Syntax
SUM(somelist)
Arguments
 somelist (list of any numeric type)
AppSheet must be given enough context to determine what type of values somelist contains, that its contents are or will be numeric. To that end, somelist must be one of the following: a column value of type List that contains numeric values; a column list (e.g., Products[Price]) for a column of a numeric type; or a constructed list (e.g., with LIST()) of a numeric type.
Return Value
The computed sum of the values in somelist.
Examples
SUM(Classes[Seat Count])
: The sum of all values in the Seat Count column of the Classes table. Equivalent to SUM(SELECT(Classes[Seat Count], TRUE))
. See also SELECT().
SUM([Discounts])
: The sum of the items in the Discounts column value, where Discounts is of type List.
SUM(LIST(1, 2, 3))
returns Number: 6
Sum Values from Select Rows
Compute the total delivery charges recorded within a reporting period:
SUM(
SELECT(
Deliveries[DeliveryCharge],
AND(
([DateDone] >= [_THISROW].[BeginDate]),
([DateDone] < [_THISROW].[EndDate])
)
)
)

SELECT(Deliveries[DeliveryCharge], ...)
gets a list of delivery charges from select rows in the Deliveries table. 
AND(..., ...)
limits the SELECT() results to only those rows that match all of the conditions. 
([DateDone] >= [_THISROW].[BeginDate])
limits the count to only rows with a DateDone no earlier than the report's BeginDate. 
([DateDone] < [_THISROW].[EndDate])
further limits the rows to those with dates before the report's end date. 
SUM(...)
totals the values from the result of the SELECT().
Common Problems
SUM(1, 2, 3)
: the arguments are not in list form. To fix, wrap them in LIST() to construct a list: SUM(LIST(1, 2, 3))
.