Units of Information: Bits, Bytes and Beyond

All data stored and processed by a computer is ultimately represented in binary — using only two values: 0 and 1. A single binary digit is called a bit.

• A bit is the fundamental unit of information in computing
• A bit can hold exactly one of two values: 0 or 1
• The symbol for bit is a lowercase b

Computers use bits because electronic circuits can reliably represent two states: off (0) and on (1). All other data — numbers, text, images, sound — is built up from combinations of bits.

A byte is a group of exactly 8 bits. The byte is the standard unit used to measure the size of files, memory, and storage.

• The symbol for byte is an uppercase B
• One byte can represent 2⁸ = 256 different values (0 through 255)

Unit	Size	Symbol
Bit	1 binary digit	b
Byte	8 bits	B

The symbol matters in questions: b means bits, B means bytes. A connection speed measured in Mb/s (megabits per second) is very different from a file size measured in MB (megabytes).

Bytes alone become impractical for describing large amounts of data. Larger units are built using SI prefixes — the same prefixes used in science (kilograms, kilometres, etc.). All prefixes use powers of 10.

Prefix	Symbol	Value	Equal to
Kilo	k	1,000	1,000 bytes
Mega	M	1,000,000	1,000 kilobytes
Giga	G	1,000,000,000	1,000 megabytes
Tera	T	1,000,000,000,000	1,000 gigabytes

Written in full:

• 1 kB = 1,000 bytes
• 1 MB = 1,000 kB = 1,000,000 bytes
• 1 GB = 1,000 MB = 1,000,000,000 bytes
• 1 TB = 1,000 GB = 1,000,000,000,000 bytes

Historically, "kilobyte" was often used to mean 1,024 bytes (a power of 2), and you may encounter this in older resources or operating systems. AQA 8525 uses the SI definitions above (powers of 10). You do not need to know the terms kibi, mebi, or gibi.

To convert from a larger unit to a smaller unit, multiply by 1,000 for each step down. To convert from a smaller unit to a larger unit, divide by 1,000 for each step up.

Worked examples:

Convert 3.5 GB to MB: 3.5 × 1,000 = 3,500 MB

Convert 750 MB to GB: 750 ÷ 1,000 = 0.75 GB

Convert 2 TB to GB: 2 × 1,000 = 2,000 GB

Convert 500 kB to bytes: 500 × 1,000 = 500,000 bytes

Step-by-step conversion chart:

Reading left to right multiplies by 1,000 at each step; reading right to left divides by 1,000. Every step in either direction is exactly a multiplication or division by 1,000.

Understanding units helps evaluate storage and transmission requirements.

Typical file sizes:

Type of file	Approximate size
A plain text document (1 page)	~5 kB
An MP3 audio file (3-minute song)	~4 MB
A high-resolution photograph	~3–8 MB
A standard definition video (1 hour)	~700 MB
A high-definition video (1 hour)	~4–8 GB
A modern AAA video game	~50–100 GB

Worked example — storage capacity:

A USB drive holds 32 GB. How many 4 MB photographs can it store?

32 GB = 32 × 1,000 MB = 32,000 MB
32,000 MB ÷ 4 MB per photo = 8,000 photographs

Worked example — download time:

A file is 600 MB. An internet connection transfers data at 50 MB per second. How long does the download take?

600 MB ÷ 50 MB/s = 12 seconds

Exam questions may ask you to compare file sizes, calculate how many files fit on a storage device, or estimate download times. Always convert to the same unit before comparing or calculating.

The distinction between bits and bytes is used throughout data representation topics.

Images: File size = (width × height × colour depth) ÷ 8 bytes. The colour depth is measured in bits per pixel; dividing by 8 converts to bytes.

Sound: File size (bits) = sampling rate × sample resolution × duration in seconds. Sample resolution is measured in bits per sample.

Transmission speeds are measured in bits per second (b/s), not bytes per second. To find how long it takes to transmit a file:

1. Convert the file size to bits (multiply bytes by 8)
2. Divide by the transmission speed in bits per second

Worked example:

A file is 2 MB. Transmission speed is 4 Mb/s (megabits per second). How long does the transfer take?

2 MB = 2 × 1,000,000 bytes = 2,000,000 bytes
2,000,000 × 8 = 16,000,000 bits = 16 Mb
16 Mb ÷ 4 Mb/s = 4 seconds

1. Confusing bits and bytes

A bit (b) is a single binary digit. A byte (B) is 8 bits. Mixing them up changes calculations by a factor of 8. A byte is 8 bits, so converting bytes to bits means ×8, and converting bits to bytes means ÷8. Check the symbol — lowercase b means bits, uppercase B means bytes.

2. Using 1,024 instead of 1,000

AQA 8525 defines prefixes using powers of 10: 1 kB = 1,000 bytes. Using 1,024 (a power of 2) gives a different answer. Unless a question explicitly states otherwise, use 1,000 per step.

3. Forgetting to convert units before comparing or calculating

You cannot directly compare 750 MB with 0.8 GB without converting one to match the other. Always bring both values to the same unit first.

4. Confusing transmission speed units

Transmission speeds are almost always given in bits per second, not bytes per second. A 50 Mb/s connection transfers 50 megabits — not megabytes — every second. To find transfer time for a file measured in bytes, multiply the file size by 8 to get bits first.

5. Forgetting that 1 byte = 8 bits in file size calculations

When converting between image/sound file sizes (which use bits in the formula) and storage capacity (which is measured in bytes), the division or multiplication by 8 is essential and easily missed.