How to know what is the exact space used by a partition of a table

Hi, Iīm developing a query to know the exact amount of space that I can use to allocate data.

I need to know what is the exact space used by a partition of a table. The user_segment table has a field called "bytes", but I think that this field represents the allocated space, not the really used.

Also, is there a way to know the free space for a tablespace?

I was looking for the tables dba_free_space, dba_segments, all_tab_partitions and user_segments, but I canīt really figure out how calculate the free space for a tablespace, or the used space for a partition.

The idea is to know the space used for a partition, and based in this, estimate the space that will be used for a future load of data, and see if this amount of data will fit into the partition (and the tablespace)

Thanks a lot for your help

Cristian Palma