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.
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.