Roll-up summary relationship rules
Key Takeaway
A roll-up summary field goes on the master side of a master-detail relationship. Lookup relationships don't support it, so objects linked only by a lookup relationship need a master-detail relationship first.
A roll-up summary field sits on a master (parent) record and calculates a value from its detail (child) records. The value can be a count of the records, or the sum, minimum or maximum of one of their fields. For example, an account can show the total of all its related invoice amounts. Because the field depends on a master-detail relationship, firm rules decide which objects qualify and what you can change later.
Why a master-detail relationship is required
The detail records must be related to the master through a master-detail relationship, so you always create the field on the master object, not on the detail object.
Lookup relationships work much like master-detail relationships, but they don't support roll-up summary fields or sharing. If two objects are linked only by a lookup relationship, you first need a master-detail relationship between the object that will display the value and the object that holds the records to summarize.
A custom object can have up to two master-detail relationships and up to 40 relationships in total. The master can be a standard object, such as Account or Opportunity, or a custom object.