International tax practitioner here. The foreign tax credit calculation under IRC Sections 901-909 is one of the most complex areas of tax law, and I see mistakes on this constantly. Let me share some common pitfalls and practical guidance.
First, you must choose between the foreign tax credit (FTC) and the foreign tax deduction. For most taxpayers, the credit is more valuable because it reduces your tax dollar-for-dollar, while the deduction only reduces your taxable income. However, you must make this election consistently for all foreign taxes in a given year. You cannot cherry-pick the credit for some taxes and the deduction for others.
Second, the FTC is limited by the Section 904 limitation, which prevents you from using foreign taxes to offset U.S.-source income. The formula is: FTC limitation = U.S. tax liability multiplied by (foreign source taxable income divided by worldwide taxable income). If your foreign tax rate is higher than your U.S. effective rate, you will have excess credits that carry forward for 10 years under Section 904(c).
Third, watch out for the sourcing rules. Income is sourced based on complex rules in Sections 861-865, and getting the sourcing wrong will mess up your entire FTC calculation. For example, personal services income is sourced where the services are performed, not where the payer is located. Dividend income sourcing depends on the residence of the paying corporation. Getting professional help with the FTC calculation is almost always worth the cost if you have significant foreign income from multiple countries.