Calculate Size Of Table With BLOB Column

when trying to calculate the occupied space for a table, I'm using DBA_SEGMENTS, which works fine as long as the table does not have a BLOB column.

As far as I can tell, the size of the BLOB data is stored with the SEGMENT_TYPE = 'LOBSEGMENT', but I cannot find a view that tells me which DBA_SEGMENT row belongs to the BLOB column in the table I'm checking.

To give you an example:

select sum(BYTES)
from DBA_SEGMENTS
where owner = user
and segment_name = 'MY_TABLE'
group by SEGMENT_NAME

returns 262144

running:

SELECT sum(length(blob_column))
FROM my_table

returns 821333

There are entries in DBA_SEGMENTS for my user with the type LOBSEGMENT, but I cannot find a way to map the correct DBA_SEGMENTS row to the table I am checking.

I'm using Oracle 10.2.0.3.0 on Redhat