Tableau measures are set to a SUM aggregation by default. You can change the aggregation for a measure by 1) right-click on the measure 2) select default properties and 3) choose an aggregation. Each time you drag the measure to the canvas it will maintain the select aggregation.
Answer:
First, this will only work on calculated fields you create:
1) Right-click on the created measure field
2) Select Default Properties
3) Aggregation and select your default aggregation for your new calculation field.
