🔢 Paper 1 · Topic 2: Data Representation
2.1e Data Units: Bits, Bytes & Beyond
Edexcel 1CP2 · GCSE Computer Science · ~12 min read · 🔒 Pro
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Bits and Bytes

A bit (binary digit) is the smallest unit of data — it is either 0 or 1. Bits are grouped together to form larger units of storage:

  • Nibble = 4 bits
  • Byte = 8 bits

Larger Units: SI vs Binary Prefixes

Edexcel 1CP2 uses the binary (IEC) prefixes, which are based on powers of 2. These differ from the SI metric prefixes (powers of 10) used in advertising and hard drive manufacturers.

UnitSymbolValueBytes
KibibyteKiB2¹⁰ bytes1,024
MebibyteMiB2²⁰ bytes1,048,576
GibibyteGiB2³⁰ bytes1,073,741,824
TebibyteTiB2⁴⁰ bytes1,099,511,627,776

Why powers of 2?

Computers work in binary, so storage is naturally structured around powers of 2. Each doubling adds one bit to the address space.

Converting Between Units

To convert, multiply or divide by 1024 for each step:

  • Bytes → KiB: ÷ 1024
  • KiB → MiB: ÷ 1024
  • MiB → GiB: ÷ 1024
  • Going up: ÷ 1024 per step
  • Going down: × 1024 per step

Example conversions:

  • 4096 bytes = 4096 ÷ 1024 = 4 KiB
  • 2 MiB = 2 × 1024 × 1024 = 2,097,152 bytes
  • 1 GiB = 1 × 1024 × 1024 × 1024 = 1,073,741,824 bytes

Calculating File Sizes

Knowing units allows you to calculate file sizes and storage requirements:

  • Text: Each character = 1 byte (ASCII) or 2 bytes (Unicode)
  • Bitmap image: width × height × colour depth (bits) ÷ 8 = bytes
  • Sound: sample rate × bit depth × duration (seconds) ÷ 8 = bytes
Exam tip: Edexcel 1CP2 uses kibibytes (KiB = 1024 bytes), mebibytes (MiB = 1024 KiB), and gibibytes (GiB = 1024 MiB). Make sure you know each unit symbol and value. You may be asked to convert between units or calculate storage requirements for images/sound.
⚠️ Common Mistakes
  • Confusing KB (kilobyte = 1000 bytes) with KiB (kibibyte = 1024 bytes) — Edexcel uses KiB
  • Forgetting that 1 nibble = 4 bits, not 8
  • Dividing by 1000 instead of 1024 when converting
  • Mixing up MiB and GiB — always move one level at a time (×1024 per step)
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Worksheet — 2.1e Data Units

8 Edexcel-style questions · AI-marked

Q1How many bits are in a nibble and how many in a byte?[2]
✅ Mark scheme
Nibble = 4 bits [1]; Byte = 8 bits [1].
Q2State the value of 1 KiB, 1 MiB, and 1 GiB in bytes.[3]
✅ Mark scheme
1 KiB = 1024 bytes [1]; 1 MiB = 1,048,576 bytes [1]; 1 GiB = 1,073,741,824 bytes [1].
Q3Convert 8192 bytes to KiB.[2]
✅ Mark scheme
8192 ÷ 1024 = 8 [1]; 8 KiB [1].
Q4Convert 3 MiB to bytes. Show your working.[2]
✅ Mark scheme
3 × 1024 × 1024 [1]; = 3,145,728 bytes [1].
Q5Explain why computer storage uses powers of 2 rather than powers of 10.[2]
✅ Mark scheme
Computers work in binary [1]; storage is naturally organized in powers of 2 as each bit doubles the addressable space [1].
Q6A bitmap image is 800×600 pixels with a colour depth of 24 bits. Calculate the file size in KiB.[3]
✅ Mark scheme
800×600×24 = 11,520,000 bits [1]; ÷8 = 1,440,000 bytes [1]; ÷1024 = 1406.25 KiB [1].
Q7A sound file is recorded at 44,100 Hz, 16-bit depth, for 60 seconds (stereo). Calculate the file size in MiB.[3]
✅ Mark scheme
44100×16×60×2 = 84,672,000 bits [1]; ÷8 = 10,584,000 bytes [1]; ÷1024÷1024 ≈ 10.09 MiB [1].
Q8State the difference between a kilobyte (KB) and a kibibyte (KiB).[2]
✅ Mark scheme
A kilobyte (KB) = 1000 bytes (SI/decimal) [1]; a kibibyte (KiB) = 1024 bytes (binary) [1].
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Mini Test — Data Units

Timed exam-style test.

← 2.1d Binary ArithmeticTopic 2Next: 2.1f Encoding →
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