Formula Field is a read only field whose value is evaluated from the formula or expression defined by us. We can define formula field on both standard as well as custom objects. Any change in expression or formula will automatically update the value of formula field.
In Salesforce formula field is a read only field, the value of formulafiled evaluate from expression defined by us. If we update any value in the expression, it automatically updates formula field value. We can create formula fields in both standard and custom object.