Last updated: 2026-05-13

Data storage units

Data storage is measured in bytes and multiples thereof, but two competing prefix systems — decimal (powers of 10) and binary (powers of 2) — assign different values to names like "kilobyte" and "gigabyte". Storage hardware manufacturers adopted decimal prefixes while operating systems historically used binary arithmetic, producing a persistent labelling mismatch: a drive marketed as "500 GB" reports as about 465 GB in Windows, and a "128 GB" device holds fewer bytes than the decimal interpretation implies.

The International Electrotechnical Commission (IEC) resolved this in 1998 by defining distinct binary prefixes — kibi (Ki), mebi (Mi), gibi (Gi), and so on — so that 1 KiB unambiguously means 1 024 bytes and 1 kB unambiguously means 1 000 bytes.

Decimal (SI) prefixes

The SI system defines storage prefixes the same way it defines any physical quantity: by powers of 1 000.

Unit	Symbol	Bytes
Kilobyte	kB	10³ = 1 000
Megabyte	MB	10⁶ = 1 000 000
Gigabyte	GB	10⁹ = 1 000 000 000
Terabyte	TB	10¹² = 1 000 000 000 000
Petabyte	PB	10¹⁵ = 1 000 000 000 000 000

Hard drive and SSD manufacturers, USB drive makers, and memory card vendors all label capacity using decimal values. A "1 TB" drive contains exactly 1 000 000 000 000 bytes.

Binary (IEC) prefixes

Computer memory and operating system address spaces work in powers of 2. The IEC binary prefixes capture this exactly.

Unit	Symbol	Bytes
Kibibyte	KiB	2¹⁰ = 1 024
Mebibyte	MiB	2²⁰ = 1 048 576
Gibibyte	GiB	2³⁰ = 1 073 741 824
Tebibyte	TiB	2⁴⁰ = 1 099 511 627 776
Pebibyte	PiB	2⁵⁰ = 1 125 899 906 842 624

Windows, Linux, and macOS historically reported file sizes using binary math but labelled them "KB", "MB", "GB". Modern Linux and macOS have largely switched to SI labels; Windows still uses binary values with "GB" labels by default.

Bits and bytes

One byte equals 8 bits. Network speeds are conventionally quoted in bits per second (bps), while file sizes and storage capacities are measured in bytes.

• Broadband speeds are advertised in Mbps (megabits per second).
• Download progress is typically shown in MB/s (megabytes per second).
• To convert: divide Mbps by 8 to get MB/s.

A 100 Mbps connection can theoretically download at 12.5 MB/s; in practice, protocol overhead reduces this further.

Decimal vs. binary capacity reporting

The conversion factor compounds across prefix steps. At the gigabyte level, the difference is about 7 %; at the terabyte level it is about 9 %.

Advertised	Bytes	Shown in Windows (GiB)
256 GB SSD	256 000 000 000	≈ 238 GiB
1 TB HDD	1 000 000 000 000	≈ 931 GiB
4 TB HDD	4 000 000 000 000	≈ 3 725 GiB

The bytes are all there — the discrepancy is entirely a labelling convention, not missing data.

RAM capacity

RAM capacity is specified by manufacturers using binary quantities (1 GB of RAM = 1 073 741 824 bytes), because memory chips are addressed in powers of 2 by design. A 16 GB RAM module therefore holds 16 GiB — in contrast to a hard drive marketed as "16 GB", which contains only 16 000 000 000 bytes.

Frequently Asked Questions (FAQ)

What is the difference between kB and KiB?

1 kilobyte (kB) = 1 000 bytes (decimal, SI prefix). 1 kibibyte (KiB) = 1 024 bytes (binary, IEC prefix). The IEC introduced the "kibi-", "mebi-", and "gibi-" prefixes in 1998 to resolve the ambiguity. File system tools and operating systems often use the binary definition while labelling it "KB", which causes confusion.

Why does my hard drive show less space than advertised?

Drive manufacturers use decimal prefixes (1 GB = 1 000 000 000 bytes), while Windows reports capacity using binary units (1 GB = 1 073 741 824 bytes) but labels them "GB". A 1 TB drive contains 1 000 000 000 000 bytes, which Windows shows as roughly 931 GB.

What is the difference between a bit and a byte?

1 byte = 8 bits. Bits are used for network speeds (Mbps, Gbps), while bytes measure file sizes and storage. To estimate a download time, divide the file size in megabytes by the connection speed in megabits per second, then multiply by 8.

How many bytes are in a GiB?

1 GiB (gibibyte) = 2³⁰ bytes = 1 073 741 824 bytes exactly. This is the amount that most operating systems call "1 GB" when reporting RAM or storage.