Default value formulas versus formula fields
Key Takeaway
A default value fills in a field once, and users can change it. A formula field is read only and stays current.
A default value formula gives a custom field a starting value, and a formula field shows a value that Salesforce calculates. To choose between them, ask whether users should be able to override the value and whether it must keep up with changes to other data.
What each one is
A default value, also called a default field value, puts a value into a custom field when a user creates a new record. You can enter a fixed value or a formula, such as TODAY() + 7. A default value formula can use values, merge fields, or expressions.
A formula field calculates its value automatically from other values, merge fields, or expressions. Users can see the result on the record detail page, but they can't see the formula behind it.