It's a huge misconception that Social Security pays seniors enough to cover their expenses completely. In reality, those benefits will replace about 40% of the average earner's pre-retirement wages, and most seniors need somewhere in the ballpark of 70% to 80% of their former earnings to keep up with their bills.

Put another way, the average senior on Social Security today gets about $18,000 a year. If you're currently living on, say, $54,000 a year, you probably won't manage to get by on a third of that, even if you're willing to cut back on expenses.

That's why it's so important to have retirement income sources outside of your Social Security benefits. Here are a few to consider.

1. An IRA or 401(k)

Saving in a dedicated retirement account is a good way to supplement your Social Security income. If your employer offers a 401(k), signing up automates your savings, as contributions will be seamlessly deducted from your paychecks on an ongoing basis. Also, many employers that sponsor 401(k)s also match some percentage of employee contributions, so you may get free money for your senior years that way.

Older couple looking at documents in front of a laptop and a calculator.

Image source: Getty Images.

If you don't have access to a 401(k), you can instead save for retirement in an IRA. You may not have an employer match, but on the plus side, you'll generally get more investment choices with an IRA than with a 401(k).

Of course, these plans vary by contribution maximums, too. Currently, 401(k)s max out at $19,500 a year for workers under 50 and $26,000 for those who are 50 or older. Today's IRA limits, on the other hand, are $6,000 and $7,000, respectively. But that doesn't mean you can't save a bundle with an IRA. In fact, if you max out at this year's levels between the ages of 35 and 65, and your investments in that account give you an average annual 7% return (which is just below the stock market's average), you'll end up with about $590,000.

2. A health savings account

Healthcare is a major expense for seniors throughout retirement. Fidelity estimates its cost at $295,000 for a typical 65-year-old couple kicking off that milestone today. That's why it pays to put money into a health savings account, or HSA. That way, you'll have a dedicated source of income to tap for medical care, and you'll be able to use your Social Security benefits to cover your remaining bills.

Currently, HSA contributions max out at $3,550 a year if you're participating just on your own behalf, or $7,100 if you're contributing on behalf of your family. If you're 55 or older, you get a $1,000 catch-up -- the same catch-up older workers get in an IRA. The only issue with HSAs is that you must be enrolled in a high-deductible health insurance plan to take advantage of one. This year, that means a deductible of at least $1,400 as an individual or $2,800 as a family.

3. Municipal bonds

Though there's nothing wrong with investing some of your money in stocks as a senior, it's also wise to focus on bonds, which are far less volatile. With bonds, you can generally look forward to regular interest payments in addition to your Social Security income. While all bonds pay interest, municipal bonds offer unique tax benefits -- namely, that they're always tax-free at the federal level, and if you buy bonds issued by your home state, you get to avoid state and local taxes on your interest income. By contrast, IRA or 401(k) withdrawals can be taxable.

Chances are, you won't manage to live on Social Security alone. Rather than figuring that out the hard way once you're already retired, line up some additional income sources in advance. You'll be thankful you did.