THE FEDERAL deficit, everyone acknowledges, is going to be over $200 billion this year--an awe-inspiring figure, more than one-fourth of total spending. The deficit is going to stay in that range next year, when the budget was supposed to be balanced under the original Reagan plan, and for as many years beyond as anyone has bothered to calculate it. Where did it come from?

You have three explanations from which to choose. The White House says that the deficits are the result of "inherited" levels of social benefits plus the need to build military strength, aggravated by an unexpected recession. The Congressional Budget Office, in the closest thing that you will find this season to a neutral view, says that it's mostly the recession at present. The House Budget Committee, run by Democrats, points to the defense buildup and excessive tax cuts.

When Mr. Reagan took office, the deficit was running just under $60 billion a year. Now it's $150 billion higher. Nearly $20 billion of it is higher interest costs. What about the changes that the Reagan administration has deliberately made?

Defense spending is now running about $19 billion a year more than it would be if Mr. Reagan had stuck to the Carter commitment of a 3 percent annual rise. Another part of the explanation is the change in taxation under Mr. Reagan--the enormous cut in 1981, the bill last year that took some of it back and the gasoline tax increase. Taken together, they mean that the federal tax system will raise $68 billion less this year than it would have raised under pre-Reagan law.

Finally, the administration has sharply reduced a lot of spending programs. That comes to about $47 billion. If you add the defense increases to the tax cuts, and then subtract the reductions in non-defense spending, you will conclude that all of the Reagan budget legislation together has increased the deficit by about $40 billion. The rest of the recent growth of the deficit is the result of the recession.

But as the budget projections run out beyond this year, the pattern changes. After 1984, all the calculations are based on the expectation of a steady economic recovery. In the past, this kind of calculation has always shown the deficit shrinking rapidly in a recovery. The dismaying thing about the present projections is that the deficits don't shrink.

As business picks up, the House Budget Committee's explanation for the expanding deficits will become increasingly relevant. The CBO's figures point in the same direction. Social benefits will represent a declining share of the budget and, for that matter, of GNP as well. But as the costs of recession go down, the structural deficit will rise. The essential reason is that defense costs will be going up while, in relation to the size of the national economy, tax revenues keep falling.