You're sailing along, socking some money away for retirement, figuring that it will be enough. You know that health care costs have been rising rapidly, but you figure that you'll be covered by your employer in retirement, and there's always Medicare. Right? Well, think again.

Did you know that Medicare only covers, on average, about half of your expected medical expenses? According to our friends at the Employment Benefit Research Institute (EBRI), if you're 65 years old and retiring now, you'll need to have saved at least $122,000 to be able to cover your health care costs in retirement. Here are the scary numbers:

How much you'll need by age 65, in 2008

Men

Women

Couples

If your premiums are covered by your former employer

$122,000

$140,000

$235,000

If you pay your own premiums

$196,000

$224,000

$376,000

Data: EBRI.

It gets even worse, because hitting these numbers only means you'll have a 90% chance of covering all your health care costs. To have a higher chance, you'll need to amass even more. Also, these numbers don't even include the possible cost of nursing-home care. It gets worse still if retirement is a decade away:

How much you'll need by age 65, in 2018

Men

Women

Couples

If your premiums are covered by your former employer

$204,000

$235,000

$394,000

If you pay your own premiums

$329,000

$375,000

$630,000

Data: EBRI.

So, how can you get to those amounts? Well, they do seem huge, but know that you can get there with some planning and discipline, if you have some time. Invest $4,000 each year for 20 years and earn an annual average of 9%, and you'll hit $220,000.

Earn a better return and you'll get there faster, true, but it's also true that over relatively short periods, such as 10 or even 15 years, even good companies (and mutual funds) can turn in lackluster results -- think of our last decade, for example:

Company

10-year average annual return

Diageo (NYSE:DEO)

6.0%

Fortune Brands (NYSE:FO)

3.8%

Lowe's (NYSE:LOW)

3.8%

Boeing (NYSE:BA)

2.9%

MGM Mirage (NYSE:MGM)

(2.6%)

CVS Caremark (NYSE:CVS)

1.3%

Cisco Systems (NASDAQ:CSCO)

(4.2%)

Data: Yahoo! Finance.

Don't mess up your retirement. If you'd like to avoid a gruesome retirement, try our Rule Your Retirement newsletter service for free, with full access to all past issues. It regularly offers recommendations of promising stocks and mutual funds, too.

Longtime Fool contributor Selena Maranjian does not own shares of any companies mentioned in this article. Diageo is a Motley Fool Income Investor pick. Try our investing newsletters free for 30 days. The Motley Fool is Fools writing for Fools.