Bit, Byte, KB, MB, GB, TB, PB

Bit

A bit is a binary digit, the smallest unit of data stored on a computer.

• A bit can hold only one of two values: 0 or 1, corresponding to the electrical values of off or on, respectively.
• Because bits are so small, you rarely work with information one bit at a time.
• Bits are usually assembled into a group of eight to form a byte.
• Everything in a computer is 0's and 1's.

Byte

A byte contains enough information to store a single ASCII character, like "A".

• 1 byte = 8 bits
• e.g.: 0 1 0 1 1 0 1 0
• One byte can store one character, e.g. 'A' or 'x' or '$'
• All storage is measured in bytes, despite being very different hardware
• Bytes and Characters - ASCII Code
• ASCII is an encoding representing each typed character by a number
• Each letter is stored in a byte, as below

• S is 83
• p is 112
• r is 114
• is is 105
• 100 typed letters take up 100 bytes
• One byte works well for individual characters, but computers are also good at manipulating numbers.
• Integers are typically stored with either 4 or 8 bytes

Kilobyte or KB

• Decimal: 1000 Bytes = 1 Kilobyte (KB) = \(10^3\)
• Binary: 1024 Bytes = 1 Kibibyte (KiB) = \(2^{10}\)
• Example: An email without images is about 2 KB

Megabyte or MB

• Decimal: 1000² Bytes = 1000 KB = 1 Megabyte (MB) = 1,000,000 Bytes = \(10^6\)
• Binary: 1024² Bytes = 1024 KB = 1 Mebibyte (MiB) = 1,048,576 Bytes = \(2^{20}\)
• Example: A high-quality digital picture is about 2-5 Megabytes.

Gigabyte or GB

• Decimal: 1000³ Bytes = 1000 MB = 1 Gigabyte (GB) = 1,000,000,000 Bytes = \(10^9\)
• Binary: 1024³ Bytes = 1024 MB = 1 Gibibyte (GiB) = 1,073,741,824 Bytes = = \(2^{30}\)
• Example: A hard drive might hold 750 GB.

Terabyte or TB

• Decimal: 1000⁴ Bytes = 1000 GB = 1 Terabyte (TB) = 1,000,000,000,000 Bytes = \(10^{12}\)
• Binary: 1024⁴ Bytes = 1024 GB = 1 Tebibyte (TiB) = 1,099,511,627,776 Bytes = \(2^{40}\)
• Example: You can buy 4 TB hard drives today.

Decimal vs. Binary Storage Units

Decimal (SI system: International System of Units)

• 1 Kilobyte (KB) = 1,000 bytes
• Based on powers of 10.
• Used by most storage device manufacturers (hard drives, SSDs, USBs).
• Example: A "500 GB" hard drive = \(500 \times 10^9\) bytes.

Here, kilo means 1,000, just like in kilometers (1 km = 1,000 meters).

Binary (IEC system: International Electrotechnical Commission)

• 1 Kibibyte (KiB) = 1,024 bytes
• Based on powers of 2.
• Used by most operating systems (Windows, Linux, macOS) when reporting file sizes.
• Example: 1 MiB (mebibyte) = 1,024 KiB = 1,048,576 bytes.

Here, "kibi" comes from kilo + binary, meaning \(2^{10}\).

Why the difference matters

If you buy a 1 TB hard drive, the manufacturer means 1 terabyte = 1,000,000,000,000 bytes (\(10^{12}\)).

But your computer may report it as about 931 GiB, because it divides by \(1,024^3\) instead of \(1,000^3\).

So the difference is in the base used:

• KB, MB, GB → Decimal (\(10^3, 10^6, 10^9\))
• KiB, MiB, GiB → Binary (\( 2^{10}, 2^{20}, 2^{30} \))

Quick Reference Table

Unit	Decimal (SI)	Binary (IEC)	Approximate Size
Bit	-	-	Single 0 or 1
Byte	8 bits	8 bits	One character
Kilobyte/Kibibyte	\( 10^3 \) (1,000)	\( 2^{10} \) (1,024)	Email without images
Megabyte/Mebibyte	\( 10^6 \) (1 million)	\( 2^{20} \) (1,048,576)	High-quality photo
Gigabyte/Gibibyte	\( 10^9 \) (1 billion)	\( 2^{30} \) (1,073,741,824)	Hard drive capacity
Terabyte/Tebibyte	\( 10^{12} \) (1 trillion)	\( 2^{40} \) (1,099,511,627,776)	Large hard drive
Petabyte/Pebibyte	\( 10^{15} \) (1 quadrillion)	\( 2^{50} \) (1,125,899,906,842,624)	Data center storage

Etymology of Storage Unit Prefixes

Kilo (\(10^3\))

From Greek "khilioi" meaning "thousand"

Used in metric system since the 1790s

Mega (\(10^6\))

From Greek "megas" meaning "great" or "large"

Also means "million" in Greek

Giga (\(10^9\))

From Greek "gigas" meaning "giant"

Pronounced "GIG-a" or "JIG-a"

Tera (\(10^{12}\))

From Greek "teras" meaning "monster" or "marvel"

Suggests something enormous

Peta (\(10^{15}\))

From Greek "penta" meaning "five"

Represents the 5th power of 1000 (1000⁵)

Exa (\(10^{18}\))

From Greek "hex" meaning "six"

Represents the 6th power of 1000 (\(1000^6\))

The "h" was dropped to avoid confusion with "hexa"

Reference

https://web.stanford.edu/class/cs101/bits-bytes.html

https://en.wikipedia.org/wiki/Kilobyte#:~:text=In%20the%20International%20System%20of,The%20unit%20symbol%20is%20kB.