In a decision that affects the prison terms of nearly 200,000 inmates in federal prisons, the U.S. Supreme Court on Monday adopted a formula for calculating “good time credit” for good behavior that results in more time served.

The Court by a 6-3 vote endorsed a long-standing Bureau of Prisons method of calculating good time credit based on the length of time actually served, not the length of the term imposed by the sentencing judge. As Justice Stephen Breyer described it in his majority opinion in the case, Barber v. Thomas, the formula preferred by the Court would result in 470 days of credit for a well-behaved prisoner serving a 10-year sentence, while the method urged by defendants would result in 540 days of credit.

That may not sound like much, but in a strongly worded dissent Justice Anthony Kennedy said the ruling will adds “tens of thousands of years” collectively to time served, at a “cost to taxpayers of untold millions of dollars.” In human terms, Kennedy added, the ruling will be “devastating to the prisoners who have behaved the best.”

Kennedy has long argued in speeches off the bench that American prison terms are too long and punishments are too severe.

Joining Kennedy in dissent were justices John Paul Stevens and Ruth Bader Ginsburg. Justice Sonia Sotomayor, who emphasized during confirmation hearings last year that she disagreed with President Barack Obama’s “empathy standard” for new justices, voted with the majority against defendants in the case.

Mary Price, vice president and general counsel of Families Against Mandatory Minimums, said the ruling “is a huge deal for so many prisoners.” As a result, she said, “we will continue to overincarcerate many, many people.” She said bills have been introduced in Congress to clarify the formula and give prisoners up to 54 days of credit for each year of a sentence, as she says the law requires.

But in announcing the opinion, Breyer said his reading of the statute is “the most natural.” Breyer’s ruling was blend of statutory interpretation and Algebra 101, and he said the “mathematically inclined” might like reading it. The method allowing 54 days of credit for each year of a sentence, he said, would improperly give a well-behaved prisoner credit for both time served and time that was not served, but was offset by past good behavior.

The case was brought by Oregon federal prisoners challenging the calculation method, but they lost at both the district court in Oregon and the U.S. Court of Appeals for the 9th Circuit.

Kennedy said that, “to a prisoner, time behind bars is not something theoretical or a mathematical concept. It is something real, even terrifying.” Kennedy added, “we should not embrace this harsh result where Congress itself has not done so in clear terms.”

Tony Mauro can be contacted at [email protected].