A method and apparatus for automatically calibrating a bale size indicator by using inputs from existing bale wrapper material monitors to determine the linear amount of wrapping material necessary to encircle the bale one time. Balers with wrapping mechanisms may include a sensor for measuring the linear amount of wrapper dispensed as a means to alert the operator when the supply of wrapper material is low. Such balers may also have additional sensors for verifying that wrapping material completely encircles the bale prior to allowing the bale to be ejected from the baler. Using inputs from each of these sensors, a control system on the baler can determine the amount of wrapping material necessary to cover one revolution of the bale, also an indication of the bale circumference, and use the measurement to calibrate the display of bale diameter also generated from the control system. The automated capability eliminates the need for an operator to manually measure the diameter of a completed bale and enter that measurement into the indicator control system.