You're doing everything you can to prepare for retirement. You're contributing to your 401(k), you've got a rough estimate of your retirement number, and you've thought about when is the best age to finally leave work for good.

Chances are, though, you're still forgetting about one thing that could significantly derail your retirement: healthcare costs.

Man sitting at a table with coins and dollar bills in front of him

Image source: Getty Images

Four out of five workers don't consider healthcare expenses when they're determining how much they'll need to save to enjoy a comfortable retirement, according to the Employee Benefit Research Institute's 2018 Retirement Confidence Survey. Among those who have already retired, the numbers aren't much better -- three out of five retirees haven't calculated how much healthcare costs impact their savings.

Not planning for healthcare expenses? It could cost you

Healthcare costs may be an afterthought for most people when planning for retirement, but they can end up being one of your most significant expenses.

The average 65-year-old couple retiring today can expect to spend around $280,000 on healthcare alone during retirement, according to a 2018 study from Fidelity Investments, which includes out-of-pocket costs such as deductibles, premiums, and copays. That number will only continue to rise, too -- in 2002, the first year that Fidelity conducted the study, the average couple could expect to spend just $160,000 on healthcare during retirement.

Also, that estimate doesn't include injuries, illnesses, or other sudden and expensive health problems that may arise, and those costs can add up quickly. For example, one of every four people age 65 and older will experience a fall this year, according to the Centers for Disease Control and Prevention, and the average hospital bill to treat a fall injury is around $30,000.

Long-term care is another potential costly expense retirees will likely encounter at some point. The average 65-year-old can expect to spend around $138,000 in long-term care costs during retirement, according to a study from the U.S. Department of Health and Human Services, and only about half of that will typically be covered by insurance (if it's covered by insurance at all). Furthermore, one in six people will spend more than $100,000 in out-of-pocket long-term care expenses, researchers found.

In other words, healthcare costs can be incredibly high, and in order to avoid being blindsided by them, it's important to be prepared.

But what about Medicare?

Many retirees (mistakenly) believe that Medicare will cover all their healthcare needs during retirement. While it's true that Medicare can help significantly with some of these costs, it doesn't cover everything.

For instance, you still have to cover all deductibles, copayments, and coinsurance. And while emergency care and hospital visits are typically covered, Medicare normally doesn't cover routine care, including most dental care, eye exams, or hearing aids or exams. So if you injure your eye and need surgery, for example, Medicare may pick up the tab, but for a routine eye exam to get your eyeglasses prescription updated, you're on your own. Even routine care can cost hundreds of dollars, and those costs add up over time.

So what can you do to prepare for these costs? One option is to take advantage of a Medicare Advantage Plan (also known as Medicare Part C). Original Medicare (or Medicare Parts A and B) is what most people think of when they think about Medicare. While Original Medicare is not completely free (most people don't pay a premium for Part A, but the standard premium for Part B coverage is $134 per month, plus you still need to pay all deductibles, copays, and coinsurance), it's typically not as expensive as an Advantage Plan.

Advantage Plans are similar to the insurance plans you likely have through your employer, as they're offered through private companies. This means costs vary widely -- with some plans, for example, you won't have a monthly premium, but you may have to pay higher copays or coinsurance. But you still get all the coverage Original Medicare offers, plus Advantage Plans also usually cover additional services (like routine care). So while these plans may cost more in some ways, they can save you money by offering greater coverage so you don't need to pay as much out of pocket.

Another option for tackling medical expenses during retirement is a health savings account (HSA). An HSA is essentially a separate retirement fund just for healthcare expenses. The money you contribute is tax deductible, and then when you withdraw the funds, they're not taxed as long as they go toward eligible medical expenses.

The downside to HSAs is that not everyone is eligible -- you need to have a qualifying high-deductible healthcare plan in order to open one. For 2018, that means you need to have a minimum deductible of at least $1,350 for individuals or $2,700 for families. There are also limits to how much you can contribute each year. In 2018, individuals are able to contribute up to $3,450 per year, while families can contribute $6,900 annually (and in both cases, you can contribute an additional $1,000 per year if you're age 50 or older).

Despite their limitations, though, an HSA can be a valuable tool for stashing away some money specifically for healthcare costs. For example, let's say you're 45 years old and are contributing $2,000 per year to an HSA. If your savings are earning an annual rate of return of, say, 6%, you'll have $77,985 stashed away by the time you turn 65 -- and that money can go a long way in helping cover medical expenses.

Saving for retirement is difficult enough as it is, but it's even more challenging when you have the uncertainties of healthcare expenses looming over you. While it's impossible to predict exactly how much you'll have to pay for healthcare during retirement, you can prepare yourself wisely.

The Motley Fool has a disclosure policy.