Government pay experts say that the average white-collar federal civil servant is due a before-taxes raise this October of $28.40 a week. That adds up to a total federal-military pay raise package costing $4.2 billion.

Despite that expert advise, President Carter still intends to limit the pay raise to around $2.7 billion for federal and military personnel. For the typical government employe that would amount to $18.46 a week before deductions.

The "average" white collar federal employe now gets $17,575.27 a year, according to government figures. That "average" is much higher in Washington with its concentration of middle and upper grade workers and officials at headquarters.

The reason for the big - nearly $10 a week - different between what "ought" to be and what probably will be is politics and policy.

President Carter months ago asked labor and industry to adopt wage-price restraints this year. To set an example, he announced that he would put a 5.5 percent pay limit on the October federal-military pay raise.

Although the law says federal white collar employes are supposed to get annual pay raises to keep them on par with private industry, it allows the president to shave that amount when he deems necessary. He has so deemed. And unless Congress over-turns the 5.5 percent pay cap he will formally announce next month, government workers will have to take less than government pay experts say they deserve.

Data collected by the Labor Department on private industry wage gains between March 1977 and March 1978, showed substantial private industry pay increases to rate paid federal workers in similar jobs, government pay experts say that translates into a sliding scale raise - averaging 8.4 percent - due government empolyes this October.

That amount, or something very close to it, is what the president's pay agents will propose to him this month. Those same pay experts expect the president to reject the 8.3 percent catch-up-with-industry raise, and to propose 5.5 percent. And they also expect Congress will accept that amount and not anger voters before the November elections by pushing for higher pay for "bureaucrats" who are this season's chief political target.

If, however, Congress or the president decided to let the full catch-up-with-industry raises go into effect, these are the amounts most employes would get on a grade-to-grade breakdown. (Bear in mind that workers in the highest pay grades, who statistically are due the largest increases, won't get anything either way. Congress says the $47,500 federal pay lid will remain on for another year).

This grade-by-grade breakdown shows the government-determined "gap" between federal and private pay. In every case, federal figures show, government worker's are this far behind their counterparts in industry:

Grade 1 employes, according to the president's pay agents, are 6.15 percent behind their industry counterparts and due that much of an increase in October. Grade 2 is 6.14 percent behind industry . . . Grade 3, 6.18 percent . . . Grade 4, 6.27 percent behind.

The gap at Grade 5 is 6.40 percent . . . Grade 6 is 6.58 percent . . . Grade 7, 6.80 percent . . . Grade 8, 7.07 . . . Grade 9, 7.39 percent and Grade 10, 7.75 percent behind.

Grade 11 lags 8.17 percent behind industry . . . Grade 12, 9.14 percent . . .Grade 13, 10.31 percent . . . Grade 14, 11.68 percent . . . Grade 15 is 13.17 percent behind . . . Grade 16, 15.08 percent . . . Grade 17, 17.12 percent and Grade 18, 19.40 percent.

It ought to be pointed out that a lot of people and employes alike, don't believe the government is behind them or anybody else. They contend that federal salaries are the best in the nation at almost any level, and that the tenure and benefits in government are - despite what one reads about stock options, bonuses and company-paid country club memberships-unmatched.

Federal officials say the figures are there. And they are accurate. They don't expect a mass exodus of government people leaving for industry, because the job market is tight. they don't expect retiremants to jump either, since many federal workers take second-career jobs when they retire. But they do say that the figures are accurate, and they will be used until Congress comes up with another way. And federal officials point out that, using this system, government employes are falling further behind industry each time a president, or Congress, decides to shave a catch-up-with-industry raise. The big salary catch-up bill will come due someday. Maybe next year. And when it does, the public is going to get a real jolt at the amount of money it will take to have their public servants catch up with them.