NLS_LENGTH_SEMANTICS and VARCHAR2 field length question

[MartyRL]

I have a database with NLS_LENGTH_SEMANTICS set to BYTE, NLS_NCHAR_CHARACTERSET is AL16UTF16 and NLS_CHARACTERSET is UTF8. I have a program that is writing data to a field defined as VARCHAR2 (234).

I am writing simplified Chinese characters to the field . When I write the data I get an error message:
ORA-12899 Value too large for column: actual 275, max 234

The # of Chinese characters in the string is 71. I thought with UTF8 I would require a maximum of 3 bytes per character (non supplemental characters) - so I would expect the actual to be 71*3 or no bigger than 213 bytes.

Why is the message telling me the actual is 275 bytes?

How can I calculate the max size I need in bytes if I know the max number of Chinese characters.

This is a large production database where only a few records will be populated with Chinese characters - so we would prefer to keep the semantics to BYTE rather than CHAR (does it make a difference in space utilized if we change to CHAR and most of the data is in english?)

[PAVB]

is your NLS_LANGUAGE set to SIMPLIFIED CHINESE? if not, how can you be sure nothing else is going in?

Don't do CHAR, that will make your columns fixed lenght.

For a quick, dirty and safe solution stay VARCHAR2 and assign a safe lenght, go 500 - who cares? VARCHAR2() is a variable size data type that will allocate exactly what is needed and not a single extra byte -other than data type overhead, off course.