select blocks, empty_blocks, num_freelist_blocks
from all_tables
where owner = 'MYSCHEMA'
and table_name = 'MYTABLE';
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The total number of blocks allocated to the table is blocks + empty_blocks + num_freelist_blocks.
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blocks is the number of blocks that actually contain data.
Multiply the number of blocks by the block size in use (usually 8KB) to get the space consumed - e.g. 17 blocks x 8KB = 136KB.