Why Does My 1TB Drive Only Show 931 GB in Windows?

Every year, millions of consumers buy a 1TB, 2TB, or 16TB hard drive or SSD and wonder why their operating system reports approximately 7% to 9% less space than advertised. This is not a defect or a missing partition—it is an outcome of two different measurement standards.

Decimal SI Standards vs. Binary IEC Standards

• Hard Drive Manufacturers (Decimal / Base 10): Use the International System of Units (SI). To a manufacturer, 1 Kilobyte = 1,000 Bytes and 1 Terabyte = 1,000,000,000,000 Bytes (10¹²).
• Operating Systems (Binary / Base 2): Computers process binary logic. Operating systems historically calculated 1 Kibibyte = 1,024 Bytes (2¹⁰) and 1 Tebibyte = 1,099,511,627,776 Bytes (2⁴⁰).

When Microsoft Windows inspects a 1,000,000,000,000 byte disk, it divides by 1,024 three times to calculate gigabytes:

1,000,000,000,000 ÷ 1,024 ÷ 1,024 ÷ 1,024 = 931.32 GiB

Because Windows incorrectly labels binary GiB as GB, users perceive missing capacity. macOS (since OS X 10.6 Snow Leopard) uses decimal 1,000-based math in Finder, correctly displaying 1 TB.

Standard Prefixes Comparison Table