New calculations by the Department of Agriculture indicate that the Reagan administration's cuts in other welfare programs may shift as much as $500 million in unanticipated costs to the food-stamp program in fiscal 1982.

Mary C. Jarratt, assistant secretary for food and consumer services, said the new cost estimates, which will be refined in coming weeks, may force the administration to ask Congress to adopt a higher cap on the program.

Jarratt's remarks to reporters yesterday were the first concession from a high-level administration official that food-stamp costs will be forced high than anticipated because of cuts in the other social programs.

Congress at the administration's behest did two main things to the food-stamp program: rewrote the eligibility and benefit rules to reduce costs, then also put a cap on total spending. The administration claimed that, as tightened up, the program would stay below the cap. Critics feared it would not, and that further eligibility cuts might then be required.

The Senate nevertheless set a cap of $10.9 billion on spending for 1982; the House Agriculture Committee, $10 billion.

The cost of the program for fiscal 1981, ending Sept. 30, has been set at $11.4 billion. Jarratt's new estimates suggest the 1982 cost could go as high as $12 billion if benefits are paid as scheduled in the new, slimmed down congressional authorization.

Jarrat said the administration's earlier estimates of 1982 food stamp costs were predicated on the success of President Reagan's economic recovery program and did not include offsetting figures from social-welfare program costs. The new estimates still assume the economic plan will succeed; if it does not, costs could go even higher.

Food-stamp eligibility rolls would increase as reductions occur in aid to families with dependent children, public-employment jobs, Social Security, trade adjustment assistance, unemployment compensation and other federal programs for low-income people.

"Our earlier budget figures did not reflect the social program changes," Jarrat said, "and we are trying to get a better handle on that now. . . . We are still in the review process and by September we hope to have better data."

She said that the administration, in order to maintain the "safety net" of assistance to neediest recipients, probably will be forced to ask Congress to raise the ceiling on food-stamp spending. She stressed that a final decision has not been made.

Congressionally ordered cuts of about $1.65 billion in the 1982 program would remove an estimated 1 million recipients from the foodstamp rolls. Jarratt said that May participation was 22.6 million recipients, a tenth of the population.