GCSE · Computer Science · AQA · Spec 8525

Kilobyte (kB)

A kilometre is 1,000 metres. A kilogram is 1,000 grams. So what's a kilobyte? Exactly what you'd guess: 1,000 bytes.

One amount, three ways to say it

Drag the amount of data
0 kB1 kB2 kBdrag to change the amount →

filled share of the 2 kB bar: 1/8. Kilobytes 0.25 kB. Bytes 250 bytes. Bits 2,000 bits

Kilobytes0.25 kBBytes250 bytesBits2,000 bits

Each section of the bar is a quarter of a kilobyte. Drag along it and watch kilobytes, bytes and bits change together.

Watch out: The two steps are different sizes. Kilobytes to bytes is ×1,000, but bytes to bits is only ×8.

Bit or byte?

BitvsByte

The Symbol row is the one that catches people out.

Focus

What it is

Bit

A single binary digit: a 0 or a 1

Byte

A group of 8 bits, such as 01001101

The insight

A byte is made of bits, so one byte is always 8 bits.

Where it sits

Bit

The fundamental unit of information. Nothing is smaller.

Byte

The unit the prefixes build on: kilobyte, megabyte and so on

Symbol

Bit

lowercase b

Byte

capital B

With kilo in front

Bit

kb means kilobits: 1,000 bits

Byte

kB means kilobytes: 1,000 bytes

The data ladder

Put the units in size order

Amount of data each unit stands for

1 · Smallest6 · Largest

  1. Gigabyte (GB)

  2. Bit (b)

  3. Megabyte (MB)

  4. Terabyte (TB)

  5. Kilobyte (kB)

  6. Byte (B)

What do you think?

1,000 or 1,024?

Three students disagree about how big a kilobyte is.

Which one is closest to what you think right now?
How sure are you?

Your turn to choose the steps

Comparing amounts in different units

Which is more data: 3 kB or 20,000 bits?

  1. You can't compare kilobytes with bits directly, so change 3 kB into bits, the smaller unit.Going down to a smaller unit, the number gets bigger.
  2. missing step
Which line is step 2?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Eight bits make a byte. A thousand bytes make a kilobyte. That's the first big step up the ladder.

What you need to know

  • A bit (b) is a single 0 or 1: the fundamental unit of information.
  • A byte (B) is a group of 8 bits.
  • 1 kilobyte (kB) = 1,000 bytes = 8,000 bits.
  • Kilo, mega, giga and tera are decimal prefixes based on powers of 10, and 1 MB = 1,000 kB.

The big picture

A bit is a single 0 or 1, the smallest unit of information. Eight bits make a byte. A kilobyte (kB) is 1,000 bytes, which is 8,000 bits. Kilo is the same decimal prefix used in other SI units, so after the byte, every step up the ladder is ×1,000.

Key points

1Going down to a smaller unit, multiply: kB to bytes is ×1,000, bytes to bits is ×8.
2Going up to a bigger unit, divide: bits to bytes is ÷8, bytes to kB is ÷1,000.
3To compare two amounts, put them in the same unit first.
4Watch the case: kb is kilobits and kB is kilobytes, which is 8 times as much.
5Historically, kilobyte has often been used to mean 1,024 bytes (a power of 2), but as a decimal prefix kilo means 1,000.

Worked example

Problem

A file is 12,000 bits. How big is it in kilobytes?

⚠ Watch out

Dividing or multiplying by 1,024 instead of 1,000, or skipping the ×8 between bytes and bits. Kilo is 1,000, and bits to bytes is a separate step of 8.

🧠

Memory hook

Little b is the little thing (a bit). Big B is the bigger thing (a byte, 8 bits). After that, kilo means a thousand, just like in a kilometre.

✓

Check yourself

Without looking back: a file is 3.5 kB. How many bits is that? (Answer: 3.5 × 1,000 = 3,500 bytes, and 3,500 × 8 = 28,000 bits.)

Flashcards

(12)
What is a bit?
A single binary digit, 0 or 1. It is the fundamental unit of information.
How many bits make one byte?
8 bits.
In kb and kB, what do b and B stand for?
b is bit and B is byte. So kb is kilobits and kB is kilobytes, which is 8 times as much.
How many bytes are in 1 kB?
1,000 bytes.
How many bits are in 1 kB?
8,000 bits (1,000 bytes × 8).
What does the prefix kilo mean?
1,000. It is the same decimal SI prefix as in kilometre and kilogram.
Kilo, mega, giga and tera are based on powers of which number?
10. They are decimal prefixes.
How many kilobytes make 1 MB?
1,000 kilobytes.
Why might you see 1 kB written as 1,024 bytes?
Historically, kilobyte has often been used for a power of 2 (1,024 is 2¹⁰). As a decimal prefix, kilo means 1,000.
Converting between units: when do you multiply and when do you divide?
Multiply going down to a smaller unit (kB to bytes ×1,000, bytes to bits ×8). Divide going up to a bigger unit.
How do you compare two amounts of data written in different units?
Convert them into the same unit first, then compare the numbers.
Order these from smallest: MB, bit, kB, byte.
bit, byte, kB, MB.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

Learning with Lightbulb is opening soon

You can use this lesson now. Join the waitlist and we'll let you know when the full Lightbulb experience is ready.

Keep me posted

More AQA GCSE Computer Science topics

See the full AQA Computer Science curriculum →

How this lesson was checked. This AQA GCSE Computer Science (specification 8525)lesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 29 September 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.