DEFINITION of 'Interest Expense'. The cost incurred by an entity for borrowed funds. Interest expense is a non-operating expense shown on the income statement. It represents interest payable on any type of borrowings – bonds, loans, convertible debt or lines of credit. It is basically calculated as the interest rate times the outstanding principal amount of the debt. Interest expense on the income statement represents interest accrued during the period covered by the financial statements, and not the amount of interest actually paid over that period.