Base 10 format
Disk space is always shown as base 10 in iStat Menus. Hard drive manufacturers usually quote the size of their HDs as base 10, where 1 GB equals 1,000,000,000 bytes. Some operating systems show disk space as base 2, where 1 GB equals 1,073,741,824 bytes. However, since Snow Leopard, macOS shows disk space as base 10.
iStat Menus 5 uses base 10 format for all disk storage figures.
A note about free space
If you’re using Time Machine, and if your Time Machine drive isn’t connected, macOS 10.7 Lion and newer OSs create local backups on your startup volume. The Finder and iStat Menus don’t report this space as used, but Disk Utility does. For more information, please read this article by Roberto Valerio.