The short answer
If you don't sign up for Medicare Part B when you're first eligible — and you don't have a valid reason to delay — you can face a lifetime late enrollment penalty. It adds 10% to your Part B premium for each full 12-month period you could have had Part B but didn't. And unlike some penalties, it doesn't disappear; you pay it for as long as you have Part B.
This is the penalty that costs people the most over time, precisely because it's permanent and it compounds with each year you delayed. The good news is that it's also very avoidable if you understand the rules.
How the math works
The penalty is 10% of the standard Part B premium for each full 12 months you were eligible but not enrolled. So if you went two full years without Part B when you should have had it, that's a 20% penalty added to your premium — every month, for life. The penalty is calculated against the standard premium, so it can grow as premiums rise.
Compare that to the Part D late enrollment penalty, which is calculated differently and is usually smaller. The Part B penalty is the one to take seriously because of its size and permanence.
When you're exempt
The key exemption is having creditable coverage from active employment. If you (or your spouse) are still working and you have group health coverage through that job, you can usually delay Part B without penalty and get a special enrollment period later. This is why so much depends on the size of the employer and whether the coverage is from current work rather than retiree or COBRA coverage.
The traps are subtle: COBRA and retiree coverage do not count as creditable for delaying Part B, even though they're real insurance. If you're weighing whether to take Part B while still working, our guide to when you need Part B with employer coverage walks through the decision.
Frequently Asked Questions
How much is the Part B late enrollment penalty?
10% of the standard Part B premium for each full 12-month period you were eligible but didn't enroll. It's added to your premium for as long as you have Part B.
Does the Part B penalty ever go away?
No. Unlike some penalties, the Part B late enrollment penalty is permanent — you pay it for the rest of the time you have Part B.
How do I avoid the Part B penalty?
Enroll when first eligible, or delay only if you have creditable coverage from current employment (yours or a spouse's). COBRA and retiree coverage don't count for delaying Part B.
Is the Part B penalty bigger than the Part D penalty?
Usually yes. The Part B penalty is calculated at 10% per year against the standard premium and is permanent, making it larger over time than the Part D penalty for most people.
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