Returns the total of all valid, non-blank values in field.
field - any related field, repeating field, or set of non-repeating fields; or an expression that returns a field, repeating field, or set of non-repeating fields.
Parameters in curly braces { } are optional.
Data type returned 
Originated in 
FileMaker Pro 6.0 or earlier
Field can be any of the following:
a repeating field (repeatingField).
a field in matching related records specified by (table::field), whether or not these records appear in a portal.
several non-repeating fields in a record (field1;field2;field3...).
corresponding repetitions of repeating fields in a record (repeatingField1;repeatingField2;repeatingField3), if the result is returned in a repeating field with at least the same number of repeats.
several fields in the first matching record specified by (table::field1;table::field2;...). You can include fields from different tables (table 1::field A;table 2::field B...).
An Invoice table has a portal showing line items.
Sum(LineItems::ExtendedPrice) totals the amounts for all items on the invoice.
A TimeBilling table has a portal showing time worked on a project. Hours is a time field.
Sum(Hours::BillableHours) returns the total number of billable hours on a project. Thus, if the portal shows 40 hours and 15:30 hours, the total billable hours are 55:30, or 55 1/2 hours.
In the following examples:
If the calculation result isn’t a repeating field:
Sum(Field2) returns 26.
If the calculation result is a repeating field:
Sum(Field2) returns a repeating field with 26 in the first repetition.
Sum(Field1;Field2;Field3) returns a repeating field with 12, 8, 7, 8.
Note  When a referenced field is a repeating field, the Sum function returns the sum of the first repetition field, then the sum of the second repetition field, and so on.
Related topics 
Functions reference (category list)
Functions reference (alphabetical list)
About formulas
About functions
Defining calculation fields
Using operators in formulas