The most underappreciated risk to prosperous retirement is your own physical prosperity.

Few topics spill more ink in personal finance than how much to save for retirement.

In some parts of the world government programs provide sufficient income and, often, healthcare, through retirement.

But in the United States, retirees are largely on their own. Social security and health programs provide minimal benefits to retirees as early as 62, and some older workers can still rely on guaranteed pensions, but for most Americans, the bulk of retirement spending must be made up by personal savings. The system is progressive: the more you make, the more the burden is on you.

Many savers think they are doing just fine—especially if they are saving the 2% or 3% rate in a 401(k) that many tax-advantaged corporate savings plans now default employees to, plus a marginal employer match. Those savers who make active decisions with their retirement savings approach 6% to 7%, on average. Take that savings level, combined with Social Security as a base, and  focus on low-fee options and a sensible asset allocation, the financial industrial complex would have us believe that—presto—we are retirement ready. But more and more evidence is surfacing that rising costs in retirement mean the amount of “replacement income” one will need over the course of retirement is increasing, from an 80-85% replacement rate closer to 100%.

That 6% figure then, even including a match from an employer, turns out to be a safety net with a gaping hole in the middle. Even progressive savers are sorely underestimating how much they need to sock away.

“If you’re not saving 10% of your own income, you are making a mistake,” said Steve Utkus, head of the Vanguard Group’s Center for Retirement Research. The most aggressive financial planners actually recommend saving away up to 15% of annual income. And the later you start, the more  you will need to put aside to replace your income in retirement. The bogey on everyone’s radar, of course, is saving enough over the course of your working life to retire comfortably, without significant financial stress in your golden years. It’s the final stage of the American dream, an ideal co-opted in countless financial services commercials and sales pitches. “With proper planning, you too can have it all!”

Why is 6% in a 401(k) not enough? It isn’t just that things cost more. Blame an even more powerful trend, a notion once seen as a blessing: longevity. Yes, the most underappreciated risk to prosperous retirement is your own physical prosperity.

“Look at the daily obituaries. Everyone dies at a different age,” said Moshe Milevsky, finance professor at the Schulich School of Business at York University in Toronto. “Longevity risk, and the uncertainty of good versus bad health, is as volatile as the stock market.”

According to the Society of Actuaries, a US and Canadian trade group that measures risk in finance, insurance, health and pensions, approximately 54% of retirees underestimate the life expectancy for the average person their own age. As a result, many people may be overestimating how much they can spend each year in retirement, and in turn will quickly deplete assets.

Those ages have been advancing while the rule of thumb on savings has barely budged and corporate safety nets have worn even thinner. Life expectancy for American males improved by more than two years per decade since 1970, to 75.7 years. For females, the increase was 1.5 years per decade to 80.8 years.

This shift in life expectancy has already destabilized both public and private pension schemes everywhere and helped accelerate the move from defined benefit retirement plans — whereby a pensioner receives some level of his final salary in perpetuity — to “defined contribution” plans put the onus on savers in the US and around the world. At the end of 2011, 3% of US private sector workers participated only in a defined benefit plan and 31% participated in a defined contribution plan and 11% had access to both.

Now, we are squarely in the era of self-determination, but the shift in savings mindset has not kept pace with the acceleration of retirement costs or the on-your-own nature of retirement income. Employee Benefit Research Institute’s Retirement Security Projection Model estimates many people will have a shortfall based on a formula that considers first spending down all retirement savings assets, other financial assets and selling your house when you’re broke. The model includes assumptions on retirement’s biggest and fastest growing expense: escalating health care costs.

For an overly simplistic view, I turned to the Retirement Planner at MarketWatch, a tool on which I was the lead editor while at SmartMoney. To demonstrate the effects of savings rates, longevity and replacement income, I entered a 25 year-old, starting out saving 3% of his $30,000 salary until retiring at 67, with annual raises in line with inflation. Assuming he receives Social Security and his investments grow at a constant 5% per year through retirement, even with no taxes, this saver hits a shortfall at age 75 with an 85% replacement rate. Flip that savings to 6%, and he could make it to 87. At 10%, he’ll have enough money to get through age 95!

See what happens when you increase the replacement rate in “Retirement Spending” to 95%!

But progress is being made in framing retirement savings in term of income replacement instead of one big pile of money. (See a recent announcement by the U.S. Employee Benefit Security Administration on Lifetime Income.) New programs and tools are shifting the conversation from magic numbers to more tangible current value equivalent, using base case, and best case, scenarios for income, saving, spending and capital markets. But it takes time—time that many people cannot afford to lose—to shift the savers’ mindset.

 “In the end, it’s not about spend less, save more, be smart,” said Milevsky. “It’s about your tolerance for longevity risk and willingness to adjust your standard of living now to compensate.”