Give 'em an inch and they'll take a kilometer.

Crusaders for wider use of the metric system are happy about an obscure section of the trade bill signed Tuesday by President Reagan, a section they say will help America measure up in a metric world.The bill designates the metric system as the preferred method of measure for trade and commerce and requires government agencies to start buying metric whenever practical.

For the U.S. Metric Association, which since 1916 has been prodding America to join most of the world in using metric measure, the bill is a milestone, or perhaps a kilometer post.

"I think the government's decision to adopt this law is pulling the country in the right direction," said Lorelle Young, association president.

"We interpret it to be saying that the metric system is highly important to the United States - to educate our youngsters in this scientific world, to put us in sync with the rest of the world and to give us the advantage of the best measurement system ever devised."

The Omnibus Trade and Competitiveness Act of 1988 gives U.S. negotiators more leeway to bargain at international trade talks, strengthens the government's ability to fight restrictive trade policies of other countries, and provides for retraining of Americans who lose their jobs because of foreign competition.

It also says that by 1993, agencies should "use the metric system of measurement in procurement, grants and other business-related activities, except to the extent that such use is impractical or is likely to cause significant inefficiency or loss of markets to U.S. firms."

Analysts at the National Association of Manufacturers don't expect much impact from the section because of the language giving agencies an economic out, said Howard Lewis, a vice president of the Washington, D.C., lobbying group.

But for the 3,500-member Metric Association, which promotes the switch nationwide from its headquarters in the Los Angeles suburb of Northridge, the bill seems likely to hasten the inevitable conversion.

Young recalled with relish that the bill's example of an economic hardship big enough to win an exemption from the metric policy was a company faced with competition from a foreign producer of non-metric goods - an unlikely situation.

Automakers and other sectors of heavy industry are gradually getting on the metric bandwagon, Young said, though lumber and machine tool industries are notable for their adherence to tradition.

"Our liquor industry hasn't suffered," she noted, "and they were one of the first to convert."