Medicare charges a late enrollment penalty when you sign up for Part B or Part D after your window to do so without one has passed. For Part B, your monthly premium may go up 10% for each full 12-month period you could have had it but didn’t sign up. For Part D, it’s 1% of a national base amount for each full month you went without Part D or other creditable drug coverage, once you’ve had a gap of 63 or more days in a row after your Initial Enrollment Period. Either one is generally added for as long as you have that coverage.
Sourced from Medicare.gov’s late enrollment penalties page, the Medicare & You handbook (CMS Pub. No. 10050), and Medicare’s Part D late enrollment penalty fact sheet (CMS Product No. 11222).
- A late enrollment penalty is added to your monthly premium. It is not a one-time fee, and for most people it lasts as long as they have that coverage.
- Part B counts only full 12-month periods; Part D counts every full uncovered month, but only once there's been a gap of 63 or more days in a row without creditable drug coverage.
- Delaying Part B generally won't bring a penalty if you've had qualifying group coverage based on current employment since you were first eligible, and sign up within that Special Enrollment Period's limits. COBRA and retiree coverage don't count.
- If a Part D penalty letter looks wrong, you have 60 days from the date on the letter to ask for a reconsideration.
The three penalties side by side
The Part A penalty applies only to people who have to pay a premium to get Part A. This post focuses on the timing rules for Parts B and D.
| Part A (only if you buy it) | Part B | Part D | |
|---|---|---|---|
| What triggers it | Not buying Part A when first eligible | Not signing up when first eligible, without a Special Enrollment Period | 63+ days in a row without Part D or creditable drug coverage, after your Initial Enrollment Period |
| How it’s figured | Premium may go up 10% | 10% of the standard premium per full 12-month period | 1% of the national base beneficiary premium per full uncovered month |
| How long it lasts | Twice the number of years you could have had Part A but didn’t sign up | Generally as long as you have Part B | Generally as long as you have Medicare drug coverage, even if you switch plans |
| Usually avoided by | Signing up in a Special Enrollment Period | A Special Enrollment Period, or enrolling in a Medicare Savings Program | Joining when first eligible, keeping creditable coverage, or getting Extra Help |
Part B: only full 12-month periods count
Part B’s penalty grows in steps. The Medicare & You handbook’s own example: a man whose Initial Enrollment Period ended in December 2021 waited until March 2024 to sign up. That’s 27 months without Part B, but only two full 12-month periods, so his penalty is 20%, added to the standard Part B premium for as long as he has Part B.
Delaying Part B without a penalty generally depends on qualifying for a Special Enrollment Period, and for the current-employment one your coverage history matters, not just what you have today. You generally need to have had group health coverage based on current employment (your own or a spouse’s, or in some disability cases a family member’s) when you were first eligible — or to have signed up for Part B during your Initial Enrollment Period — and to have kept that coverage or Part B every month since. If so, you can sign up any time you’re still covered, or during the 8 months after the employment or the coverage ends, whichever happens first. This Special Enrollment Period isn’t available if you’re eligible for Medicare only because of End-Stage Renal Disease. COBRA and retiree coverage are not current employment, and they don’t extend that 8-month clock. If you’re weighing this, my post on working past 65 covers the employer-size rule and the separate Part D decision, and a warning for anyone with an HSA: enrolling in Medicare, even Part A alone, affects your ability to contribute, and if you sign up for premium-free Part A after 65 it can start up to six months earlier (but not before the month you turned 65). Confirm the timing with your tax professional or IRS Publication 969 before you enroll.
Medicare also recognizes some exceptional situations for a Special Enrollment Period, including a natural disaster or emergency, an employer or health plan error, and losing Medicaid coverage, among others. Medicare decides whether a situation qualifies.
If none of that applies, the General Enrollment Period runs January 1 through March 31 each year, and coverage starts the first day of the month after you sign up. The penalty may still apply. Part B enrollment happens through Social Security, or the Railroad Retirement Board if your benefits come through them, so that’s also where to ask if a Part B penalty looks wrong.
Part D: every full month counts, once a 63-day gap opens
Part D works differently. After your Initial Enrollment Period, the penalty can apply if you go 63 or more days in a row without Part D or other creditable prescription drug coverage. Creditable means drug coverage expected to pay, on average, at least as much as Medicare’s. Once that gap opens, Medicare counts each full uncovered month as 1%.
The 1% is taken of the “national base beneficiary premium,” a figure that can go up or down each year, so the penalty amount can change each year too. In the handbook’s example, the percentage stayed the same from one year to the next while the amount moved with the base. Medicare’s fact sheet walks through someone who went 17 full months without coverage: their penalty is 17% of that year’s base amount, rounded to the nearest 10 cents and added to their monthly premium.
Extra Help changes the math. You aren’t charged a Part D late enrollment penalty while you get Extra Help, and uncovered months before you qualified don’t count toward a penalty later. If you think you might qualify, my post on Extra Help and Medicare Savings Programs explains the programs and how to apply.
Why the Annual Enrollment Period doesn’t stop the count
Here’s a timing detail worth knowing before October 15. The Annual Enrollment Period (AEP) runs October 15 through December 7, and someone who has gone without drug coverage can join a Part D plan then. That coverage generally starts January 1, so the uncovered months keep counting through December. Medicare’s own example shows it: a person without coverage from August 2024 who joined in the fall of 2025, with coverage starting January 1, 2026, was charged for 17 months, August 2024 through December 2025.
So if another enrollment period you qualify for would start your drug coverage sooner, it can reduce the number of uncovered months counted. For a Special Enrollment Period, timing and plan choices depend on the specific event, so check the rules before counting on one. The Medicare Advantage Open Enrollment Period (January 1 – March 31) is a separate window with its own rules. If you already have a drug plan, my post on re-checking Part D every fall is the better read for AEP.
If a Part D penalty looks wrong
For example, someone could have had drug coverage through work for years, not kept the paperwork, and received a penalty letter after joining a Medicare drug plan. If that coverage was creditable, reconsideration is how to show it.
Use the reconsideration form that comes with the penalty letter, and return it within 60 days of the date on the letter. If you’re late, you’ll need to explain why. Include proof, like a creditable coverage notice from an employer or union plan. A Medicare contractor not connected with your plan reviews it and generally decides within 90 days, though it can take up to 14 more days in some cases. Until then, you still have to pay the penalty with your premium. By law it’s part of the premium, and a plan can disenroll you for not paying it. If the contractor finds the penalty wrong, your plan removes or adjusts it and tells you whether you’ll get a refund of what you paid during the review.
Before you delay anything
The risky assumption is that other coverage automatically counts. Sometimes it does and sometimes it doesn’t, and the difference is set by Medicare’s rules, not by how good the coverage feels. If you’re approaching 65 or leaving a job, my turning-65 checklist lays out the order of steps, and my Medicare page walks through the basics of Parts A through D.
If you’d like a second set of eyes on your own timing, you can book a time to talk. I work with multiple carriers, and we can go through your dates before you make a decision.
