Open file explorer. Right click your C. drive. Left click on "Properties" at the bottom of the menu. It will tell you the amount of disk space used and free space in GB's. It will also give the capacity of the drive.
Used space- 29,155,983,360 bytes, 27.1 GB
Free Space- 789,245,952 bytes, 752 MB
Capacity- 29,945,229,312 bytes 27.8 GB