🔢 Component 1 · 1.2 Memory & Storage
1.2.3 Units of Storage
OCR J277 · GCSE Computer Science · ~10 min read
Notes
──
Video
──
Slides
──
Worksheet
──
Quiz

The Bit — The Smallest Unit

All digital data is stored and processed as binary — sequences of 0s and 1s. The smallest unit of data is a bit (binary digit). A single bit can hold either 0 or 1.

The Nibble and Byte

  • A nibble = 4 bits
  • A byte = 8 bits

A byte is the fundamental unit used to measure file sizes and storage capacity. One byte can represent 2⁸ = 256 different values (0–255).

Storage Units — The Full Scale

UnitSymbolValueApproximate real-world size
Bitb1 binary digit (0 or 1)—
Nibble—4 bitsOne hex digit
ByteB8 bitsOne character of text
KilobyteKB1,024 bytesA short text document
MegabyteMB1,024 KB = 1,048,576 bytesA 3–4 minute MP3 song (~4 MB)
GigabyteGB1,024 MBA 2-hour film (~1–4 GB); phone RAM
TerabyteTB1,024 GBA large hard drive
PetabytePB1,024 TBLarge data centres
Exam tip: OCR J277 uses the binary prefix system (1 KB = 1,024 bytes, not 1,000). Always use 1,024 in calculations unless told otherwise. A common calculation: "Convert 5 GB to MB: 5 × 1,024 = 5,120 MB."

Key Conversion Rules

  • ×1,024 when converting down (e.g. GB → MB: multiply by 1,024)
  • ÷1,024 when converting up (e.g. MB → GB: divide by 1,024)
  • 1 KB = 1,024 B; 1 MB = 1,024 KB; 1 GB = 1,024 MB; 1 TB = 1,024 GB

Worked Conversions

QuestionWorkingAnswer
Convert 3 GB to MB3 × 1,0243,072 MB
Convert 2,048 MB to GB2,048 ÷ 1,0242 GB
Convert 4 KB to bytes4 × 1,0244,096 bytes
Convert 1 TB to GB1 × 1,0241,024 GB
How many bits in 1 byte?—8 bits
How many bytes in 4 KB?4 × 1,0244,096 bytes

Why Binary Uses Powers of 2

Computers use binary (base 2), so storage units are based on powers of 2:

  • 2¹⁰ = 1,024 ≈ 1,000 → this is why 1 KB ≈ 1,000 bytes but exactly 1,024 bytes
  • 2²⁰ = 1,048,576 ≈ 1 million → 1 MB
  • 2³⁰ ≈ 1 billion → 1 GB
  • 2⁴⁰ ≈ 1 trillion → 1 TB

File Size Examples

File typeTypical sizeUnit
Text document (1 page)~5–20KB
A single high-res photo~3–8MB
MP3 song (3 min)~3–5MB
HD film (2 hours)~1–4GB
Smartphone storage64–512GB
Desktop HDD1–10TB
⚠️ Common Mistakes
  • Using 1,000 instead of 1,024 — always use 1,024 in OCR J277 calculations
  • Confusing bits (b) and bytes (B) — 1 byte = 8 bits; a lowercase b usually means bits
  • Forgetting the nibble is 4 bits (half a byte)
  • Going the wrong direction in conversions — to go to a bigger unit, divide; to go to smaller, multiply
  • Forgetting the order: Bit → Nibble → Byte → KB → MB → GB → TB → PB
✅ Notes completed!
▶
Video coming soon

What's in this video

  • • Bit, nibble, byte — definitions and sizes
  • • The full scale from KB to PB with 1,024 multipliers
  • • Worked conversion examples
  • • Why computers use powers of 2
Click slide or press arrow keys to navigate

Worksheet — 1.2.3 Units of Storage

8 questions · 17 marks · Submit to reveal mark schemes

Q1State how many bits are in: (a) a nibble (b) a byte.[2]
✅ Mark scheme
(a) 4 bits [1]; (b) 8 bits [1].
Q2Convert 6 GB to MB. Show your working.[2]
✅ Mark scheme
6 × 1,024 [1] = 6,144 MB [1].
Q3Convert 3,072 MB to GB. Show your working.[2]
✅ Mark scheme
3,072 ÷ 1,024 [1] = 3 GB [1].
Q4Convert 2 TB to GB.[1]
✅ Mark scheme
2 × 1,024 = 2,048 GB [1].
Q5A storage device holds 512 GB. Convert this to TB.[2]
✅ Mark scheme
512 ÷ 1,024 [1] = 0.5 TB [1].
Q6List the storage units in order from smallest to largest: Gigabyte, Byte, Terabyte, Kilobyte, Bit, Megabyte, Petabyte, Nibble.[3]
✅ Mark scheme
Bit → Nibble → Byte → Kilobyte → Megabyte → Gigabyte → Terabyte → Petabyte. [1 mark per two consecutive pairs correct, up to 3]
Q7A photo has a file size of 4 MB. How many bytes is this?[2]
✅ Mark scheme
4 × 1,024 × 1,024 [1] = 4,194,304 bytes [1]. (Accept 4 × 1,024 KB = 4,096 KB; 4,096 × 1,024 = 4,194,304 bytes.)
Q8Explain why 1 KB = 1,024 bytes rather than 1,000 bytes.[3]
✅ Mark scheme
• Computers use binary (base 2) [1]
• 2¹⁰ = 1,024 — the closest power of 2 to 1,000 [1]
• Because all computer storage is based on powers of 2, not powers of 10 [1]
?
out of 17 — self-mark above
Topic Quiz
Question 1 of 15
You scored
out of 15
🎯

Mini Test — 1.2.3 Units of Storage

10 questions · 10 marks · 10 minutes

  • • 5 multiple choice + 5 short answer / calculation
  • • Mark schemes shown after submission
Card 1 of 15
Click to reveal definition
🎉
Session complete!
TermDefinition
← 1.2.2 Secondary Storage 1.2 Memory & Storage 1.2.4a Binary & Denary →
🔒
Unlock Everything
Subscribe to unlock all 48 OCR J277 lessons, worksheets, quizzes and key terms.
£7.99/month
or £59/year
Subscribe now →