IRR

The internal rate of return (IRR) is the interest rate received for an investment with payments and income occurring at regular intervals (i.e. monthly, annual). Payments are expressed as negative values and income as positive values. Amounts can vary, but intervals need to be the same. The first value is negative, since it represents an outflow. Excel uses iteration to arrive at a result, starting with the guess (if provided) or with .1 (10%) if not. If an accurate IRR can't be calculated after a fixed number of iterations, the #NUM error is returned. A better guess will prevent this error.