It depends. For earned income the calculation is usually what is reported on your W-2: the amount of income you earned in each state (or while living in each state). However, for unearned income such as interest you may not have any clear-cut documentation to determine the allocation numbers. When this is the case use percentages. Divide the number of days you lived in one of the states by 365, and then multiply this percentage by the amount of income in question (interest, dividends, etc.). This is the amount of that income that is allocated to the first state, and then you subtract that amount from the base amount of that income category to report the allocation to the other state. Keep a worksheet of your calculations to show what method you used in case the state questions why you reported what you did.
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