Column length when migrate VARCHAR2 data from Oracle -

as know have 2 types of varchar2 in oracle

e.g. col1   varchar2(20 byte)      col2   varchar2(20 char) 

when migrate data different database (e.g. netezza) should correct length.

col1 should 20  col2 should 20 or 20x4= 80 ?  

because 1 char = 4 bytes

thanks

to idea how bytes data takes up.

 select max(lengthb(column)) table 

this give max length in bytes of values in column specified. it's highly unlikely characters take 4 bytes.