Americans can safely weigh several pounds more than their doctors and diet counselors may have been telling them.

This cheerful news will be announced sometime this year, when the Metropolitan Life Insurance Co., publisher of the "desirable weight" tables that have been the national standard for two decades, issues new tables based on latest findings.

The guidelines will be increased by 10 to 15 pounds, on the average, for men of all ages and five to seven pounds for women 25 and younger, insurance industry sources forecast.

There will be little change for women over 25, "perhaps just a few pounds, five at most at some ages," one source estimated.

Just the same, "These figures are going to raise a lot of furor," Dr. W. Henry Sebrell, former director of the National Institutes of Health and consultant to Weight Watchers, predicted last week. "There are thousands of fat people waiting to see them."

The precise numbers for various categories are still being calculated. But these new "desirable" or "optimal" or "acceptable" weights--a name for them has not yet been picked--will still recommend only moderate amounts of body fat for any given age and body build.

Most authorities still believe it is healthiest to be slightly underweight--perhaps 5 to 15 pounds below average--at most ages, despite the contention of a few that it is actually better to be somewhat overweight.

"At the same time," said Edward A. Lew, president of the Society of Actuaries, "your doctor is soon going to be telling you, 'You can weigh a little more than we've been saying. And if you're average or slightly above average, forget about weight. A few extra pounds are not going to affect you very much.'

"The food companies are going to love this," Lew said. "It says we can indulge sometimes. But it still doesn't mean it's good to be fat. It just means that for those only 5 or 10 percent overweight, the average difference in length of life may be just a matter of months, perhaps only three to six months."

The merits of modest underweight versus overweight were hotly debated at a two-day workshop sponsored last week by the National Institutes of Health and the federal Centers for Disease Control.

Most of the participants agreed that the overweight suffer more high blood pressure, diabetes, heart and blood vessel disease and other disabilities, and, on the average, die earlier.

But the participants also heard the latest statistics showing that American men and young women--though not older white women--have been getting both taller and heavier in recent years, at the same time death rates have been falling.

These are the statistics that will be reflected in the new "Met Life" tables due out "we hope by June," Frederic Seltzer, the company's assistant chief actuary, said in an interview.

The tables will promptly be posted in thousands of doctors' offices and diet clinics, in place of a 1959 set of Metropolitan Life guidelines almost all doctors and insurers accept as a standard.

The new numbers will be based not on the experience of Metropolitan policyholders alone, however, but on two larger studies.

One is a study of 4 1/2 million policyholders of many insurance companies, followed between 1954 and 1972 by the Association of Life Insurance Medical Directors and the Society of Actuaries. The old, more restrictive weight tables were based on a similar 1935 to 1954 study.

"Now we've measured a new generation. It's a heavier generation, but it's also a healthier generation, for many reasons," said Lew, a retired Metropolitan Life vice president and actuary who headed both studies.

In addition, 750,000 initially healthy men and women were followed for l2 years by the American Cancer Society.

"No other studies begin to approach these numbers, and the results of both studies are much the same," Lew said. "It's healthier to be somewhat underweight, somewhat less than average."

Any "recommended" weights, the experts emphasized, are only averages, not to be taken as good guides for everyone. Some ethnic groups naturally tend to be tall and slim, others short and thickset.