The military’s health care budget in coming years is likely to cost billions of dollars more than expected, according to congressional investigators, because the Pentagon has used “unrealistic” and “questionable” methods of calculation.

The $15 billion Defense Health Program is expected to rise 18 percent, by 2003, according to the Defense Department’s calculations.

But the General Accounting Office, the investigative arm of Congress, said in a new report that the budget could rise by anywhere between 18 percent and 56 percent in that period.

The medical program, which accounts for 6 percent of the Pentagon’s budget, is used by 6.3 million personnel, retirees and dependents. There are 8.2 million people eligible.