At least one-quarter of the U.S. economy has suddenly gone idle amid the coronavirus pandemic, an analysis conducted for The Wall Street Journal shows, an unprecedented shutdown of commerce that economists say has never occurred on such a wide scale.

The study, by the economic-analysis firm Moody’s Analytics, offers one of the most comprehensive looks yet at how much of the world’s largest economy has shut down in the past three weeks. It also analyzes counties big and small—from Manhattan to tiny Gilpin County, Colo.—to estimate...