Retirement: It's something everyone has to do eventually, and that many people look forward to. Unfortunately, retirement is also a major source of worry for millions of Americans -- largely because of concerns about where, exactly, cash is going to come from during retirement.

Concerns about financial security in retirement are not unfounded. Many Americans are woefully unprepared for retirement, with too little savings, unrealistic ideas about when retirement will occur, and no clear plan for paying for healthcare in retirement. In fact, these four jaw-dropping stats about retirement show that Americans are in a whole lot of trouble when it comes to their post-working years.

An old-fashioned alarm clock and a stack of coins sitting next to a jar labeled "Retirement"

Image source: Getty Images.

Some jaw-dropping retirement stats

17%: That's the number of workers very confident in their ability to live comfortably in retirement, according to the Employee Benefit Research Institute. While another 47% are "somewhat confident," having money as a senior isn't something you want to be unsure about. You need to become confident, by figuring out exactly how much money you'll need for retirement and making a plan to save the necessary funds.

25%: That's the percentage of current retirees who indicate some of their income comes from working for pay during retirement. It's a problem that the number is this low, because two-thirds of current workers expect work for pay will be a major or minor source of retirement income. Unfortunately, you can't count on your ability to continue to bring in a paycheck after you hit retirement age. Many people either are too sick to work, or struggle to find work, and have no choice but to live off their Social Security benefits and their savings. So, you need to make sure you have enough to support you without outside income. You also need to be aware that if you haven't yet hit your full retirement age and you work while on Social Security, those benefits can be reduced -- so even if you find a job, you may not be able to bring in much extra money.

40%: That's the percentage of current retirees who indicate their healthcare costs have been higher than anticipated. For far too many retirees, healthcare expenses aren't just a little higher than planned -- the costs are in a whole different ballpark. In fact, future retirees responding to one Nationwide Retirement Institute survey thought they'd spend 20% of their Social Security benefits on healthcare, but the average retiree who claims Social Security benefits at 64 could actually end up spending almost 65% of Social Security benefits on medical expenditures.

65: This is the median age at which workers expect to claim Social Security benefits. But the median age at which current retirees claimed benefits was 63. If you end up claiming Social Security earlier than anticipated, this can cause some big problems. Benefits will be lower if you claim earlier, because you receive a reduction for each year you retire before full retirement age (which, for people born after 1960, is 67). There's a big difference between claiming benefits at 62 versus at 65 -- and if you're planning on higher benefits but don't get them, this could lead to a financial shortfall. To err on the side of caution, assume you'll get the lowest possible benefits when factoring in how much income Social Security will actually provide you.

The numbers show: You need to know your retirement number

The only way to be confident about your retirement security is to know how much you'll need to live on -- including healthcare costs -- and how much you need to save to produce the desired income. Then, save enough to hit your target number.

Plan for much of your income in retirement to come from investments, don't plan to over-rely on Social Security, and start saving ASAP. That way, you can join the elite 17% who are confident about their financial futures as a retiree -- and hopefully you can enjoy your golden years with no money worries at all.