Millions of seniors count on Social Security for retirement income. But the less you know about the program, the more scary surprises you might be in for. Here are a few Social Security facts that may give you a fright.

1. Filing early could slash your benefits by 30%

The monthly Social Security benefit you're entitled to in retirement is based on your personal wage history. From there, you can collect that benefit in full once you reach full retirement age, or FRA.

FRA hinges on the year you were born. If that year is 1960 or later, your FRA is 67.

Social Security cards

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You're allowed to file for Social Security as early as age 62. But for each month you claim benefits ahead of FRA, that income stream takes a hit. If you file one year early, you'll lose 6.67% of your benefits. But if you file five years early, you'll slash your benefits by a whopping 30% -- for life.

2. Your benefits will replace only 40% of your income

Many people expect to fall back on Social Security once they retire and don't really worry so much about building savings of their own. But in reality, your benefits may not provide as much of a financial cushion as you'd expect.

If you're an average earner, you can expect Social Security to replace about 40% of your pre-retirement income. But most seniors need about 70% to 80% of their former earnings to live comfortably. And so if you don't come into retirement with savings of your own, you might really struggle to keep up with your expenses.

3. Benefits could be cut in a little over 10 years

Social Security needs money to operate, and it gets a lot of that money from payroll taxes. But in the coming years, that revenue is expected to shrink as baby boomers leave the workforce and too few people come in to replace them. As such, Social Security will owe more in benefits than it collects in revenue.

The good news is that the program has trust funds it can raid to cover its shortfall -- that is, until that money runs out. According to the program's Trustees, that's expected to happen by 2034. And at that point, Social Security may need to slash benefits across the board, leaving current and future recipients in the lurch.

Know what to expect from Social Security

All of these tidbits of Social Security information are far from comforting. They're also reality.

If you want to avoid financial stress during your retirement, it's important to understand how Social Security works and what to expect from the program. Filing for benefits too soon could leave you with a lot less income for life, and relying too heavily on those benefits could leave you in a position where you're unable to pay your bills.

While there's nothing you can do individually to prevent benefit cuts, you can compensate for them by socking money away for retirement in an IRA or 401(k) plan. That's something you should be doing even if lawmakers find a way to avoid a reduction in benefits.

If you play your cards right with Social Security, it could end up serving you very well throughout retirement. But if you don't read up on the program and plan around its shortcomings, your senior years could end up being much darker than you'd like them to be.