President Carter's apparent success in trimming his proposed fiscal 1980 budget deficit below $30 billion is based in part on a decision to rejuggle the way the government estimates its interest payments, officials said yesterday.

Sources said at least $2 billion of Carter's proposed "savings" stems from changing the procedure for estimating interest rate levels for this year and next, and not from spending cutbacks. But that total could change, depending on the economy.

Officials involved in the decision to change procedures say they are aware the move could stir criticism that the administration is "playing games" with the budget. Altering interest rate projections is a ploy often used in previous years.

However, they defend the step as "more realistic" in the face of an expected easing of interest rates early this spring, insisting that using the current "peak" rates would only result in distortions later if interest rates fall as predicted.

They also say the changes will bring the administration's procedures into line with congressional budget practices. The House and Senate Budget committees have criticized the White House in previous years for using the old procedures.

The shift could prove crucial to Carter's ability to meet his deficit target. The president is planning a $533 billion budget, with a deficit of just under $30 billion. Using the old interest rate method would push that to $32 billion.

Under previous practice, the White House estimated the coming year's interest payments by using the rates that were in effect when the budgt was completed -- projecting them ahead for several years, no matter what the outlook.

Under the new procedures, however, budgetmakers are using interest rates calculated by pegging them to the projected inflation rate, which the administration is prediciting will decline this year and next.

The result is that officials now can use significantly lower interest-rate projections than would have been possible under the former procedure.Authorities say the new budget will be based on interest rates of between 7.5 and 8.5 percent, rather than the 9.3 percent Treasury bill rate now in effect.

Budgetmakers insist their computation of the lower interest rate was purely a mechanical one and was not intended as a forecast of what interest rates might be. Officials said there was no consultation with the Federal Reserve Board on where interest rates might go.

One snag the administration could hit is that interest rates may prove higher than officials projected, if only because inflation could end up significantly higher than the White House is predicting Private forecasters expect an 8 percent inflation rate, compared to 7.4 percent used in the budget calculations.

The previous procedure caught the administration in a bind last year when interest rates rose sharply, soaring to 7.2 percent early in the year rather than the 4.4 percent level predicted in the fiscal 1978 budget by the Ford administration.

The Ford projections were criticized by the congressional budget committees, whose own estimates turned out to be closer to the mark. The Ford administration's estimates fell $4 billion short of the interest the government had to pay.

Officials acknowledged yesterday the Carter administration probably would not have changed the procedures if the result had not been to push total spending lower. However, they denied vigorously that the move was designed as a political ploy, and insisted it was sound budgetary practice.