As stated above, hard drive manufacturers advertise capacity in metric GB, but software traditionally uses binary GB. Software that uses binary should use the unambiguous prefixes: MiB, GiB, etc. to refer to binary units.
If you want a authoritative answer, run "sudo lshw -C disk" in a terminal to "list hardware, just disks". It will show you the raw capacity of your drive in both binary (GiB) and metric (GB). "sudo lshw -C volume" will show you how your disk is divided up. I think the Ubuntu installer defaults to 2 partitions, a swap/virtual memory partition of a couple GB (for when RAM is low), and a large partition for everything else.