WHEN PRESIDENT Reagan unveils his budget on Wednesday, the numbers -- as usual -- will leave most people confused. At the risk of adding to that confusion, let's examine some of the administration's premises about revenues and spending.

Reagan is fond of turning aside pleas from those who want to raise taxes to reduce federal budget deficits by saying that tax revenues, as a share of the gross national product, are as high as they ever were.

"We're getting the same percentage of gross national product in tax revenues at the lower rates than we've been getting before," Reagan declared at his press conference last month. Moreover, higher taxes would be "a block in the path of progress and economic growth," he said.

At the same press conference, the president also reiterated another familiar budget theme: "I have never thought that Social Security plays a part in the deficit. It doesn't, because Social Security is supported by its own tax and that tax can't be used for anything else. So it's playing games to pretend that Social Security is a part of the budget and can affect the deficit."

In striking both of these themes, Reagan himself is playing a sort of game -- by refusing to recognize that federal revenues remain so high as a share of GNP partly because Social Security payroll taxes have gone up sharply.

The picture painted by the president looks considerably different if Social Security is removed, as he suggests it should be. By the same logic, budget calculations should also exclude the Medicare portion of the payroll tax and all Medicare spending other than the portion financed from general government revenues. (Interest paid by the government into the Social Security and health insurance trust funds is treated in the budget as an intragovernmental transfer and as such does not affect total spending.)

To see what happens when these items are extracted from both sides of the budget ledger, let's start with receipts.

Federal government receipts in fiscal 1985 were equal to 18.6 percent of GNP. According to recent estimates by the Congressional Budget Office and the Office of Management and Budget, they will be 18.5 percent this year. Contrary to Reagan's assertion that "we're getting the same percentage" of GNP in revenues as before the tax cuts, the average figure for the four years of the Carter administration was a percentage point higher, 19.5 percent.

But the president has also frequently been critical of the fact that during the Carter years federal receipts were rising relative to the size of the economy. His standard for comparison is the 18.6 average for the decade of the 1960s and the first half of the 1970s.

Even during that 15-year period, Social Security payroll taxes -- including those for Medicare after the mid-1960s -- were going up as a share of GNP while revenue from all other receipts were falling. The roughly stable overall total masked those offsetting trends.

In the first half of the 1960s, receipts were 18.4 percent of GNP, with payroll taxes accounting for only 2.4 percentage points of the total. During the first half of the following decade, the total had crept up to 18.8 percent, but the payroll tax take was up to 4.3 percent of GNP.

In other words, between the early 1960s and early 1970s, receipts from taxes other than the payroll tax dropped from 16 percent to 14.5 percent of GNP.

Since then, the payroll tax has continued to rise. It now represents almost one-third of federal revenues and is equal to about 6.1 percent of GNP. Other receipts will be equal to 12.4 percent of GNP this fiscal year, according to the CBO and OMB figures.

Reagan has succeeded in bringing total revenues back down to 18.5 percent from the 19.5 percent level that prevailed in the Carter years. However, payroll tax receipts have gone up from 5.1 percent of GNP then to 6.1 percent now. Thus, all other receipts, as a share of GNP, have dropped about 2 percentage points over the past five years.

On the spending side of the budget, the CBO and OMB estimate that fiscal 1986 outlays will be equal to 23.4 percent of GNP even after the first round of spending cuts under the new Gramm-Rudman-Hollings deficit reduction law. Given the lower level of revenues, this will leave a budget deficit of 4.9 percent of GNP, or well over $200 billion.

Now let's assume that Social Security and payroll-tax-financed Medicare outlays were dropped from the spending side, just as the above calculations dropped them from the revenue side.

Again, the best place to start is in the first half of the 1960s. Average federal outlays in those years were equal to 19.1 percent of GNP while spending on Social Security -- Medicare was not yet on the books -- accounted for 2.6 percentage points of the total. Thus, non-Social Security spending was equal to 16.5 percent of GNP.

Where do the figures stand today? Social Security and the payroll-tax-financed portion of Medicare account for 6 percentage points of the federal spending total of 23.4 percent of GNP. That means that all other spending will be equal to 17.4 percent of GNP, with 0.5 percentage points representing Medicare spending paid out of general revenues.

So taking Social Security and the payroll-tax-financed portion of Medicare out of both the revenue and spending sides of the budget, and stating the figures as shares of GNP, leaves this comparison: [See Chart].

The figures show that spending has gone up a little while revenues have fallen a lot. That is quite a different budget reality than the one the president likes to paint. And it challenges the frequent Reagan assertion that the basic deficit problem is not too little revenue but overspending.

Reagan's new budget will underscore once again his determination not to increase taxes but to reduce deficits by concentrating on cutting spending that falls outside defense, interest payments and Social Security. His budget reflects the view that the payroll tax and the programs it supports should be treated separately.

But that approach yields hard evidence that -- compared to the 1960s, Reagan's own reference point -- the deficit problem reflects a shortage of revenue, rather than overspending. Say, about three parts revenue shortage to one part added spending.