About 84 percent of full-time workers in the nation's large and medium-size private companies are covered by private retirement pension plans in addition to Social Security, according to a Labor Department survey.

In all but a handful of cases, the entire cost of the pension is paid by the employer, the survey of worker fringe benefits showed.

Most workers also were found to have group health insurance, usually financed by the employer, group life insurance and reasonably generous vacation and sick-pay benefits. The study, made public this month, examined benefits received by 21 million full-time employes in large and medium private companies last year.

One purpose was to furnish information to the Office of Personnel Management so that it could compare compensation of federal workers with those in private industry.

The government is under pressure to bring federal workers' benefits into line with those of workers in equivalent private-sector jobs. In general, pensions of the private-industry workers are inferior to the federal employe retirement system.

But other fringe benefits and salaries, which may be higher in private industry in some cases, also need to be taken into account in comparisons.

Public-sector workers, as well as private workers in agriculture, education and health services, were not included in the survey; nor were part-time workers or employes of companies with fewer than 50, 100 or 250 workers, depending on the industry. The study did not represent the nation's entire labor force of 107 million workers because smaller companies usually can't afford a broad package of fringe benefits.

For example, while 84 percent of those in the larger-company survey have private pension coverage, the figure for the total labor force in March, 1981, was about 45 percent, according to the Census Bureau.

Most employes of the federal government, the nation's largest employer, are in agencies similar to the bigger, more affluent private companies. Here are the key findings of the survey of benefits in the nation's big bellwether private companies:

* Eighty-four percent of full-time workers were covered by private pension plans. Of those workers, only 7 percent helped contribute; for the rest, the employers financed the pensions entirely.

About two-fifths were in plans requiring workers to stay on the job until age 65 to receive full benefits; other plans permitted full benefits at 60 or 62 after a specified term of service; and about an eighth of the workers were in plans that allowed retirement at any age after 30 years of service.

Only about 3 percent of the workers were in plans with automatic cost-of-living adjustments, but nearly two-fifths were in plans that made occasional ad hoc increases.

* Ninety-seven percent of the workers were covered by health insurance plans, and 93 percent had plans covering family members as well. Employers paid the entire cost of the plan three-quarters of the time for the worker and just under half the time for family members.

For virtually all workers covered, the plans--with a variety of deductibles, co-payments and the like--included hospital and surgical care, doctor visits, X-ray and laboratory work, prescription drugs and a variety of other services. Sixty-eight percent of the workers were covered for dental care, 62 percent for post-hospital extended care and 22 percent for vision care.

* Ninety-six percent of the workers had group life insurance; employers paid for it entirely four-fifths of the time. Coverage usually equaled between one and two years' salary.

* On the average, employes received 10 paid holidays a year, 16 days of vacation after 10 years and 21 days after 20 years.

* Ninety-three percent of all employes had income protection for short-term disability, either through sick leave or insurance. On the average, those with five years of service got from five to 15 weeks of sick leave a year with full pay. About two-fifths of employes were provided with long-term disability insurance, and an additional 49 percent had plans providing immediate disability retirement benefits.

* Eighty-four percent of the employes worked a 40-hour week, 7 percent 37.5 hours, 3 percent between 37.5 and 40 hours, and 3 percent 35 hours. And 76 percent had formal rest time daily--about 20 to 30 minutes in most cases.