Yes it's about 100% space of the index in the log file.
It uses log space because it's a transaction. Anything that changes data/structure/etc requires log space. Log space is not the same thing as the data space. Log space is regarding transactions and recovery.
In case relevant: we run Log Backups every 2 minutes during index rebuild (normally ours are every 10 minutes) because Index Rebuild puts more strain on our Log files than any other activity in our databases. We also only rebuild indexes that are fragmented (rather than "Rebuild all indexes every time" <sigh!>