(Bloomberg) -- Student debt levels have have mushroomed, and Americans have saved little for retirement.

Read: Student loan debt haunting America's workers

The relationship between those two things is not obvious. So just how does having a big slug of student debt actually hurt the ability to prepare for a comfortable retirement?

To find out, Alicia Munnell, director of the Center for Retirement Research at Boston College (CRR), and her colleagues tweaked a measure developed by CRR to quantify America's retirement prospects.

Their National Retirement Risk Index (NRRI) uses data from the Federal Reserve's Survey of Consumer Finances to compare how much income people are on track to replace in retirement with how much income they will need to maintain their standard of living.

Read: Class of 2015 can't retire till they're 75

The index shows that 52 percent of Americans are at risk of a shortfall in retirement, based on data that includes an average student debt load of $18,000.

If you raise that debt burden to $31,000—the 2013 average for recent college students—and increase the universe of people with student loans to mirror the current situation, the outlook looks grimmer.

Read: 5 signs of employee financial stress

When CRR updated its calculation to reflect more current student debt loads, the at-risk percentage rose to just above 56 percent. "Our latest study finds that student debt can have a big impact on retirement preparedness by reducing a young worker's 401(k) savings and delaying a home purchase," said Munnell.

To put that 4.6 percentage point increase in context, the paper notes that "a 19.6-percent across-the-board benefit cut in Social Security (exempting current retirees) to eliminate the program’s long-term financing shortfall would raise the NRRI by 10.7 percentage points."

The authors conclude that "extrapolating the effects of the growth in student debt into the future has an impact that is roughly half as large as a huge and unprecedented cut in the nation’s main source of retirement income."

The NRRI's calculation assumes that everyone can work until age 65.

It looks at what retirement income would be if a person were to use all available financial assets to buy an annuity. (Financial assets include money from taking out a reverse mortgage on a home.)

Instant Insights /

Here's the toll that student debt can take on retirement

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