Many workers don't save for retirement and instead plan to fall back on Social Security during their golden years. The problem, however, is that those benefits were never designed to sustain seniors on their own. In a best-case scenario, they'll replace about 40% of your pre-retirement earnings, assuming you bring home an average income. Most seniors, however, need about twice that much to live comfortably, which means that money needs to come from somewhere -- namely, retirement savings.

Unfortunately, a large number of Americans are falling seriously short in that regard, with an estimated 42% of working adults having no money set aside for the future whatsoever. The reason? For some, it boils down to an overreliance on Social Security and a missing dose of reality. In other words, some folks think they don't need to save on their own because they'll get by on whatever Social Security pays them, all the while assuming that their living costs will drastically drop once they stop working.

Detail of a U.S. hundred-dollar bill

IMAGE SOURCE: GETTY IMAGES.

Here's the thing, though: When we think about the expenses we face during our working years and compare them to what we're facing in retirement, there's not much of a difference. Once you stop working, you'll still need to pay to have a roof over your head, a means of getting around town, and a reasonable supply of food and personal care items. You'll also need to cover the cost of healthcare, which could increase substantially once you're no longer covered by a subsidized health plan through work. And that's why workers are told to expect to need about 80% of their pre-retirement income to live comfortably.

But some people who aren't saving for retirement aren't clueless about Social Security's buying power and the cost of senior living. Rather, they're struggling to eke out the money to save. And if you're a low or middle earner, that makes sense. Even if you're living a relatively frugal lifestyle, with limited wages, maxing out an IRA or 401(k) year after year is extremely challenging, if not close to impossible.

The good news, however, is that you don't have to max out to help your future self. In fact, contributing a mere $100 a month to a retirement plan could be just the thing to salvage your golden years.

What can $100 a month do for you?

When you're working with a $30,000-a-year salary, saving 15% to 20% of your earnings -- which is what most financial experts recommend -- may not be feasible. But if you set a lower target of $100 per month, you're more likely to meet that goal. And while it won't allow you to retire rich, it'll certainly help. Here's what your total savings might look like if you were to sock away $100 a month over various periods of time:

If You Start Saving $100 a Month at Age:

Here's What You'll Have by Age 70 (Assumes a 7% Average Annual Return):

25

$343,000

30

$240,000

35

$166,000

40

$113,000

45

$76,000

Data source: author.

Now let's be clear: $76,000 isn't a whole lot of money to retire with, especially when you consider that most seniors are advised to withdraw around 4% of their nest egg each year to cover living expenses. That translates into about $3,000 a year, which, when added to the current average annual Social Security benefit of roughly $17,500, equals only $20,500 a year of income.

On the other hand, $343,000 is a nice sum of money to work with, and that's what you stand to accumulate if you start saving $100 a month consistently over your entire career. When we apply a 4% withdrawal rate to $343,000, we get close to $14,000 a year of income. That, plus $17,500 from Social Security, translates into over $31,000 -- a far more livable income level.

Another thing: That 7% average annual return is actually a couple of percentage points below the stock market's average. If you invest in an IRA or 401(k) for multiple decades and load up on stocks, you're likely to see that sort of return, if not a higher one.

If you want to avoid struggling financially in retirement, you'll need income outside Social Security to get there. Even if your earnings leave much to be desired, you can still build a substantial nest egg with just $100 a month. The key, however, is to save that $100 consistently, and for the duration of your working years, to ensure that you don't fall short down the line.

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