Social Security isn't designed to sustain retirees without other income. Those benefits pay the average senior around $1,500 a month, or $18,000 a year, and for many older Americans, that's not enough income. It's for this reason that workers today are encouraged to save independently for retirement, and when we think about doing that, we usually land on tools like IRAs or 401(k) plans. But there's another powerful long-term savings tool you may have access to without even realizing it: a health savings account (HSA).

HSAs make it easy and cost-effective to set aside funds for near-term and future medical expenses. Yet among savers who have an HSA, only 19% are using it to sock away money for retirement, according to a white paper published by Optum Bank and Empower Retirement.

Older man sitting on couch reading  magazine

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If you have the option to participate in an HSA, it pays to make the most of that account. And that means using it to set yourself up for a more secure retirement.

Put your HSA to work

With an HSA, you can allocate money on a pre-tax basis to cover medical expenses both now and in the future. But the best way to maximize that account is to contribute more than what you expect to need for near term medical costs.

The reason? HSA funds never expire, and any money you don't use immediately can be invested for added growth, the same way you're able to invest the money you save in an IRA or 401(k). Now, imagine you put an extra $2,400 a year into an HSA over 20 years, and invest it at an average annual 5% return (which is a fairly conservative return). You'll wind up growing that extra money into over $79,000, which you can then use as a source of retirement income.

And remember, you get a triple tax break on the money you put into an HSA. Your contributions are tax-free, investment growth in that account is tax-free, and withdrawals are tax-free, as long as they're used to cover qualified medical expenses.

Speaking of medical expenses, you're apt to have your fair share of them as a senior, from Medicare premiums to deductibles to copays. Having funds set aside for that purpose will help you better manage your money in retirement, and it will also put less of a strain on your Social Security income and savings.

Of course, not everyone is eligible for an HSA. To participate, you must be enrolled in a high-deductible health insurance plan and meet other requirements. You can check out this HSA guide to learn more. But if you have an HSA, don't just use it to set aside funds for immediate medical expenses. Instead, use it as another way to sock away money for retirement. Even if you don't end up using that money for medical expenses as a senior, you won't be penalized if you withdraw HSA funds for non-healthcare purposes once you turn 65. The only benefit you'll lose in that case is tax-free withdrawals, but that's really no different than taking withdrawals from a traditional IRA or 401(k).

Ultimately, using your HSA as a retirement savings tool gives you more financial flexibility later in life. And that's an unquestionably important thing to have.