Spec & Goals 3 min
AQA Spec 3.3.3 · Units of information
By the end of this lesson you can:
- State the size of a bit, nibble and byte.
- Order the larger units — kB, MB, GB, TB — and convert between them.
- Calculate how many values a given number of bits can represent.
Warm-Up 5 min
Last lesson you grouped 4 bits into a nibble to read hexadecimal. Now we name the bigger groups of bits too.
Quick starter
You learned a nibble is 4 bits. How many bits do you think are in a byte?
Reveal the answer
A byte is 8 bits — two nibbles. It is the basic unit computers use to store one character.
Key Concept — from bits to terabytes 14 min
Everything in a computer is stored as bits. We group bits into larger units so the numbers stay manageable.
The larger units
Above the byte, each unit is bigger than the last. AQA uses decimal prefixes.
| Unit | Symbol | Size |
|---|---|---|
| Bit | b | 1 binary digit (0 or 1) |
| Nibble | — | 4 bits |
| Byte | B | 8 bits |
| Kilobyte | kB | 1000 bytes |
| Megabyte | MB | 1000 kB |
| Gigabyte | GB | 1000 MB |
| Terabyte | TB | 1000 GB |
How many values can bits store?
Each extra bit doubles the number of patterns. With n bits you can represent 2ⁿ different values.
| Bits (n) | Calculation (2ⁿ) | Values |
|---|---|---|
| 1 | 2¹ | 2 |
| 2 | 2² | 4 |
| 4 | 2⁴ | 16 |
| 8 | 2⁸ | 256 |
Worked Example — three calculations 12 min
1. How many bits are in 2 bytes?
One byte is 8 bits. So 2 bytes = 2 × 8.
Answer: 16 bits.
2. Convert 3000 kB to MB
1 MB = 1000 kB, so going up a unit means dividing by 1000.
| Step | Working | Result |
|---|---|---|
| kB → MB | 3000 ÷ 1000 | 3 MB |
Answer: 3000 kB = 3 MB.
3. How many bits store N different values?
Find the smallest n where 2ⁿ ≥ N. Suppose a school stores one of N = 100 different house colours.
| Bits (n) | 2ⁿ | Enough for 100? |
|---|---|---|
| 6 | 64 | No (64 < 100) |
| 7 | 128 | Yes (128 ≥ 100) |
Answer: 7 bits, because 2⁷ = 128 is the first power of 2 that reaches 100.
Try It Yourself 12 min
Goal: State how many bits are in 4 bytes.
Hint: multiply the number of bytes by 8.
Goal: Convert 2 GB to MB.
Hint: going down a unit (GB → MB) means multiplying by 1000.
Goal: A canteen menu has 20 different dishes. Calculate the smallest number of bits needed to give each dish a unique code.
Hint: find the smallest n where 2ⁿ is 20 or more.
📝 Exam Practice 10 min
Answer the way the examiner expects — the command word and the marks tell you how much to write.
State how many bits are in one byte.
Mark scheme
- 8 (1).
Convert 5 MB to kB.
Mark scheme
- 5 × 1000 = 5000 kB (1).
State how many different values can be represented by 1 byte.
Mark scheme
- 256 (1) — because a byte is 8 bits and 2⁸ = 256.
Which unit is the largest?
A) kilobyte B) megabyte C) gigabyte D) terabyte
Mark scheme
- D — terabyte (1).
Recap & Key Terms 3 min
A bit is one binary digit; 4 bits make a nibble and 8 bits make a byte. Larger units (kB, MB, GB, TB) each multiply by 1000 in AQA. With n bits you can store 2ⁿ different values.
- Bit
- A single binary digit, 0 or 1 — the smallest unit of data.
- Nibble
- A group of 4 bits.
- Byte
- A group of 8 bits; stores one character.
- Kilobyte
- 1000 bytes (AQA decimal prefix).
- Megabyte
- 1000 kilobytes.
- Gigabyte
- 1000 megabytes.
- Terabyte
- 1000 gigabytes.
Homework 1 min
Task (≤ 15 min): Convert 4 GB to MB, and calculate the smallest number of bits needed to give a unique code to each of the 30 students in a class. Show your working.
Model answer
4 GB → MB: 4 × 1000 = 4000 MB.
30 students: 2⁴ = 16 (too few), 2⁵ = 32 (enough). So 5 bits.
Award marks for: correct GB → MB conversion (1), 2⁵ = 32 ≥ 30 shown (1), answer 5 bits (1).