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

Read: Student loan debt haunting America'sworkers

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

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

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

Read: Class of 2015 can't retire till they're75

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

If you raise that debt burden to $31,000—the 2013 averagefor recent college students—and increase the universe of peoplewith student loans to mirror the current situation, the outlooklooks grimmer.

Read: 5 signs of employee financialstress

When CRR updated its calculation to reflect more currentstudent debt loads, the at-risk percentage rose to just above56 percent. "Our latest study finds that student debt can have abig impact on retirement preparedness by reducing a young worker's401(k) savings and delaying a home purchase," said Munnell.

To put that 4.6 percentage point increase in context, thepaper notes that "a 19.6-percent across-the-board benefit cutin Social Security (exempting current retirees) to eliminate theprogram’s long-term financing shortfall would raise the NRRI by10.7 percentage points."

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

The NRRI's calculation assumes that everyone can work until age65.

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

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