A late start on Part B or Part D adds a surcharge to your premium for the rest of your life. Enter the ages you actually enrolled and see the monthly penalty, the annual cost, and the lifetime bill — plus how many months until the next 10% step.
Situation: Dana retires at 65 and keeps her employer's COBRA plan for 24 months because it feels familiar. She enrols in Medicare at 67.
Rule / rate: COBRA is not "creditable coverage" for Medicare penalty purposes, and there is no special enrolment period when it ends.
Calculation: 24 months late = 2 complete periods x 10% x $202.90 = $40.58 a month on Part B, plus 24 months x 1% x $36.78 = $8.80 on Part D if the drug coverage was also non-creditable.
Situation: Marcus keeps contributing to his HSA after 65 and delays both Part B and Part D for 3 years to stay eligible.
Rule / rate: You can keep an HSA only if you are not enrolled in Medicare; delaying enrolment to fund the HSA triggers the Part B penalty.
Calculation: 3 periods x 10% x $202.90 = $60.87 a month on Part B, plus 36 x 1% x $36.78 = $13.20 on Part D.
Situation: Priya enrols 20 months after her IEP ended and assumes the penalty is pro-rated.
Rule / rate: Part B does not pro-rate. Only complete 12-month periods count.
Calculation: 20 months late = 1 period = 10% x $202.90 = $20.29 a month. The remaining 8 months add nothing.
| Situation | Part B penalty / mo | Part D penalty / mo | Extra per year | Over 20 years |
|---|---|---|---|---|
| 20 months late to Part B only | $20.29 | $0.00 | $243.48 | $4,869.60 |
| 3 years late to Part B only | $60.87 | $0.00 | $730.44 | $14,608.80 |
| 18 months late to Part D only | $0.00 | $6.60 | $79.20 | $1,584.00 |
| 5 years late to both | $101.45 | $22.10 | $1,482.60 | $29,652.00 |
Part B counts only complete 12-month periods, so 20 months late earns one period and an 10% surcharge; 18 months late earns nothing yet. Part D counts every single month at 1% of the $36.78 national base premium, rounded to the nearest dime. Both penalties are added to your premium for as long as you keep the coverage, with no expiry date.
The Part B late enrolment penalty is 10% of the standard premium for every full 12 months you could have had Part B but did not. In 2026 the standard premium is $202.90, so one period adds $20.29 a month and five periods add $101.45 a month. The percentage is locked in when you finally enrol, then applied to whatever the standard premium becomes in later years — so the dollar penalty grows with premiums over time.
| Months late | Periods charged | Penalty on a $202.90 premium |
|---|---|---|
| 1–11 | 0 | $0.00 |
| 12–23 | 1 | $20.29 |
| 24–35 | 2 | $40.58 |
| 48–59 | 4 | $81.16 |
| 60+ | 5+ | $101.45 and up |
The Part D late enrolment penalty is 1% of the national base beneficiary premium — $36.78 in 2026 — for every month without creditable drug coverage. Unlike Part B, it counts months, not years. The fee is recalculated each year against the new base premium, rounded to the nearest ten cents, and it is added to your plan's own premium, which is billed separately.
You avoid the penalties only while you have coverage at least as good as Medicare: active employer group health coverage through your own or a spouse's current job (with 20+ employees), TRICARE, the VA, or a creditable drug plan. Retiree coverage, COBRA, and most individual market plans do not qualify, and there is no eight-month special enrolment period when COBRA ends.
File Form SSA-44 for a Part B penalty if you had creditable coverage through active employment, and ask Social Security or the Medicare plan for reconsideration for Part D. Keep the employer's creditable-coverage notice — it is the document that decides the appeal.
A late-enrolment surcharge is not a one-off fine. It is added to your premium for every month you remain enrolled, and because the percentage is applied to the standard premium, it grows as Medicare premiums rise. A single 12-month lapse today typically costs well over $5,000 across a 20-year retirement — more than most people would ever save by waiting.
⚠️ Important: Penalty rules depend on when you first became eligible, whether you had creditable coverage, and on annual premium changes that are not knowable in advance. This calculator applies 2026 figures and a flat premium assumption over your horizon; the real Part B premium is reset every year and the penalty percentage rides on top of it. Confirm your situation with Social Security before making an enrolment decision.