Excel has a built-in function to determine the length of a string:

=LEN(string)

For example, if you have a string in cell A1, you can use the following formula to determine its length:

=LEN(A1)

This function will return the number of characters in the string, including spaces. If you only want to count the number of letters in the string, you can use the following formula:

=LEN(TRIM(A1))

This will remove any leading or trailing spaces from the string before counting the number of characters.