President-elect Ronald Reagan may soon find that he simply does not have enough money to fulfill his campaign promises on the economy: to give big tax breaks to business and individuals, increase defense spending and simultaneously fight inflation.

Already, Reagan advisers are raising their estimates of how deeply the federal budget would have to be cut next year to accommodate tax cuts without too big a budget deficit.

At the same time, pressures are building to push up defense spending even more than promised before the election. Incoming chairman of the Senate Armed Services Committee John G. Tower (R-Tex.) believes the defense budget should rise by 9 to 13 percent a year, after allowing for inflation.

Reagan does not just have to find the money to fulfill his campaign promises. He has to do it without upsetting financial markets or fueling inflation.

Although the post-election stock market boom is going strong, interest rates are higher now than on Nov. 4. They will not come down if financial markets see a ballooning budget deficit next year, even if Wall Street financiers were Republican to a man.

And inflation, which Reagan has to bring down if he is to satisfy the electorate, is still running high. Soaring food prices, on the way after the bad harvest, together with rising oil prices, will give another twist to the spiral in coming months.

Democrats accused Reagan during the campaign of having an unworkable and inflationary economic plan. He countered with figures that did add up -- giving him a balanced budget by 1983 -- but which will translate into hard and limited choices in office. They relied on two key assumptions.

The first was that a Reagan administration could cut total federal spending sharply from its projected levels. But the "untouchables" -- defense spending, Social Security benefits, other entitlements and interest payments on the debt -- add up to more than two-thirds of the budget. The rest would have to be cut at an annual rate of maybe 15 to 20 percent, which many budget experts doubt is possible, at least in the near term.

The second assumption was that the planned big tax cuts would unleash such growth in the economy that eventually they would help pay for themselves; any deficits would be temporary. But money markets will almost certainly not wait for "eventually" before hitting the panic button. Even if they do wait, the planned spending cuts, if achieved, will work against the growth stimulus from the tax cuts.

In addition, Reagan based his figuring on a set of numbers (from the Senate Budget Committee and the Congressional Budget Office) that now look over optimistic. Higher inflation and more disaster spending than expected are major reasons. The Reagan figures show federal spending at $633 billion in fiscal 1981, rather than the $648 billion used last week by the House Budget Committee or the even higher numbers being discussed by economics on the Hill and in the administration.

The Reagan figures estimated spending by 1985 a full $100 billion lower than recent figures produced by a congressional economist. Revenues were projected to be higher, by about the same amount, than the Carter administration estimates. Just these differences in forecasts (which are more like guesses) would wipe out a $200 billion potential budget surplus.

"If spending estimates go up, why, then, spending reduction efforts have to be increased also," Caspar Weinberger, head of the Reagan task force on spending control, said yesterday. He reiterated the Reagan aim of holding federal spending to $620 billion next year. That would mean cuts of almost $30 billion from the latest congressional estimate of spending in fiscal 1981, rather than the $13 billion talked of in the campaign.

Budget experts wonder whether cuts of even $13 billion can be made in 1981. The financial year, on which the budget deficit and spending is reckoned, will be two-thirds over when Reagan takes office. By the time the new administration has "figured out what they're doing" and taken action to curb spending, the year will be half over, according to one congressional economist.

That would mean that spending totals would have to be cut at an annual rate of 3 to 4 percent for the rest of the year to achieve the 2 percent 1981 cut promised as a minimum.

But most of the budget simply cannot be cut on short notice. And Republicans are committed to let some large parts of it go on growing. Defense spending accounts for more than 20 percent of the 1981 budget, and Reagan is pledged to increase it.

Social Security payments, Medicare, unemployment compensation and other entitlement programs account for a further 40 percent or more. Reagan has said the great bulk of these will not be cut.

In fact, many of these programs have to grow just for the benefits to stand still. The number of retired persons is growing, for example. So the "real" cost, after inflation, of keeping their benefits level is growing, too. Fast growing medical costs also keep the Medicare bill rising in real terms.

Interest payments on the federal debt are down for about 10 percent of the 1981 budget, by recent congressional figuring. But economists agree that with the recent rise in rates, all the budget projections for this nonnegotiable program are way too low.

A further 10 percent or so is due to go in grants to state and local governments. Apart from the political difficulties in cutting back this money -- which include revenue sharing, grants for highway construction and sewage plants -- it would be difficult for a new administration to have an impact on actual spending in fiscal 1981.

Only about a fifth, or at best a quarter, of the budget can therefore be cut without major legislation, or reneging on campaign promises. To get the 3 or 4 percent cut overall, the "touchables" would have to be slashed by 15 or 20 percent, according to Bowman Cutter, outgoing deputy director of the Office of Management and Budget.

Weinberger says it can be done -- if Congress and the president have the will. No details have yet been given of how the administration plans to go about it. Reagan said over and over in the campaign that getting rid of fraud, waste and mismanagement could cut 2 percent from spending. Few experts believe that figure. "Real fraud and waste tends to be penny-ante stuff," Cutter said. "What waste really tends to be is someone else's program."

Other obvious possible cuts, such as reducing civil servants' travel expenses, are also not lucrative. Nondefense travel is expected to cost less than $2 billion in the current fiscal year, with nondefense transport of things, rather than people, adding less than another $1 billion.

The only expenses that are paid out regularly and quickly and can therefore be cut quickly, according to a congressional economist, are pay and benefits. Reagan will probably try to cut the first anyway, although he has promised an increase in military pay.

But, the economist said, even a 10 percent cut in the $40 billion federal payroll (excluding military pay) would probably save only $1 billion in 1981. The full year saving of $4 billion would be whittled down by the cost of termination contracts, and the fact that the administration would be starting part way through the financial year.

This economist projects federal spending, with no policy changes except a 5 percent real increase in defense spending, at about $730 billion in fiscal 1982. The Reagan fact sheet, which Weinberger said gave the figures the president-elect would like to reach, showed federal spending after the Reagan cuts at $682 billion in fiscal 1982. So 1982's cutting exercise may be no easier than 1981's.

Some of Reagan's advisers would be happy to see a short-term rise in the budget deficit if that were the price of large tax cuts. But they are in a minority. The others stick to the belief that taxes must be cut more carefully next year, preferably along the campaign lines of a 10 percent across-the-board cut in personal tax rates combined with more generous and simpler cuts in business taxation.

This will cost only about $20 billion next year if it is not made retroactive, and is passed by the summer. But the cost soars in later years. Reagan has supported further 10 percent rate cuts in both 1982 and 1983. Weinberger said yesterday that there has been "no weakening of Reagan's desire" for the full three years of cuts.

By 1982 the net annual cost of these cuts, together with some business tax reductions, would be between $50 billion and $55 billion, taking into account the reflow to the Treasury of some of any tax cut. Even if only the first round of personal tax rate cuts is enacted, the revenue loss by 1982 could be $40 billion to $45 billion.

By 1985 Reagan estimated the annual cost of his tax cuts at just under $200 billion. The administration put it at considerably more.

But long before then the need to restrict the budget deficit is likely to curb the tax cutters, and perhaps the defense spenders as well. Latest estimates put the 1981 deficit at about $40 billion, with just a small tax cut included. Figures for 1982 depend crucially on Reagan's policy. But the starting point is probably around the same figure.

Financial markets may tolerate a larger deficit from Republican than from Democrats, but they will draw the limit somewhere. And the Federal Reserve Board, whose chairman, Paul Volcker, has already spoken against a large tax cut in 1981, will not allow the money supply to expand to pay for such tax cuts. The result, unless Reagan achieves the unlikely if not the impossible, on the spending side will be smaller tax cuts, higher interest rates or both.