Is there a way to find out how much space is available within an allocated extent? In particular, if I delete a number of records, they will be marked as deleted and available for reuse. I want to be able to find out how much space these deleted records are taking up and therefore how much space in the allocated extents will be reused before new extents start getting allocated. I don't need to reclaim the space, I just want to know how much there is.

Thanks for any advice.

I am not sure any onstat commands available to calculate how much space freed after delete records.

Easy way to get an estimate :
(Number of records deleted * record length) + (Number of records deleted * length of each index record )

Hope this will give you a rough estimate.

Cheers!!
Sanjit