Why does my 1 TB hard drive only show 931 GB in Windows?

Drive manufacturers use decimal SI units where 1 TB = 1,000,000,000,000 bytes. Windows computes in binary IEC units (Gibibytes) where 1 GiB = 1,024^3 bytes. 1,000,000,000,000 / 1,073,741,824 equals approximately 931.32 GiB.

Storage Units Converter (Decimal vs. Binary)

Convert drive manufacturer sizes (Decimal / SI) into true Operating System capacity (Binary / IEC).

Drive Manufacturer / Nominal Size (Decimal)

A manufacturer labeled "1 TB" drive will register as approximately 931.32 GiB in Windows OS file systems.

Equivalent Binary (IEC) & Exact Byte Capacities:

Exact Bytes (B)

1,000,000,000,000 B

Kibibytes (KiB - 2¹⁰)

976,562,500 KiB

Mebibytes (MiB - 2²⁰)

953,674.32 MiB

Gibibytes (GiB - 2³⁰)

931.32 GiB

Tebibytes (TiB - 2⁴⁰)

0.9095 TiB

Pebibytes (PiB - 2⁵⁰)

0.000888 PiB

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

Decimal (SI Base 10)	Bytes	Binary (IEC Base 2)	Bytes	Discrepancy
1 Kilobyte (KB)	1,000	1 Kibibyte (KiB)	1,024	2.4%
1 Megabyte (MB)	1,000,000	1 Mebibyte (MiB)	1,048,576	4.9%
1 Gigabyte (GB)	1,000,000,000	1 Gibibyte (GiB)	1,073,741,824	7.4%
1 Terabyte (TB)	1,000,000,000,000	1 Tebibyte (TiB)	1,099,511,627,776	9.9%
1 Petabyte (PB)	1,000,000,000,000,000	1 Pebibyte (PiB)	1,125,899,906,842,624	12.6%