Millions of seniors today collect monthly benefits from Social Security. And for some older Americans, those benefits are their primary or even sole source of retirement income.

You might assume that Social Security will suffice in covering your retirement expenses in full. But if you operate under that assumption, you might end up woefully cash-strapped during your senior years.

Don't overestimate your Social Security income

A lot of people are able to trim their spending in retirement, whether by downsizing their homes, giving up a car in the absence of having a commute, or relocating to a less expensive part of the country. And you might assume that if you're willing to do these things, then you'll be able to retire on Social Security alone.

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But one thing you should know is that Social Security will generally replace about 40% of your pre-retirement wages if you're an average earner. And while you may not need 100% of your former income once your retirement becomes official, there's a big gap between 40% and 100%.

As a general sort of rule, seniors tend to need about 70% to 80% of their former earnings to live comfortably. But there are of course outliers.

Your needs may or may not fall into that range depending on the sort of retirement you want. But either way, living on 40% of what you used to earn is probably far from ideal.

So if you don't want to spend your entire retirement pinching pennies, you'll need to recognize that Social Security will only replace a modest portion of your former salary. And you'll need to build up some savings to give yourself the spending power in retirement that you want.

There's an easy way to know what benefit you're in for

Some people enter retirement without any clue as to what sort of monthly benefit they're entitled to from Social Security. A better bet? Create an account on the Social Security Administration's website and review your most recent earnings statement. It should give you an estimate of your future benefit right there.

Of course, the farther away retirement is, the less accurate that estimate might be. But if you're, say, in your 40s, it's a good starting point.

Go in armed with knowledge

Thinking you'll be fine to retire on Social Security alone is a mistake that could ultimately make your life miserable when you're older. Rather than sentence yourself to a cash-strapped retirement, get an estimate of the Social Security income you're in line for, and then take steps to build up some savings so you're able to supplement your benefits once you start collecting them.

Meanwhile, if you're on the cusp of retirement and have no idea what to expect from Social Security, it's imperative that you get some details on what your benefits look like before resigning from your job. If that number is smaller than expected, you can at least pivot at the last minute and delay your retirement by a few years. But if you make your retirement official and then realize how little Social Security is paying you, you might end up in a really tough situation.