There are a good number of retired Americans today who effectively live on Social Security alone. And many people with savings still get the overwhelming majority of their retirement income from the monthly checks Social Security sends them.

But if that’s your plan for retirement, I’m here to tell you that it’s not a great one. There’s a big reason you can’t afford to retire on only Social Security — and it may not be the reason you think.

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Benefit cuts are a problem, but they’re not the only problem

If you’ve been reading the news, you may be aware that Social Security is at risk of having to cut benefits broadly. The reason stems from the fact that the program gets most of its revenue from payroll taxes. But in the coming years, that revenue stream is expected to shrink as baby boomers wrap up their time in the workforce and stop earning paychecks.

Social Security can use its trust funds to keep up with scheduled benefits for a good number of years, even in the absence of adequate payroll tax revenue. But once the program’s trust funds run out, benefit cuts will be a real possibility.

Recent estimates have Social Security’s trust funds running dry in about a decade from now, so that’s the rough timeline. There’s wiggle room, though, depending on what labor force participation rates look like in the coming years.

But all told, benefit cuts are something all current and future Social Security recipients need to plan for. And they’re a good reason not to rely on just Social Security for retirement income.

The real reason you can’t retire on Social Security alone

While benefit cuts have the potential to be a real problem for current and future retirees, they’re not the main reason it’s a bad idea to plan to exist on Social Security only in retirement. Rather, the primary reason that’s a poor choice is that Social Security will only replace a limited portion of your former paycheck to begin with.

Seniors are commonly advised that they’ll need 70% to 80% of their former income to live comfortably in retirement. But if you earn an average salary, Social Security will only replace about 40% of your pre-retirement wages. That could leave you with a huge shortfall.

And sure, it may be possible to get by on less than 70% to 80% of what you used to earn if you maintain a frugal lifestyle. But is a 60% pay cut really doable? Probably not. Or, it may be technically be doable but lead to a penny-pinching lifestyle you aren’t happy with.

Have a better plan

There’s nothing wrong with counting on Social Security to provide a decent chunk of your retirement income. But benefit cuts aside, it’s not prudent to plan to retire on only those monthly checks.

Even if lawmakers manage to avoid Social Security cuts, you could end up being forced into a very limited lifestyle if you don’t have at least one other income stream at your disposal. So whether it’s savings, investments, or a part-time job you commit to holding down, make sure to have a second option for paying your bills.

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