Millions of seniors count on Social Security to pay the bills in retirement. If you're expecting to rely heavily on your benefits, you'll need to read up on how Social Security works to make the most of the program. With that in mind, here are a few things you probably didn't realize about it.

1. It'll only replace about 40% of your preretirement income.

Many people assume that they'll manage to get by on Social Security alone in retirement, so they don't save properly during their working years. The reality, however, is that Social Security is only designed to replace about 40% of your preretirement income if you were an average earner. If your earnings were above average, your benefits will replace an even smaller percentage. Most seniors, however, need closer to 70% or 80% of their previous earnings to live comfortably, which means that if you're banking on those benefits alone to pay the bills, you're headed for trouble.

Social Security card on an angle with bottom half blurred out.


The good news is that if you're still working, you can shift your priorities and fund a retirement plan so that you have another income source with which to cover your expenses as a senior. Workers 50 and older can contribute up to $7,000 a year to an IRA or $25,000 to a 401(k). Even if you can't manage to max out either contribution, saving something for the remainder of your career will help ensure that you don't fall as glaringly short in retirement. For example, socking away $500 a month over 10 years will leave you with $75,000, provided your investments generate a conservative 5% average annual return during that time.

2. It's designed to pay you the same lifetime total regardless of when you file.

Though your Social Security benefits are calculated based on what you earned during your top 35 working years, the age at which you file for them will have an impact, too. If you file at full retirement age (66, 67, or somewhere in between, depending on your year of birth), you'll get the exact monthly benefit your earnings record entitles you to. If you file ahead of full retirement age (you can claim benefits as early as 62), you'll reduce your monthly benefits. And if you delay benefits past full retirement age, you'll boost them by 8% a year up until you turn 70.

That said, the program is actually designed to pay you the same total lifetime benefit regardless of the age at which you choose to file, provided you live an average life expectancy. The logic is that filing early will reduce your monthly benefit amount, but you'll collect more individual payments. Filing after full retirement age will boost your monthly benefits, but you'll receive a smaller number of individual payments.

There's one important caveat, though: You must live an average life expectancy to mostly break even under these different scenarios. If your health is poor going into retirement, it generally pays to claim benefits on the early side, since doing so increases your chances of coming away with a higher lifetime total. And if your health is outstanding, the opposite is true -- it generally makes sense to delay benefits, because if you live longer than the average senior, you'll come away with more money in total.

3. It might pay you retirement benefits even if you never work.

Even if you never earned money a day in your life, if you are or were married to someone who's eligible for Social Security, you may be entitled to spousal benefits based on his or her record. Your spousal benefits can amount to up to 50% of your current or former spouse's benefits at full retirement age. This means that if your spouse is entitled to a monthly benefit of $2,000, you'd get $1,000.

Keep in mind, however, that you can't file for spousal benefits until your spouse or former spouse starts taking Social Security him- or herself. If you're still married and are eager to get your hands on that money, you'll need to establish a filing strategy jointly with your spouse that works for both of you.

The more you know about Social Security, the better you'll be able to take advantage of it. Keep reading up on the program's various rules, especially as retirement nears.