A method of measuring the quantity of wrap remaining on a round baler supply roll uses the existing wrap measuring apparatus of a round baler. An on-board microprocessor is programmed to calculate the minimum amount of wrap required for each bale, subtract the amount of wrap used to wrap a bale from the amount of wrap on the supply roll before wrapping the bale, and signal the operator if the amount on the roll is insufficient to wrap a bale. The microprocessor can also be programmed to signal the operator if the amount of wrap on the roll is insufficient to wrap a number of bales specified by the operator.