SPEAKING of the federal budget deficit, you also need to know that there is a large, and growing, off-budget deficit. Beginning in the early 1970s, successive presidents and Congresses have agreed that certain outlays shouldn't be counted in the budget. There have been various elegant attempts to justify the practice, but the basic purpose is merely to hold down the deficit by declining to count part of it.

This year the off-budget outlays will run about $16 billion.Since the budget deficit this year is currently calculated at something over $40 billion, the total federal deficit, on and off the budget, is over $56 billion. Similarly, when you consider next year's deficit, remember that there will again be at least $16 billion in off-budget outlays.

This fiscal leakage is in the form of government loans that are carried on no budget. The $16 billion is the amount by which new lending exceeds the repayments of old loans. In its effects on the economy, this lending is like spending. It is an outlay and, like any other outlay, it tends to encourage inflation.

The practice got started under President Nixon. Although he always talked like the most tightfisted of fiscal conservatives, in practice he was anything but. Under President Ford, there was a rapid expansion of direct loans off-budget. Under President Carter, the expansion has shifted to guaranteed loans. The administration prefers to leave the impression that these loans are cost-free to the public unless the borrower defaults. In fact, vast amounts of this guaranteed lending is being picked up by the U.S. Treasury.

There are a couple of things very wrong with this practice, as the Congressional Budget Office points out in its recent report on federal lending. It's not only that off-budget lending leads to important omissions in the government's basic accounts. Worse, the lending is exempt from the scrutiny and control imposed by the budget process. Some of this lending is doubtless justifiable. But how about those interest-free loans currently offered to college students, regardless of their need and their families' income? Offered as a budget item, that one would not have a high chance of surviving a routine budget hearing.

This large area of off-budget outlays now threatens to get even larger. The aid to the Chrysler Corporation was in the form of loan guarantees. The administration's bill to speed up synthetic fuel production will depend heavily on much larger loan guarantees.

Circumventing the budget is bad in principle. The corrective is the simple and obvious one of putting these outlays back in the budget where they belong.