A question for today: What weighs 6 1/2 pounds, will cost $500 billion and means a piece of pie for virtually every American?

The answer: President Carter's proposed federal budget for fiscal 1979.

In the driest of terms, the budget is a proposal for an allocation of resources, as the fiscal people put it, between the public and private sectors.

What that describes, after the last penny has trickled down through the spout, is the way that Uncle Sam transfers money from John Doe's left pocket to John Doe's right pocket.

Doctors, patients: teachers, pupils; prosecutors, jailbirds: bankers, debtors; corporate chieftains, assembly-line workers - you name it all end up with a slice of the pie.

More than one of every 20 working Americans are on Uncle Sam's payroll. The soldiers who defend you, memebers of Congress who represent you, bureaucrats who serve you will be paid about one-seventh of the budget.

Almost one-fourth of all the money Uncle Sam spends consists of bebefit checks and other aid to the elderly. Add to that federal benefits paid to veterans, disabled coal miners and railroad pensioners, the jobless and welfare recipients. Include their food, health and housing assistance. It comes out to two-fifths of the budget, and about one-seventh of the personal income Americans have available to spend.

Or take your cities and states. One out of every four dollars they spend will come from the federal coffers. It is money that eventually reaches the policeman, the nurse's aide, the welfare mother, the housing and sewer builder.

Coming from another direction one-fourth of the budget is ticketed for defense - $126 billion worth. The bulk of it - 55 percent - goes not to guns, but to people, in the form of salaries and pensions.

Consider aches and pains and illness. If all the money spent on health care in United States, close to $3 of every $10 will come from the federal treasury. Partly that is aid to the sick, partly it is income for doctors.

Howabout the federal debt? Interest payments on the debt will be about $40 billion. Putting it another way, almost $1 of every $12 in President Carter's budget is earmarked for creditors. They include banks and lending institutions, individual citizens and - to the tune of $5 billion - foreigners.

These are rough numbers, of course. Before it becomes graven in stone, Carter's budget will undergo some transformation - every presidential budget does - as Congress and the special pleaders sink their teeth into it and rearrange spending priorities.

So the fine-print details change, give a billion here or a billion there, but the effect is the same. More Americans than not depend on the federal budget for all or part of their directly day-to-day income.

The six-plus pounds of documents that comprise the federal budget for fiscal 1979 make for some of the most imaginable. It is a mass of figures and tables that never seems to add up.

But behind almost every line of the budget is a person who will receive some sort of assistance from the government, or a service that the government will provide to the public.

And there is a hidden hooker in this scheme of something for everyone - the tax structure that determines who pays how much into Treasury that in turn dispenses the largess.

A for-instance: the mortgage interest payments John Q. Homeowner is allowed to deduct from his income tax liability are a more piece of the pie.

Another for-instance: the depreciation credits and operating writeoffs granted to wealthy corporations, thus reducing income to the treasury, are also a piece of the pie.

One way to contemplate the federal budget is as if it were an onion. Beneath each successive layer is a function or a service of government, each in its way having an impact on quality of life, public and private policy, bread and butter.

The health-care portion of the budget is an onion that becomes increasingly as each layer is peeled off.

With the advent of Medicare and Medicaid, federal spending on health programs has grown dramatically during the past decade. Next fiscal year alone it will increase 11.4 percent. In terms of money, that will be an additional $6.5 billion, lifting the outlay to $63.4 billion.

This is to say that almost one-eight of all federal spending will go into health programs - largely for the aged, the poor and the disabled.

But that is not money that stays in their pockets or that even reaches them. It is provide the services: The doctors and nurses, the hospitals and clincs, the drugstores and pharmaceutical firms.

Carter's budget estimates that $24.9 billion will be spent on hospital care. The enormousness of that can be viewed another way. of every $20 spent by Uncle Sam, $1 goes to hospitals.

That pie is sliced into other pieces. Doctors will receive $5.9 billion - roughly a fifth of all their income. Nursing homes will get $4 billion.Drugs will cost $600 million.

Federal health-care programs and the money that pays the bill reach every corner of society, from the sick and disabled to the healers and dealers.

In every section of the big budget summary, comparable examples are found.

Farmers, to pick a case, are in the news, protesting federal farm policy and what they feel are low prices. This budget will provide more than $4 billion in crop loans for them and more than $1.9 billion in direct subsidies - that is, money that goes into their jeans with no obligation to repay the government.

Much of that direct subsidy money will go to wheat farmers because of marketing with their 1977 crop. The Agriculture Department is estimating that wheat producers will collect slightly more than $1 billion in direct aid to lift their income to the federally guaranteed "target" prices.

That, as the onion is peeled, converts into handsome chunks of income for the banks, the seedmen, the fertilizer companies, the tractor and combine dealers who service the wheat grower and his family.

Buy, buy, American pie.