As Washington considers closing a profusion of tax breaks in its quest to reduce record budget deficits, a study suggests that some targets might produce far less cash than is commonly thought.

Tax preferences for retirement plans, such as 401(k) accounts, rank among the biggest money losers for the federal government, with official projections showing a loss of about $600 billion to federal coffers over the next five years. But a study set for release this week suggests that the accounts would have to be closed, or contributions sharply limited, to realize much in the way of budget savings — a move that could undermine some of the nation’s most popular retirement tools.

“Given the questions about the future of Social Security, this is no time to be making bad policy decisions about private savings,” said Brian Graff, executive director of the American Society of Pension Professionals and Actuaries, which conducted the study.

The analysis highlights the murky math surrounding hundreds of tax breaks that collectively deprive the U.S. Treasury of more than $1 trillion each year. Known as “tax expenditures,” the list includes dozens of popular credits, deductions and other policies that benefit many special interests, but also millions of ordinary taxpayers.

The most expensive tax breaks from the government’s standpoint are the tax-free treatment of employer-provided health benefits and the mortgage-interest deduction for homeowners. Those provisions allow taxpayers to keep nearly $300 billion a year that would otherwise be collected in taxes, according to administration estimates.

Although that figure gauges the value of the policies to taxpayers, it says nothing about how much of that cash could be recovered through repeal or reform, tax experts say. For example, if the mortgage interest deduction were eliminated, people would probably shift their investments to other tax-preferred vehicles, thereby denying the tax man a portion of his expected rewards, said Ed Kleinbard, a former director of the congressional Joint Committee on Taxation, which, along with the Treasury, estimates the value of federal tax expenditures.

The estimates are “an accurate measure of the extent to which the preference is being used by taxpayers,” said Kleinbard, who teaches law at the University of Southern California. “But that’s a completely different question than how much revenue could be raised if the preference were eliminated.”

Tax preferences for retirement savings are particularly tricky, Kleinbard said, because they are not typically straightforward tax breaks, whose entire value is realized each year. In the case of 401(k)s, for example, taxpayers defer taxation on their contributions until they retire. In the meantime, they accumulate interest on their contributions tax free.

To determine the loss to the government, the Joint Committee on Taxation and the Treasury look at taxes lost each year because of new 401(k) contributions, as well as the interest earned on existing contributions, and then subtract taxes paid on distributions from 401(k)s. The difference ranks 401(k) retirement plans as the third most expensive provision on the administration’s list of tax expenditures, projected to deprive the Treasury of an estimated $67 billion in 2012.

Because 401(k)s have only been around for a few decades, however, the value of the contributions and interest earnings still far exceeds the value of distributions to retirees, Graff said. That means traditional methods dramatically overstate the amount of revenue that ultimately will be lost.

The study says that a more accurate measure of retirement savings incentives would estimate the value of the tax breaks over the lifetime of the accounts. By that measure, the study shows, the loss to the government would be as much as 77 percent lower than official estimates.

“If the system were mature, then it might not make much difference. But the system is still growing,” said Judy Miller, the society’s director of regulatory affairs. Because “the taxes being deferred are far more significant than the amounts being paid out,” she said, the official estimates are “very distorted.”

Graff noted that President Obama’s fiscal commission recommended slashing the total amount people could contribute to retirement plans as part of its proposal to rein in the national debt. Lawmakers, he said, have also told the group that everything is on the table.

Kleinbard said he has seen no move in Congress to limit retirement incentives such as the 401(k), the nation’s biggest retirement savings program with 70 million participants. “I do think they are being a bit paranoid,” he said of the society.

Still, Roberton Williams, a senior fellow at the nonpartisan Tax Policy Center, said there are good arguments for reexamining tax breaks for retirement savings, which disproportionately benefit the wealthy.

“You could design these things in ways that would encourage people to save for retirement, but in a much more equitable way that would actually impact behavior,” Williams said. “The rich are saving anyway. It’s the poor guy that needs the incentive.”