Social Security checks are set to get a substantial boost next month as the 8.7% cost-of-living adjustment (COLA) for 2023 kicks in. This will add $147 to the average senior's monthly check, and some could get a lot more. But others may get smaller benefits than they expect in 2023. 

Below, we'll take a look at three reasons why. Some of these may be familiar to seniors who are already claiming Social Security, while others could come as a surprise to current beneficiaries.

Person holding check and talking on the phone.

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1. Medicare Part B premiums come out of your checks

Adults 65 and older who are on Medicare have their Medicare Part B premiums automatically withheld from their checks each month. Currently, that's $170.10 per month for most people, but it will fall to $164.90 in 2023. Some high earners pay more than this. 

But some argue that this automation is a benefit rather than a drawback. If the government didn't take your Medicare premiums out of your Social Security checks, you'd get a bill for them each month. This way, you have one fewer monthly payment to worry about. 

2. The federal government can garnish your checks

Most people have only their Medicare Part B premiums withheld from their Social Security checks, but others have even more taken out if they're behind on some of their financial obligations. Those who owe back taxes, child support, or alimony will have some of their checks garnished until these debts are paid off.

But you won't lose your whole check regardless of how much you owe. The government will withhold only up to 15% of each check for unpaid federal taxes. The most you can lose for unpaid child support or alimony is 65% of your checks. And to pay that much, you'd need to be more than 12 weeks behind on payments and not supporting another spouse or child other than the one you're ordered to pay child support or alimony to.

It's pretty tough to argue with the IRS about unpaid taxes, but if you don't believe you should owe the child support or alimony, you can always file an appeal with the court that filed the garnishment order. However, unless the decision is overturned, you'll continue to lose money from your Social Security checks each month.

3. You could owe taxes on your benefits

The IRS has taxed the Social Security benefits of seniors who have earned over a certain income since 1984, but the threshold for benefit taxation hasn't changed over the years. This means that over time, more and more seniors have had to give a portion of their benefits back to the government. And we can expect this trend to continue in 2023 as all benefits are expected to rise thanks to the COLA.

The Social Security benefit taxation formula is a little beyond the scope of this article. But basically, if your provisional income -- your adjusted gross income, plus any nontaxable interest you have, and half your annual Social Security benefits -- exceeds $25,000 for a single adult or $32,000 for a married couple, you will owe something.

You could owe your state government some of your Social Security benefit as well. Currently, 12 states tax Social Security benefits of some of their seniors. But each has its own rules for determining who owes these taxes and how much they'll pay. Seniors who live in one of these states should reach out to their state department of taxation for more information.

None of this is meant to scare you. But it's important to be aware of where else your checks could go so you can budget appropriately. If none of the three situations above apply to you, you can spend your entire benefit freely. But if you think you could lose money to one or more of the things above, you may need to reduce your spending or draw more upon your personal savings to make up the difference.