The penalty is 10% for each full year
For each full 12-month period you could have had Part B and did not, the penalty is generally 10% of the standard premium. In 2026 that standard premium is $202.90. One full year is about $20.29 extra, for an estimate of $223.19 a month. Two full years is about $40.58 extra, for an estimate of $243.48 a month. A delay under 12 months is generally not a penalty year. The Part B penalty calculator uses the same estimate. It is not a bill.
A missed window is not always a penalty
You may owe nothing extra if you had coverage from a current employer with 20 or more employees, or another situation Medicare treats as a Special Enrollment Period. COBRA usually does not count as that employer coverage. Get the employer’s answer in writing. Read still working at 65 and COBRA vs Medicare at 65 before you assume you are late.
Income can add a second amount
An extra amount some people with higher incomes pay on top of the Part B and Part D premiums. Social Security decides it. It is not a plan fee. is separate from the late penalty. It is an extra amount on Part B, and on Part D, when income from two years earlier is over a threshold. You can owe the late penalty, An extra amount some people with higher incomes pay on top of the Part B and Part D premiums. Social Security decides it. It is not a plan fee., both, or neither. The 2026 Part B deductible is $283. What is IRMAA explains the income piece. Confirm both numbers on Medicare.gov.
What to do next
Write down the month your coverage should have started and whether you or a spouse was still working. Then call Social Security. This website cannot see your record and cannot remove a penalty. If you are still inside a window, the enrollment worksheet shows which one may apply. Medicare at 65 collects the timeline pages in one place.