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Data Size Units: SI (KB, MB) vs IEC (KiB, MiB)

Resolve the confusion between decimal gigabytes (1000) and binary gibibytes (1024).

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Have you ever plugged in a new 1 TB hard drive only to see your computer report only ~931 GB of free space? This common discrepancy stems from the difference between SI (decimal) and IEC (binary) standards.

SI Units (Decimal, Base 10)

Standardized by the International System of Units (SI), these prefixes use powers of 10:

  • 1 Kilobyte (KB) = 1,000 bytes (103)
  • 1 Megabyte (MB) = 1,000,000 bytes (106)
  • 1 Gigabyte (GB) = 1,000,000,000 bytes (109)
  • 1 Terabyte (TB) = 1,000,000,000,000 bytes (1012)

Hard drive manufacturers, network speed tests, and macOS use SI units.

IEC Units (Binary, Base 2)

Standardized by the IEC in 1998, these prefixes represent powers of 2:

  • 1 Kibibyte (KiB) = 1,024 bytes (210)
  • 1 Mebibyte (MiB) = 1,048,576 bytes (220)
  • 1 Gibibyte (GiB) = 1,073,741,824 bytes (230)
  • 1 Tebibyte (TiB) = 1,099,511,627,776 bytes (240)

Operating systems like Windows traditionally calculate storage using powers of 2 (1024), but label them with SI symbols, causing the perceived "missing space".