About half the Fortune 500 companies have begun conversion to the metric system, but consumers still oppose it by two to one.

In a recently released progress report on metrication, the General Accounting Office observed that the primary reason for coneversion in this country is the belief it is inevitable, rather than an assessment of any certain benefits to be derived from it. (A mere 5 percent of the Fortune 500 felt that shifts in measurement would significantly promote international trade.)

Given the costs, which the GAO estimates in the billions of dollars, the government watchdog agency recommended a go-slow attitude toward metrication until Congress can decide whether the use of the metric systems warrants the effort and the expense necessary.

The GAO instigated the report, officials insist. Yet its point of view is shared by two conservative Republican congressmen, Eldon Rudd of Arizona and Philip M. Crane of Illinois. They introduced separate bills this past session to abolish the U.S. Metric Board on the ground that conversion should proceed at the pace of the marketplace without government intervention. The bills were defeated by a 4-1 margin.

The U.S. Metric Board, which began functioning only recently, shot back at GAO, insisting that by passing the Metric Conversion Act of 1978, Congress had committed its support for voluntary conversion to metric measurement. American industry is converting not because of any "inevitability syndrome," but because it feels the United States' interest lies in getting in step with the rest of the world, the board said. Finally the board noted the GAO had failed to point out that conversion costs are a one-time investment, whereas benefits are continuous.

The board's chairman, Louis Polk, sought to reassure the business community last week at a Chamber of Commerce meeting. "The board has no mandate to cram anything down anyone's throat and it has no intention of doing so," he said. He suggested that companies convert in conjunction with other changes so as to reduce costs.

Public criticism of U.S. conversion to the metric system has increased during the past year, according to the American National Metric Council, a private group helping companies to convert.

A precipitating event seemed to have been the National Highway Traffic Safety Administration's decision in June 1977 that all speed limits and other highway information would be converted to meters and kilometers by 1982. Of the more than 5,000 comments received, 98 percent were negative. Then there was the National Weather Service's decision to switch public weather reports to Celsius only by June 1980.

A public opinion poll conducted for the GAO showed that half the people responding believed they would not benefit from conversion. The remainder was split between those who believe they would benefit and those that had no opinion. While the public is once again questioning whether the confusion and the cost are really worth it, big business goes determinedly ahead with conversion. Small business has taken little positive action, the survey showed. Only 12 or 13 percent of the 1,400 small firms polled indicated they had converted to metric or were in the process. They cited the costs of dual inventories during conversion and employe training in metrication.

Apare from the pharmaceutical industry, which has used the metric system for years, the automotive industry -- really General Motors Corp. -- leads the way. In fact, the report noted, GM's competitiors and suppliers told the GAO that if it were not for its leadership, conversion in the United States would be at a relative standstill.

GM's target date for predominantly metric passenger cars is 1982; Chrysler, the late 1980s; Ford and American Motors, the early 1990s. GM also plans metric equivalents for the measurements of trucks by 1982. Moreover, GM has found that conversion is not so expensive as it had anticipated. In 1976, it estimated that its costs would be only 3 or 4 percent of the original 1966 estimate.

Most of the activity, as might be expected, is with multinational corporations, but two major industries, aviation and petroleum, have adopted the attitude that because the world is used to the U.S. system, there would be little benefit in changing.

There seems to be little momentum for conversion in railroads, maritime transport, construction, domestic appliances, business paper and groceries. A considerable number of products packaged in customary U.S. sizes now bear metric equivalents. This is called "soft" conversion. The wine and distilled spirits industry, at the instigation of the Bureau of Alcohol, Tobacco and Firearms, has begun the first complete national metric conversion of a consumer product. This is a "hard" conversion, a switch to liter and milliliter bottles. The price increases that have accompanied the conversion have fueled the GAO's fires against government-led conversion.

GAO also criticized conversion in sports, calling the benefits "nebulous." Spokespersons for horse racing, basketball and tennis reported no plans, the GAO said, and professional football has stated that it will declare itself exempt. Though there is lettle interest from pro sports, college and high school athletics are converting.

This goes along with the metric education policy of the federal government. Since 1972, the Department of Health, Education and Welfare has provided $6.3 million in grants, and 13 states have set 1980 as teaching predominantly in metric. Thus "there may be a generation of children who were primarily educated in metric trying to function in the customary system in the United States," the GAO concludes.

In the case of metrication, the GAO recommended, the government should be a follower of private industry and not a leader.