The pocket calculator is always there, tucked into the coat of his striped, three-piece suit, but J. Peter Grace doesn't really need it. He squints up at the ceiling, the brain starts clicking, and the numbers roll off the tongue with an auctioneer's rapid-fire cadence.

"It's 63, 81 of 30 and it's 90 at 50 gross, 50 gross, convert gross to taxable, you multiply by .77 and take away $3,400. So actually 50 gross is 35 taxable income. You got 90.9 percent of all the taxable income is flown through there."

Percentage points, decimal points, income brackets, interest rates, cash accelerations, floats, liabilities.

Add it up, multiply it, divide it, subtract it. It all comes out the same: Uncle Sam is headed for the poorhouse. Broke, busted, bankrupt.

Nobody says it more convincingly than Grace, the industrial tycoon who contends the government is facing the "mathematical certainty" of financial catastrophe--unless it starts applying the business practices that made him a multizillionaire.

Who, after all, is going to bail out a bankrupt government? Chrysler?

For the past 15 months, the chairman of W.R. Grace & Co. has been the driving force and evangelizing spirit behind the President's Private Sector Survey on Cost Control, an amalgam of 162 Brooks Brothers executives and 1,300 business-efficiency wizards whose mission is to lead the government safely off the path of profligacy and perdition.

"The deficit went 1.6 in '65, 45 in '75 and 207 now." Grace doesn't bother with the word "billion." It interrupts the flow of figures. "And that's a differential growth rate between outlays and revenues. Now if that goes on in 1990, it's 600, and two trillion in 2000 . . . . I mean, anything over a trillion a year is chaos. And it just has to happen."

Even with higher taxes? On rich people, for instance? "If you took 100 percent of everybody's income over $75,000, you'd run the government for 14 days," Grace said.

The brink-of-doom statistics are everywhere in Grace's Pennsylvania Avenue office suite, where desks are littered with computer printouts and copying machines are always warm.

But, like any good evangelist, Grace has a plan for salvation.

With a donated staff, an infusion of private-sector cash and the imprimatur of President Reagan, his band of business executives has amassed more than a million pages of documents and recommendations, all of which will be distilled into a 300-or-so-page report to be delivered to Reagan Dec. 9.

The report will call for about $300 billion in potential savings, added revenues and cash-flow benefits along with "a lot of very interesting systemic and management organization changes" that "will help cure the problem that created the thing in the first place."

The cost-cutting proposals will range from big-ticket projects (like a shakeup of the civil service retirement system) to nickel-and-dime items (like charging for firewood from national forests).

The survey honored no sacred cows. Even the military pension plan was grist for the mill. There was no nit too small to pick, no piece of red tape too small to be measured.

"I turned 70 in May, I became eligible for Social Security," Grace said. "I got my first check, and it was wrong." About $1,200 a year short, to be exact. That might not bother most men who make $800,000 a year, but then Grace is a man who wears two watches and works long division in his head.

In any case, two financial specialists spent several days verifying his suspicion, finding out in the process that the Social Security computers were two years behind and the whole business of determining eligibility was a hopeless morass of paper work.

More figures, this time from J.P. Bolduc, senior vice president of W.R. Grace & Co. and his boss's right-hand man on the cost-cutting survey: "The Social Security manual is 25,000 pages long. It stands 6 1/2 feet high, costs $44.3 million to print and $66 million to distribute. There are 12,000 pages of changes every year, and employes get one hour a week to familiarize themselves with the changes. That's 16 pages a minute."

By Grace's reckoning, the survey's recommendations can be achieved within three years, assuming the unswerving support of the White House and Congress.

Which isn't likely, as even Grace concedes.

The recommendations trickling out from the survey's office already have drawn salvos from interest groups, members of Congress and federal agencies.

Consumer, health and low-income advocacy groups accuse Grace of pernicious penny-pinching for his proposals to cut back federal feeding programs. Senior officials at the Environmental Protection Agency call proposals to combine research laboratories and turn construction grants over to the states "over-simplified" and "ill-advised."

The Agriculture Department's assistant secretary for natural resources, John B. Crowell Jr., warned colleagues to keep their mouths shut about proposals to sell off forest lands and the like, "even though some of the recommendations are not thoroughly thought out and their adoption is unlikely."

Grace dismisses the complaints with a shrug. "They're going to have to take major measures, and if there are better major measures than the ones we're recommending, I don't know about them," he said. "Maybe there are. There's always something better under the sun. But if there are, I haven't seen them, and so I'm saying that our recommendations will be ageless, irrefutable, sensible, and they will have to ultimately be adopted."

It is a belief said with enough force to sweep all of workaday Washington in its path.

Grace on food stamps, for example: "Let's put it this way. I mean, you get something for nothing, you know . . . and anybody who's getting something for nothing is going to scream like hell if anybody even tampers with that . . . . If anybody even talks about it, they're saying they're trying to take food out of the mouths of starving people. So they have this propaganda bit damn well organized."

Grace on bloated spare-parts bills from defense contractors: "You have to start with the theory that everybody is a potential S.O.B."

And on the excesses of Congress: "If you ship military personnel's personal property and goods to Alaska or Hawaii, there's a rider in the legislation that says you may not get competitive bidding. Just for those two areas. That cost $68 million over three years, see? . . . . Utterly ridiculous. Some guy says, 'Look,' you know, boom boom, 'I'll vote for that if you vote for this.' "

Congress is at the square root of a good many of Grace's mathematical scenarios. But he insists he has no particular knife out for the nation's lawmakers.

"I'm sympathetic to a congressman. They're under terrible pressure. They have to get elected every two years in the House, and they have to get silly sometimes. And, probably, if I was a congressman, I'd be silly."

Meanwhile, Grace has a little pressure of his own to contend with. There is one little pool of budgetary red ink that even the best financial minds in the private sector haven't yet figured out how to drain.

The cost-cutting survey is about $500,000 in the hole.

"Somebody," Grace said, "is going to have to pick up the deficit."