A lot of near-retirees and retirees alike are aware that Medicare isn't free. Although most enrollees do not pay a premium for Part A, which covers hospital care, there are monthly premiums that need to be paid for Part B, which covers outpatient care.

Each year, Medicare sets a standard monthly premium for Part B. This year, it's $202.90.

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But some Medicare enrollees have to pay a lot more for Part B. If you're one of them, your costs could easily double.

Medicare isn't reserved for low-income seniors only. But if you earn too much money, you might have to pay more for Part B.

Higher-income seniors can be subject to income-related monthly adjustment amounts, or IRMAAs. They're basically surcharges on your premiums, and they apply not just to Part B, but Part D as well.

But while Part D IRMAAs could drive your costs up, they're lower than Part B IRMAAs. Part B IRMAAs could add hundreds of dollars a month to your premiums.

This year, IRMAAs apply to Medicare enrollees with a modified adjusted gross income (MAGI) of over $109,000 for singles, or $218,000 for couples filing jointly. But IRMAAs are tiered and increase by income level.

If you're single with a MAGI of $120,000, you'll have $81.20 a month added to your Part B costs, bringing your total monthly premiums to $284.10. But if you're single with a MAGI of $150,000, your IRMAA will be $202.90. That means you're looking at double the cost of the standard monthly premium for Part B -- $405.80.

The highest IRMAA applies to singles with a MAGI over $500,000 and joint filers with a MAGI over $750,000. In that case, the IRMAA is $487, and the monthly cost of Medicare Part B is a whopping $689.90 per month.

You might think that if you have a healthy retirement income, IRMAAs are unavoidable. But that's not necessarily the case.

You should know that Roth retirement plan withdrawals do not count toward your MAGI. So if you have your savings in a Roth IRA or 401(k), distributions won't drive you closer to IRMAAs.

If you don't have any Roth savings, it could pay to convert your traditional retirement accounts to Roth accounts before you enroll in Medicare. But you'll need to be careful with your timing.

IRMAAs are based on your MAGI from two years prior. If you do a large Roth conversion at 63, it could result in higher costs if you enroll in Medicare at 65.

Thanks to IRMAAs, the cost of getting Medicare coverage may be much higher than expected. Understanding how IRMAAs work, and how to avoid them, could save you a boatload of money in the course of your retirement.

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The Sneaky Reason Your Medicare Costs Could Double was originally published by The Motley Fool