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GB vs GiB: why your drive looks smaller

Convert between decimal (GB, TB) and binary (GiB, TiB) units and see what Windows, macOS and the box will show.

Box, macOS, iOS1 TB
Windows shows931.3 GB

Decimal (SI)

KB
1,000,000,000
MB
1,000,000
GB
1,000
TB
1
PB
0.001

Binary (IEC)

KiB
976,562,500
MiB
953,674
GiB
931.3
TiB
0.9095
PiB
0.000888

1,000,000,000,000 bytes

Why a 1 TB drive shows 931 GB

The same 1 TB bar cut two ways: into ten 100 GB pieces, and into 9.31 pieces of 100 GiB, each 7.4 % bigger Sold as 1 TB: ten blocks of 100 GB (10¹¹ bytes) 100 100 100 100 100 100 100 100 100 100 Windows: the same bytes in 100 GiB blocks (2³⁰ × 100 bytes) 100 100 100 100 100 100 100 100 100 31 = 931.3 GiB, shown as “931 GB” same length: no bytes missing

Two counting systems share the same names. The SI prefixes (kilo, mega, giga, tera) mean powers of 1,000, as they do for metres and grams. A terabyte is 1,000,000,000,000 bytes. Drive makers, network speeds, phone makers, macOS and iOS count this way.

Computer memory is built in powers of two, so early software counted a "kilobyte" as 1,024 bytes. In 1998 the IEC gave those binary units their own names: kibibyte (KiB), mebibyte (MiB), gibibyte (GiB), tebibyte (TiB). Windows still counts in binary units and labels them KB, MB, GB, TB.

So the 1,000,000,000,000 bytes of a "1 TB" drive divided by 1,073,741,824 bytes per GiB is 931.32. Windows shows "931 GB". Nothing is lost: it is the same number of bytes, counted with a bigger unit. The gap grows with each prefix: 2.4 % at kilo, 4.9 % at mega, 7.4 % at giga, 10 % at tera.

Memory (RAM) is the exception that keeps the confusion alive: a "16 GB" memory stick really is 16 GiB, because memory chips come in powers of two.

What common sizes show as

Advertised size, binary value, and what each system shows
Sold asIn GiBWindows showsmacOS / iOS show
16 GB14.9 GiB“14.9 GB”16 GB
32 GB29.8 GiB“29.8 GB”32 GB
64 GB59.6 GiB“59.6 GB”64 GB
128 GB119.2 GiB“119.2 GB”128 GB
250 GB232.8 GiB“232.8 GB”250 GB
256 GB238.4 GiB“238.4 GB”256 GB
500 GB465.7 GiB“465.7 GB”500 GB
512 GB476.8 GiB“476.8 GB”512 GB
1 TB931.3 GiB“931.3 GB”1 TB
2 TB1,862.6 GiB“1.81 TB”2 TB
4 TB3,725.3 GiB“3.63 TB”4 TB
8 TB7,450.6 GiB“7.27 TB”8 TB
16 TB14,901.2 GiB“14.55 TB”16 TB
Decimal and binary prefixes, in bytes
DecimalBytesBinaryBytesBinary is bigger by
kilobyte (KB)1,000kibibyte (KiB)1,0242.4 %
megabyte (MB)1,000,000mebibyte (MiB)1,048,5764.9 %
gigabyte (GB)1,000,000,000gibibyte (GiB)1,073,741,8247.4 %
terabyte (TB)1,000,000,000,000tebibyte (TiB)1,099,511,627,77610 %
petabyte (PB)1,000,000,000,000,000pebibyte (PiB)1,125,899,906,842,62412.6 %

Worked answers

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