Data Storage Units (Bit, Nibble, Byte, KB, MB, GB, TB, PB)

Understanding data storage units and being able to convert between them is a fundamental requirement for OCR J277 Section 2.6. This lesson covers all the units you need to know and how to convert between them.

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The Basic Units

Unit	Symbol	Size
Bit	b	The smallest unit of data — a single 0 or 1
Nibble	—	4 bits (half a byte)
Byte	B	8 bits

A bit is a single binary digit. A nibble is 4 bits (one hexadecimal digit). A byte is 8 bits (two hexadecimal digits) and is the standard unit of data storage.

OCR Exam Tip: Note the difference between lowercase 'b' (bit) and uppercase 'B' (byte). This distinction matters in technical specifications, especially for network speeds (measured in bits per second) vs storage (measured in bytes).

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Larger Units

Each subsequent unit is 1024 times the previous one (because 2^10 = 1024):

Unit	Symbol	Size in bytes	Power of 2
Kilobyte	KB	1,024 bytes	2^10
Megabyte	MB	1,048,576 bytes	2^20
Gigabyte	GB	1,073,741,824 bytes	2^30
Terabyte	TB	~1.1 trillion bytes	2^40
Petabyte	PB	~1.1 quadrillion bytes	2^50

Conversion Multipliers

From	To	Operation
Bits	Bytes	Divide by 8
Bytes	KB	Divide by 1024
KB	MB	Divide by 1024
MB	GB	Divide by 1024
GB	TB	Divide by 1024
TB	PB	Divide by 1024

To go in the opposite direction (e.g., KB to bytes), multiply by 1024 instead.

flowchart LR
    Bit[Bit - 1 binary digit] -->|x4| Nibble[Nibble - 4 bits]
    Nibble -->|x2| Byte[Byte - 8 bits]
    Byte -->|x1024| KB[Kilobyte - 1024 B]
    KB -->|x1024| MB[Megabyte - 1024 KB]
    MB -->|x1024| GB[Gigabyte - 1024 MB]
    GB -->|x1024| TB[Terabyte - 1024 GB]
    TB -->|x1024| PB[Petabyte - 1024 TB]

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Real-World Examples

Unit	Approximate real-world equivalent
1 bit	A single on/off switch
1 byte	A single ASCII character
1 KB	A short text message (~1,024 characters)
1 MB	A 1-minute MP3 audio file
1 GB	About 250 MP3 songs
1 TB	About 250,000 MP3 songs or 500 hours of video
1 PB	About 1,000 TB — used by large data centres

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Conversion Examples

Example 1: Convert 4,096 bytes to KB

4,096 / 1024 = 4 KB

Example 2: Convert 3 MB to bytes

3 x 1024 = 3,072 KB 3,072 x 1024 = 3,145,728 bytes

Or directly: 3 x 1024 x 1024 = 3 x 1,048,576 = 3,145,728 bytes

Example 3: Convert 2,400,000 bits to KB

2,400,000 / 8 = 300,000 bytes 300,000 / 1024 = 292.97 KB (approximately 293 KB)

OCR Exam Tip: When converting between units, always show each step of your working. Write "divide by 8" or "divide by 1024" at each step to make your method clear.

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Why 1024 and Not 1000?

Computers work in binary (powers of 2). The closest power of 2 to 1000 is 2^10 = 1024. This is why:

• 1 KB = 1024 bytes (not 1000)
• 1 MB = 1024 KB (not 1000)
• And so on

However, hard drive manufacturers often use 1000 instead of 1024, which is why a "500 GB" hard drive appears as about 465 GB in your operating system. The operating system uses 1024-based units, while the manufacturer uses 1000-based units.

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Data Capacity of Storage Devices

Device	Typical capacity
Floppy disk (historical)	1.44 MB
CD	700 MB
DVD	4.7 GB
Blu-ray disc	25-50 GB
USB flash drive	8-256 GB
SSD	128 GB - 4 TB
HDD	500 GB - 16 TB
Cloud storage	Virtually unlimited

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File Size in Context

File type	Typical size
Plain text email	1-5 KB
Web page (HTML only)	50-100 KB
JPEG photograph	2-5 MB
MP3 song (4 min)	3-5 MB
1080p video (1 min)	100-200 MB
4K video (1 min)	300-500 MB
Full video game	30-100 GB
Operating system	15-30 GB

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Practice

1. Convert 5 GB to MB: 5 x 1024 = 5,120 MB
2. Convert 16,384 bytes to KB: 16,384 / 1024 = 16 KB
3. Convert 48 bits to bytes: 48 / 8 = 6 bytes
4. How many bits in 2 KB? 2 x 1024 x 8 = 16,384 bits
5. A file is 3,072 KB. How many MB is this? 3,072 / 1024 = 3 MB

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Worked Example: How Many Songs Fit on a 64 GB USB Drive?

This is an exam-favourite style of question: calculating capacity using storage units, requiring careful chaining of divisions by 1024.

Question: A student has a 64 GB USB flash drive. Each MP3 song is approximately 4.5 MB. The flash drive also has 1.2 GB of firmware overhead that is unavailable to the user. How many complete songs can the student store on the drive?

Step 1: Find the usable capacity.

Total capacity: 64 GB. Overhead: 1.2 GB. Usable: 64 - 1.2 = 62.8 GB.

Step 2: Convert usable capacity to MB.

Each GB = 1024 MB. Usable in MB = 62.8 x 1024 = 64,307.2 MB.

Step 3: Divide by the size of each song.

64,307.2 / 4.5 = 14,290.48 songs (approximately).

Since we can only store complete songs, round down: the student can store 14,290 complete songs.

Step 4: Verify with a sanity check.