Medicare Late Enrollment Penalty Calculator

Both Medicare late enrollment penalties share a common purpose — encouraging people to enroll when first eligible rather than waiting until they need care — but they're calculated in meaningfully different ways, and conflating the two formulas is one of the most common Medicare mistakes.

The Part B penalty is period-based: for every full 12-month period you were eligible for Part B but didn't enroll (and didn't have a qualifying exception, like active employer coverage), your premium permanently increases by 10% of the standard premium. Critically, this counts complete 12-month periods only — 11 months late costs nothing extra under this formula, while 13 months late already triggers the full 10% for that first period, even though it's barely past the one-year mark. The percentage is fixed for the life of the penalty; it's the dollar amount that changes each year, since the underlying standard premium ($202.90 in 2026) is recalculated annually. This makes the Part B penalty a moving cost that scales with the plan's own premium growth — a 20% penalty from years spent uninsured will always represent 20% of whatever that year's premium happens to be, not a dollar figure frozen at the original calculation.

The Part D penalty works on a different, more granular clock: 1% of the national base beneficiary premium for every single full month without creditable prescription drug coverage, once a gap reaches 63 or more consecutive days. Because it's month-based rather than 12-month-period-based, there's no rounding-down benefit — 14 months without coverage means a 14% penalty, not a 12% penalty rounded down to the nearest full year. Like the Part B penalty, the percentage is locked in permanently, but the dollar figure it's applied against — the national base beneficiary premium — is recalculated by CMS every year and has generally trended upward, meaning a Part D penalty tends to cost slightly more in absolute dollars each year even though the percentage never changes.

What both penalties share is permanence and the absence of a statute of limitations: there's no point at which either penalty simply expires after enough years of paying it. The only way either penalty goes away is if you become eligible for a program that provides it automatically (certain low-income subsidy programs for Part D, for example) — otherwise, a decision made at 65 can still be adding to a premium bill at 85.

The practical takeaway is that the safest default, absent a genuinely qualifying exception, is enrolling in both Part B and a Part D (or creditable-equivalent) plan during your Initial Enrollment Period. The exceptions exist precisely for people who have equivalent coverage elsewhere — active employer group coverage for Part B, any creditable drug plan for Part D — not as a general grace period for anyone who simply didn't get around to signing up.