SUM Function

The SUM function returns the total value of the numbers in a list, table or cell range.
This function will add up anything that has a numerical value.
This includes numbers and dates.
Logical values and text are ignored.

Using Cell Ranges

(same as count)

Using Individual Values

(same as count)

Using Arrays

Status Bar Sum

The sum is only displayed in the status bar when you select at least 2 cells that contain numerical values.
If any of the selected cells contain an error, the sum will not be displayed.

Excluding Hidden Rows

This function includes hidden rows. Row 5 has been hidden and is included
SS
If you want to add up the numerical values excluding hidden rows you can use the AGGREGATE function.
You can use SUBTOTAL but AGGREGATE is recommended.

Ignoring Error Values

you can use the AGGREGATE function.

Adding Dates & Times

Summing Formulas

full list