Count the uncovered months
Medicare counts the full months after your Initial Enrollment Period ended when you had neither a Part D plan, a Medicare Advantage plan with drug coverage, nor other creditable prescription drug coverage.
Free Medicare estimate tool
Medicare Part D Late Enrollment Penalty Calculator
Estimate the Part D late enrollment penalty from the months you went without creditable drug coverage. The calculator shows every step of Medicare's published formula, so you can check the number against a penalty notice or plan for it before you enroll.
Official sources checked September 27, 2026Private: nothing you enter is submitted.
Estimate your Part D late enrollment penalty
Count the full months you went without Part D or other creditable drug coverage after your Initial Enrollment Period ended. The calculator applies Medicare's published 1%-per-month method.
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The result shows the penalty percentage, the monthly amount added to your Part D premium, and a 12-month total for the selected year.
How the Part D late enrollment penalty is calculated
Medicare uses the same method for everyone. The penalty is a permanent surcharge, not a one-time fee, so the monthly figure matters more than it first appears.
Multiply by 1% of the base premium
Each uncovered month adds 1% of the national base beneficiary premium. The base premium is a national figure, not your plan's premium, and CMS publishes a new one every year.
Round and add it to your premium
The result is rounded to the nearest $0.10 and added to your monthly Part D premium for as long as you have Medicare drug coverage. It is recalculated each year when the base premium changes.
Worked example
Suppose your Initial Enrollment Period ended in March 2024, you had no creditable drug coverage, and your Part D plan started in June 2025. The uncovered months are April 2024 through May 2025: 14 months. In 2026 the estimate is 14% × $38.99 = $5.4586, rounded to $5.50 a month, or $66.00 over twelve months — on top of the plan's own premium.
The 63-day rule
A penalty is only assessed if, at any time after your Initial Enrollment Period ended, you went 63 or more days in a row without Part D or other creditable drug coverage. A short gap between two plans usually does not trigger it. If you are unsure, compare the exact start and end dates on your coverage notices.
What counts as creditable drug coverage?
Creditable coverage is drug coverage expected to pay, on average, at least as much as Medicare's standard Part D coverage. Many employer, union, TRICARE, VA, and Indian Health Service plans qualify. Plans are required to tell you each year whether their drug coverage is creditable — keep those notices, because they are how you prove months should not be counted.
Part D and Part B penalties are separate
The Part B late enrollment penalty uses a different formula — generally 10% for each full 12-month period you could have had Part B but did not. This calculator estimates the Part D penalty only. Use the Medicare enrollment timeline to check your Initial Enrollment Period and avoid both.
Check my Medicare enrollment datesWhen you may not owe the penalty
- You qualify for Extra Help (the Low-Income Subsidy) with Medicare drug costs.
- You had creditable drug coverage for the months in question and can document it.
- You did not go 63 or more consecutive days without creditable coverage.
- Medicare agrees on reconsideration that the penalty was applied in error — you generally have 60 days from the date on the penalty letter to ask.
Part D penalty questions
Is the Part D late enrollment penalty permanent?
Generally, yes. The penalty is added to your monthly Part D premium for as long as you have Medicare drug coverage, even if you change plans. The dollar amount can change each year because it is based on that year's national base beneficiary premium.
What is the national base beneficiary premium for 2026?
The national base beneficiary premium used for 2026 penalty calculations is $38.99. It was $36.78 in 2025 and $34.70 in 2024. It is a national reference figure, not the premium of any particular plan.
Does Medicare Advantage with drug coverage count?
Yes. A Medicare Advantage plan that includes prescription drug coverage counts as Medicare drug coverage, so months enrolled in one are not uncovered months.
Can I avoid the penalty if I had employer coverage after 65?
If your employer or union drug coverage was creditable, those months are not counted. Ask the plan for its creditable coverage notice and keep it; Medicare may ask you to show it when you enroll in Part D.
Is this calculator's result my official penalty?
No. It is an educational estimate using Medicare's published formula. Medicare and your drug plan determine whether a penalty applies and the exact amount, and they may have coverage records you have not entered here.
Already facing a penalty, or about to enroll late?
Enrolling now stops new months from adding to the penalty. Compare Part D and Medicare Advantage drug coverage by your actual prescriptions and pharmacy, then confirm the plan's formulary before you enroll.
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