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Is there a law or regulation that governs the maximum allowable interest amount that can be charged on credit cards or in agreements where credit is extended?
In the U.S., each state has its own local usury law. This website has a separate page for each state summarizing the local usury law and provides a reference to the local statute. The rules aren't simple: some set absolute limits, some appear to be pegged to something like the Prime Rate, some states don't have a general usury limit, the rules don't apply to certain loans because of the type of loan or lender, etc. There are US Federal laws dealing with usury, primarily in the context of racketeering -- the RICO Act lets the Feds go after racketeers that violate local usury laws beyond certain parameters.