Why Your Hard Drive Shows Less Space Than Advertised

You buy a 1 TB hard drive, plug it in, and your computer reports 931 GB. You did not get cheated — the discrepancy comes from different measurement systems.

The two systems: - Decimal (SI): Used by storage manufacturers. 1 KB = 1,000 bytes, 1 MB = 1,000 KB, 1 GB = 1,000 MB, 1 TB = 1,000 GB. A "1 TB" drive = 1,000,000,000,000 bytes. - Binary (IEC): Used by operating systems. 1 KiB = 1,024 bytes, 1 MiB = 1,024 KiB, 1 GiB = 1,024 MiB, 1 TiB = 1,024 GiB.

The math: 1,000,000,000,000 bytes (1 TB decimal) ÷ 1,073,741,824 (1 GiB binary) ≈ 931 GiB. The drive holds exactly 1 TB by the manufacturer's definition and 931 GiB by the OS's definition. Nothing is missing.

Why manufacturers use decimal: It produces larger, more marketable numbers. A drive labeled "1 TB" sounds better than "0.909 TiB" or "931 GB." This practice dates back to the early days of storage when the difference was negligible (a few KB on a 1 MB drive). The gap grows with each unit step — at the PB level, the difference is ~10%.

Other space consumers: The filesystem itself (NTFS, APFS, ext4) consumes some space for metadata and organizational structures — typically 0.1–0.5% of total capacity. Pre-installed recovery partitions and system files on OEM computers consume additional space.

Use Tooler's Data Storage Converter to understand the exact relationship between decimal and binary storage units.