3.3.3 Units of Information — AQA 8525

Bits and Bytes

The smallest unit of data in computing is a bit (binary digit) — it holds a single 0 or 1. Bits are grouped together into larger units:

Unit	Abbreviation	Number of bits
Bit	b	1
Nibble	—	4
Byte	B	8

A byte can store 2⁸ = 256 different values (0–255).

The Units Ladder

Each step up the ladder multiplies by 1024 (which is 2¹⁰). This is because computers are binary — 1 KB = 2¹⁰ bytes = 1024 bytes.

Terabyte (TB)

= 1024 GB

↑ ×1024

Gigabyte (GB)

= 1024 MB

↑ ×1024

Megabyte (MB)

= 1024 KB

↑ ×1024

Kilobyte (KB)

= 1024 bytes

↑ ×1024

Byte (B)

= 8 bits

Converting Units

To convert down (e.g. GB → MB): multiply by 1024.
To convert up (e.g. MB → GB): divide by 1024.

Worked examples

Question	Working	Answer
2 GB in MB?	2 × 1024	2048 MB
3072 KB in MB?	3072 ÷ 1024	3 MB
0.5 TB in GB?	0.5 × 1024	512 GB
How many bits in 4 bytes?	4 × 8	32 bits

Data Quantities in Context

Unit	Typical real-world example
1 byte	One ASCII character (e.g. 'A')
~1 KB	A short plain text email
~1 MB	A small photo or one MP3 second
~1 GB	A feature film (compressed)
~1 TB	About 1000 feature films or 250,000 photos

Calculating File Size

For many file size calculations in the AQA spec, the formula is:

File size = number of items × size per item

For example, a text document with 5000 characters encoded in ASCII (1 byte per character) would be 5000 × 1 byte = 5000 bytes ≈ 4.9 KB.

Exam tip: In AQA exams, always use 1 KB = 1024 bytes (not 1000). Answers using 1000 may not gain marks. Prefixes: K=kilo, M=mega, G=giga, T=tera — each is ×1024 of the previous.

⚠️ Common Mistakes

Using 1 KB = 1000 bytes — always use 1024 in computing (binary context)
Confusing bits (b) and bytes (B) — a byte is 8 bits, not the same thing
Forgetting that converting up (to a larger unit) means dividing by 1024

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Key points

Bit = 1 binary digit; byte = 8 bits; nibble = 4 bits
1 KB = 1024 bytes; 1 MB = 1024 KB; 1 GB = 1024 MB; 1 TB = 1024 GB
Convert down: ×1024; convert up: ÷1024
File size = number of items × bits per item

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Worksheet — 3.3.3 Units of Information

8 questions · 14 marks

Q1How many bits are in one byte? How many bits are in a nibble?[2]

✅ Mark scheme

Mark scheme

8 bits in one byte [1]; 4 bits in a nibble [1].

Q2State the number of bytes in: 1 KB, 1 MB, 1 GB.[3]

✅ Mark scheme

Mark scheme

1 KB = 1024 bytes [1]; 1 MB = 1024 KB = 1024 × 1024 = 1,048,576 bytes [1]; 1 GB = 1024 MB [1].

Q3Convert 5 GB to megabytes.[1]

✅ Mark scheme

Mark scheme

5 × 1024 = 5120 MB [1].

Q4Convert 2048 KB to megabytes.[1]

✅ Mark scheme

Mark scheme

2048 ÷ 1024 = 2 MB [1].

Q5A text file contains 4000 characters. Each character is stored using 1 byte. Calculate the file size in kilobytes.[2]

✅ Mark scheme

Mark scheme

4000 × 1 = 4000 bytes [1]; 4000 ÷ 1024 ≈ 3.9 KB (accept 3.9 KB or 4000 B) [1].

Q6Explain why in computing 1 KB = 1024 bytes rather than 1000 bytes.[2]

✅ Mark scheme

Mark scheme

Computers work in binary (base 2) [1]; 2¹⁰ = 1024 is the nearest power of 2 to 1000, so kilo in computing means 1024 rather than the decimal 1000 [1].

Q7A storage device holds 2 TB. How many GB is this?[1]

✅ Mark scheme

Mark scheme

2 × 1024 = 2048 GB [1].

Q8A song file is 4.5 MB. A student claims it would take about 45 MB to store 10 songs. Is the student correct? Show your calculation.[2]

✅ Mark scheme

Mark scheme

10 × 4.5 = 45 MB [1]; yes, the student is correct [1].

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Bits and Bytes

The Units Ladder

Converting Units

Worked examples

Data Quantities in Context

Calculating File Size

Key points

Worksheet — 3.3.3 Units of Information

Mini Test — 3.3.3 Units of Information