The arithmetic of the impending U.S.-Soviet nuclear arms talks in Moscow helps to explain why the Carter administration's initial discussions with the Soviet leaders are guaranteed to be difficult.

State Department officials acknowledged yesterday that the U.S. delegation may encounter some heated reaction from Soviet leader Leonid I. Brezhnev. They primarily attribute the expected chilly reception to President Carter's strong stand on human rights. The more troublesome Soviet reaction, however, other sources concede, is likely to come over the President's nuclear proposals.

President Carter's call for substantial reduction in U.S. and Soviet nuclear forces, which is yet to be spelled out, automatically raises Soviet hackles, if reductions are to be equal for both countries, as the Carter administration intends.

To the Soviets, this looks like a lopsided 2-to-1 cut in U.S. favor from existing force levels.

The reason is that the Soviet Union now has more intercontinental bombers and missile launchers than the United States. By American count, the Russians have 2,540 of these strategic nuclear weapons, compared with 2,128 for the United States.

At Vladivostok, in 1974, the United States and the Soviet Union agreed on equal ceilings for these forces, 2,400 on a side. That requires a modest cut in Soviet force levels, and permits an increase in the U.S. level.

President Carter Thursday disparaged the Vladivostok ceilings as far too high, and no real arms control at all. Instead, he proposed "actual substantal reductions if the Soviets will agree!"

The still-secret details will be presented to Brezhnev by Secretary of State Cyrus R. Vance when talks begin Monday in Moscow. The Vance delegation left last night.

It is possible, however, to approximate, mathematically, the undisclosed U.S. figure.

The Carter administration has criticized as too low a token out of 10 per cent in the 2,400 level, discussed last year by Brezhnev and former Secretary of State Henry A. Kissinger.

On the opposite site, administration sources say a cut to 1,500 would be too drastic at this stage. This means the U.S. proposal will be between those two figures, perhaps a 25 per cent reduction, or about 1,800 strategic weapons on a side.

By coincidence, at least, Sen. Henry M. Jackson (D-Wash.), a prime figure in strategic arms limitation debates, in 1974 proposed cutting the Vladivostok force ceilings to 1,700.

A cut to 1,800 for example, would mean reducing existing Soviet forces by 740, but cutting U.S. forces only 328.

This disparity, Jackson and other critics of previous U.S. policy, contend would be at least equalized by the larger size of Soviet missile "throw-weight," or payload, which ultimately might enable the Russians to surpass the United States in multiple nuclear warheads. This has been the core of the recent claims of impending Soviet military superiority.

At present, however, what preoccupys Soviet leadrs is the great U.S. advantage in existing multiple warheads. The United States has more than a 3-to-1 edge, with 8,500 strategic bombs and missile warheads to hit Soviet targets, compared with 4,000 Soviet bombs or missile warheads.

This, from a Soviet perception, is only one of the arguments are United States will be debating against in the Moscow talks, in addition to Soviet alarm over developing American long-range cruise missiles.

Only if the Russians agree to "substantial reductions" in the Vladivostok force levels, State Department officials confirmed uesterday, will the United States agree to limits on cruise missiles and the Soviet bomber known as Backfire.

As a consequence, U.S. officials anticipate a prickly exchange in Moscow over the goals set by President Carter.