Roll Up Summary

Define roll-up summary fields on the object that is on the master side of a master-detail relationship. If a relationship does not already exist, first create a master-detail relationship between the master object that displays the value and the detail object containing the records you are summarizing.

When Roll-Up Summary is grey out that means you cannot create this type of field on this object because it is not the master in a master-detail relationship.

Setup > Customize > Accounts > Fields

Account Custom Field & Relationships > New (Create new roll up summary field)

Select (Roll-Up Summary) > Next

Enter the details

Field Label: (Mandatory Field)

Field Name: (Auto Populates and Mandatory Field)

Next

Select the object on the detail side of a master-detail relationship. This object contains the records you want to summarize.

Summarized Object: Opportunities

Roll-Up Type

            Count: Total the number of related records.

Sum: Totals the values in the field you select in the Field to Aggregate option. Only number, currency, and percent fields are available.

Min: Displays the lowest value of the field you select. Only number, currency, percent, date, and date/time fields are available.

Max: Displays the highest value of the field you select. Only number, currency, percent, date, and date/time fields are available.

Select Roll-Up Type > Count

Filter Criteria > All records should be included in the calculation > Next

Select the profiles to which you want to grant access to this field via field level security> Next

Select the page layout you want to add this field and then save.

Roll-Up field created

On account detail page you can see Roll Up Summary field “Count Opportunity” counting the number of opportunities associated to that account.