Storage Units Reference

Decimal (SI) — what drives are sold in

1 kB

1,000 bytes

1 MB

1,000,000

1 GB

1,000,000,000

1 TB

1,000,000,000,000

Binary — what Windows shows (mislabelled)

1 KiB

1,024 bytes

1 MiB

1,048,576

1 GiB

1,073,741,824

1 TiB

1,099,511,627,776

The gap compounds — stated both ways

kilo

unit 2.4% bigger · reads 2.3% smaller

mega

unit 4.9% bigger · reads 4.6% smaller

giga

unit 7.4% bigger · reads 6.9% smaller

tera

unit 10.0% bigger · reads 9.1% smaller

1 TB drive

shows as 931 GB in Windows

4 TB drive

shows as about 3,725 GB in Windows

⚠ RAM is the exception

16 GB of RAM really IS 16 GiB

⚠ Network speeds are BITS

100 Mbps ≈ 12.5 MB/s

b vs B

lowercase = bits, uppercase = bytes, x8

Nothing is missing from your drive

A 1 TB drive holds a trillion bytes, exactly as advertised. Windows divides that by 1,024³ instead of 1,000³ and prints the result as '931 GB'. The standard name for what it is showing is GiB. If it printed that, the complaint would not exist — the arithmetic on both sides has always been right.

Why you only notice it on big drives

The gap compounds with every prefix. A binary unit is bigger than its decimal namesake by 2.4% at kilo, 4.9 at mega, 7.4 at giga, 10.0 at tera — which means the same bytes read 2.3, 4.6, 6.9 and 9.1 percent smaller when divided into binary units. A text file loses nothing worth seeing. A 4 TB drive shows as about 3,725 'GB', which looks like a scandal and is a division.

The eight-times trap

Storage is bytes; network speed is BITS. A 100 Mbps line carries about 12.5 MB per second, so a 1 GB download needs at least 80 seconds at full speed. Lowercase b is bits, uppercase B is bytes — a single character worth a factor of eight.