 65, you are enrolled in Part A and Part B automatically and a card arrives in the post. Everyone else has to apply, and Social Security does not send a reminder that the window has opened.
If you are still working, you can usually wait
Employer coverage from a job you or your spouse currently hold changes the timetable. While that coverage continues you can delay Part B without a penalty, and when the job or the coverage ends - whichever comes first - a special enrollment period gives you eight months to sign up. That period starts the month after the coverage or the employment ends.
Two traps sit inside this rule. Retiree coverage and COBRA are not coverage from current employment, so they do not hold the penalty off and they do not extend the eight months - the clock starts when you stop working, whatever you take afterwards. And if the employer coverage ends during your initial enrollment period rather than after it, no special enrollment period applies at all.
Missing both windows means waiting for January
If you have no special enrollment period and your initial one has closed, the only remaining route is the general enrollment period, which runs from 1 January to 31 March every year. Coverage then begins the month after you enrol, so someone who realises in April has to wait until the following January to apply and until February to be covered.
That gap is the real cost of a missed deadline for most people - not the penalty but the months with no Part B at all, during which doctor visits, outpatient care and most of what Part B pays for are entirely your own bill.
What the Part B penalty actually costs
The Part B late enrollment penalty is 10% of the standard premium for each full 12-month period you could have had Part B and did not. It is not charged for partial years: eleven months late costs nothing extra, thirteen months late costs 10%. The surcharge is then added to your monthly premium for as long as you hold Part B.
Because it is a percentage of the standard premium rather than a fixed dollar amount, the penalty rises every year the standard premium rises. Two narrow exceptions exist - people who qualify for a Medicare Savings Program, and people who had Medicare through disability before turning 65 - but for most people the surcharge is permanent.
Drug coverage has a separate penalty of its own
Part D works on different arithmetic. Its penalty is 1% of the national base beneficiary premium - $38.99 for 2026 - for each full month you went without Medicare drug coverage or other creditable drug coverage after becoming eligible. Fourteen uncovered months means a 14% surcharge, and it follows you even if you later change plans.
The trigger is going 63 days or more in a row without creditable coverage. If your employer or retiree plan includes drug coverage, check the notice it sends each autumn confirming whether that coverage is creditable, and keep it - that notice is what prevents the penalty being applied later.
What our calculator works out
Our Medicare calculator works out what Part B will actually cost you, including the income-related surcharge that applies above $109,000 of income for a single filer and $218,000 for a couple filing jointly. Since the late enrollment penalty is a percentage of the standard premium, knowing your premium is also what tells you what a delay would add.
What this does not tell you
This is an estimate of cost, not a decision about enrolment. The Social Security Administration decides when your enrollment periods run and whether a penalty applies, and it uses its own record of your coverage history to do it.
The calculator covers the Part B premium and the income-related surcharge. It does not compute a late enrollment penalty, model Part D or Medicare Advantage premiums, or account for a Medicare Savings Program paying your premium for you.
Official sources
What you can do next
Work out what Part B will cost you before your window opens, not after. If you are approaching 65 and still working, the one thing worth confirming now is whether your employer coverage is from current employment - that single fact decides whether you can safely wait or need to enrol in the next few months.