WASHINGTON — Determining which financial companies are so large that their failures could pose a risk to the nation’s financial system should be done using a “transparent and replicable” formula that makes it “clear to the financial firms, the markets and the public” what institutions are covered, a Federal Reserve governor said Friday.

The governor, Daniel K. Tarullo, said “systemically important financial institutions,” a category specified in the sweeping overhaul of financial regulation enacted last year, would be required to carry substantially higher levels of capital on their books. The levels would act as a cushion against losses, perhaps as much as twice the level specified in new international banking standards.

“The failure of a systemically important financial institution, especially in a period of stress, significantly increases the chances that other firms will fail,” Mr. Tarullo said. But because those firms have “no incentive to reduce the chances of such systemic losses,” higher capital requirements are necessary “to make those large, interconnected firms less prone to failure.”

Wall Street is bracing for guidelines as to how federal regulators will decide which companies fall under the “systemically important” designation. Mr. Tarullo’s remarks, before a group at the Peterson Institute for International Economics here, offered some of the first public details on how regulators are thinking about the rules. The law requires that banks, nonbank financial firms like hedge funds, insurance companies or other institutions that greatly affect the financial system, to be subject to an additional capital requirements to prevent a repeat of the 2008 financial crisis. The crisis was made worse by the interdependence of many of the largest financial outfits.