The exchange rate that should be used to convert foreign interest or foreign dividends depends on the nature of each transaction. If the money is received sporadically during the year such as at the end of the year or on a specific date you should use the spot exchange rate from a reliable source such as www.oanda.com. If the money is received evenly throughout the year you can use the IRS yearly foreign exchange rates as published on the IRS website (https://www.irs.gov/Individuals/International-Taxpayers/Yearly-Average-Currency-Exchange-Rates).
About US Tax Practice GmbH: US Tax Practice offers U.S. tax services in Switzerland. It is the go-to tax preparation service and compliance practice for U.S. tax payers. Located in Lenzburg, Switzerland, the company was founded by Patrick Evans, a U.S. tax accountant (CPA, CGMA) and U.S. citizen determined to work with his clients’ best interests in mind. US Tax Practice GmbH services include U.S. Income Tax Preparation, U.S. Tax Compliance, Foreign Bank Account Reporting (FBAR), Tax Planning and Optimization, and Expatriation.