Medicare Part B Late Enrollment Guide (2026): Penalties, SEPs, and Creditable Coverage Proof

Best for people aging off employer coverage, retiring after 65, or comparing COBRA vs Medicare timing. Missing Part B without qualifying coverage triggers a lifelong premium penalty—plan enrollment dates before the employer plan ends.

Standard Part B enrollment windows

• Initial Enrollment Period: three months before through three months after Part B eligibility month (usually 65).
• Special Enrollment Period: up to eight months after employer group coverage ends if you had creditable coverage.
• General Enrollment Period: Jan 1–Mar 31 annually with July 1 effective date if you missed other windows.

Late enrollment penalty math

• Roughly 10% of the Part B premium for each full 12-month period you could have enrolled but did not.
• Penalty usually lasts as long as you have Part B—it is not a one-time fee.
• Part D has a separate late penalty—see /guides/medicare-part-d-formulary-guide.

Scenario: retire at 67 with delayed Part B

An employee works past 65 with large-group employer coverage, then retires without enrolling in Part B within eight months. Two full years of delay can add ~20% to Part B premium for life—often thousands over retirement. Enroll during the employer SEP with HR proof of coverage dates.

Scenario: COBRA vs Medicare at 65

COBRA is not creditable for delaying Part B without penalty in most cases—Medicare is primary at 65 for most workers. Compare Medigap guaranteed issue windows in /guides/medicare-advantage-vs-medigap-buying-guide before choosing COBRA only.

Buying checklist

• Request written creditable coverage letter from HR before leaving employer plan.
• Enroll Part B online or at Social Security before COBRA ends if Medicare is primary.
• Pair with Medigap timing: /guides/medicare-supplement-buying-guide-2026.
• Plan letters reference: /guides/medicare-medigap-plan-letters-deep-guide-2026.

FAQ

Q: Can I delay Part B if I have retiree coverage? A: Only if coverage is creditable primary insurance—verify with SSA, not HR alone.

Q: Does the penalty apply if I had VA coverage? A: Rules differ—VA is not always creditable for Part B delay; confirm before declining Part B.

Q: Can the penalty be removed later? A: Generally no—it attaches for as long as you have Part B.

Related guides

• Advantage vs Medigap: /guides/medicare-advantage-vs-medigap-buying-guide
• Compare Medicare products: /insurance/medicare