Healthcare costs are one of the biggest wild cards in retirement. They can also be extremely difficult to plan for.

Sure, you can read up on what the average retiree spends on healthcare expenses (hint: it's $172,500, according to Fidelity). But even so, you don't know what your health will end up looking like. And if you end up with chronic conditions or other issues, healthcare could easily become one of your biggest retirement bills, if not the single biggest.

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If that worries you, you're not alone. But here's a strategy that could make future healthcare costs much easier to manage.

Use this powerful savings tool

You could boost your IRA or 401(k) contributions so there's more money available for healthcare expenses in retirement. But you might feel better knowing you have a dedicated pool of funds to cover healthcare costs during your senior years. That's where a health savings account (HSA) comes in.

What makes HSAs so great is that they offer three unique tax breaks:

• Contributions are tax-free.
• Investment gains are tax-free.
• Withdrawals are tax-free as long as the money is used to cover qualifying healthcare expenses.

Best of all, HSA funds never expire. You could make contributions to an HSA in your 20s and carry that money all the way into your 60s.

In fact, let's say you have health insurance that's HSA-compatible for a while and that by age 35, you have a $17,000 balance. If you then leave that money alone until age 65 and invest it at an 8% yearly return, which is a bit below the stock market's average, you'll have roughly $171,000 available for your future healthcare needs.

Another thing you should know is that if you end up in the enviable position of having too large an HSA balance during retirement, your money won't have to go to waste. Once you turn 65, you can withdraw HSA funds for non-medical purposes without a penalty. In that situation, you'll simply owe taxes on your withdrawals similar to what would happen with a traditional IRA or 401(k).

A simple strategy that could make a big difference

Funding an HSA and letting the balance grow is a great way to arm yourself against future healthcare costs. So it pays to see if your health insurance plan meets the requirements to participate in an HSA.

And if that's not the case this year, check again next year, and every year after that. HSA requirements change annually, and it pays to keep track so you can enjoy the benefits these accounts have to offer.