The sum rollup type populates the total from adding a column in all related records, and can calculate the sum of any numeric type including whole number, decimal or currency

The calculation type of rollup and sum as the rollup type are entered in the first section for these calculated field. This example below will populates a custom field on the account entity with the sum of all completed orders

The remaining configuration in the second section details the related type and field to rollup. This example also includes a filter to only include completed order

This screenshot visualises an example account with 2 orders summed by the example calculated field