Your journey through Image Representation
Pixels, resolution, colour depth, and the file-size formula. Seven stages from theory to mastery. Click any stage card to dive in — or continue where you left off.
Where you are in Topic 1.3B
Start with Book Notes
Read the theory and terminology to get grounded.
Choose any stage
Green cards are stages you've already visited. Grey cards are new territory. Click any card to jump straight to that stage.
Mistake Tracker
Notes
Stats
Bookmarks
Knowledge Vault
Add entry
Entries
Image Representation Lab — pixels to file size
How digital images are stored as pixels: colour depth, resolution, and the file-size formula Cambridge tests in almost every paper.
What is Topic 1.3B?
Digital images are made of pixels. Each pixel stores one colour. Colours are stored as binary numbers. The number of bits per pixel is the colour depth, and the total number of pixels is the resolution. Multiply them together and you get the file size in bits.
Cambridge test this in almost every paper — the definition of a pixel, the file size formula, and the effect of changing resolution or colour depth. Learn the definitions cold and the formula becomes trivial.
Note: text is covered in 1.3A Text Representation Lab and sound in 1.3C Sound Representation Lab. This lesson (1.3B) covers images only.
By the end of 1.3B you can…
- Define pixel as the smallest component of an image, storing a single colour
- Define resolution as the number of pixels in an image (usually width × height)
- Define colour depth as the number of bits used to represent each pixel's colour
- State the file size formula: width × height × colour depth (in bits)
- Convert bits to bytes by dividing by 8 (and to KiB / MiB when required — see 1.4)
- Explain the effect of increasing resolution (more detail, larger file)
- Explain the effect of increasing colour depth (more colours, larger file)
- Describe how a computer stores a digital image (bitmap of pixel values + metadata)
Cambridge-approved terms
| Term | Definition |
|---|---|
| Pixel | The smallest component of a digital image. Each pixel stores a single colour. |
| Resolution | The number of pixels in an image, typically given as width × height. |
| Colour depth | The number of bits used to represent the colour of each pixel. Also called bit depth. |
| Bitmap | An image stored as a grid of pixel values in a file, each pixel encoded in binary. |
| Metadata | Extra data stored alongside pixel values — width, height, colour depth, file format, etc. |
| File size (bits) | width × height × colour depth — the raw bit count of the pixel data. |
The three tables you must know cold
Colour depth → colours representable
| Colour depth (bits per pixel) | Colours representable | Common use |
|---|---|---|
| 1 | 2 (black/white) | Monochrome bitmap |
| 2 | 4 | Basic grayscale |
| 4 | 16 | Very simple graphics |
| 8 | 256 | Indexed colour images |
| 16 | 65,536 | High colour |
| 24 | ~16.7 million | True colour — standard for photos |
Rule: n-bit colour depth = 2ⁿ possible colours per pixel.
File size formula
| Quantity | Formula |
|---|---|
| File size in bits | width × height × colour depth |
| File size in bytes | (width × height × colour depth) ÷ 8 |
Effect of changing parameters
| Change | Effect on image quality | Effect on file size |
|---|---|---|
| Increase resolution 2× per side | More detail, sharper | ×4 (2 × 2) |
| Increase colour depth from 8 to 24 | Many more colours | ×3 (24/8) |
| Decrease colour depth from 24 to 8 | Fewer colours, may posterise | ÷3 |
Watch each calculation done step by step
Worked example 1 · File size of a small photo
An image is 100 pixels wide by 200 pixels tall with a colour depth of 24 bits per pixel. Calculate the file size in bytes.
Worked example 2 · Effect of doubling resolution
An image is 400 × 300 pixels at 24-bit colour. If BOTH dimensions are doubled to 800 × 600, what happens to the file size?
Worked example 3 · Effect of changing colour depth
An image uses 8-bit colour depth and has a file size of 200 KiB. If the colour depth is changed to 24 bits, what is the new file size?
Traps that cost marks every session
Trap · Pixel defined as "all the colours in the image" or "all the squares"
A pixel is a single component of an image, storing a single colour. Not "all the colours". Not "all the squares". The stronger Cambridge responses said: "the smallest component of an image" or "a single square that stored a single colour".
Cited: 2024 examiner report — verbatim: "Fewer candidates could define a pixel. Some responses describe a pixel as all of the colours in the image, or all of the squares that make up the image. The stronger responses commonly defined it as the smallest component of an image, and some responses accurately described it as a single square that stored a single colour."Trap · Colour depth = "the colours in the image"
Colour depth is the number of bits per pixel — not the number of colours actually used in the image. A 24-bit image CAN hold ~16.7 million colours but may only use a few. Always say "bits per pixel".
Cited: recurring patternTrap · Answer given in bits when the question asks for bytes (or KiB)
Read the question. Bytes = bits ÷ 8. KiB = bytes ÷ 1024. Wrong unit = zero marks even with correct arithmetic.
Cited: recurring exam-technique flagTrap · "Higher resolution improves quality" without saying HOW
Saying "improves quality" is vague. State the mechanism: more pixels means more detail / sharper edges / less pixelation. Same discipline for colour depth: "more bits per pixel means more distinct colours can be shown".
Cited: recurring "explain how" patternTrap · Confusing resolution with colour depth
Resolution = number of pixels (width × height). Colour depth = bits per pixel. They're independent quantities, and BOTH appear in the file size formula. Don't swap them.
Cited: recurring definition mix-upTrap · Forgetting metadata in the "how an image is stored" answer
Cambridge accept a 5–6 point answer: the image is stored as a bitmap — each pixel has a colour stored as binary, with metadata (width, height, colour depth) at the start of the file. Omit metadata and you lose the mark.
Cited: recurring "6-point storage" patternMark-scheme templates
These are the exact phrasings Cambridge accept. Learn them.
Check yourself — tap to reveal (0/6)
Answer these in your head, then tap to see the model answer. Aim for 5 or 6 right before moving on.
If you can answer 5 of the 6 knowledge checks above, you're ready to move on. Tap 🎓 Learn for the interactive content, then 🎮 Activities for hands-on drills.
How digital images work
Pixels
A digital image is made of pixels — the smallest component of an image. Each pixel is a single point of one colour.
Think of a mosaic: each tile is one colour. Stand back and you see a picture.
Resolution
The number of pixels per unit area (e.g. per inch) OR the total dimensions (width × height in pixels).
Higher resolution = more pixels = more detail = larger file.
Colour depth
The number of bits used to represent each pixel's colour.
1-bit = 2 colours, 8-bit = 256, 16-bit = 65,536, 24-bit = ~16.7 million.
How an image is stored — the 6-point answer
Cambridge typically awards 3 marks. Any 3 from these 6 points:
| # | Point |
|---|---|
| 1 | Made up of pixels |
| 2 | Each pixel stores one colour |
| 3 | Set number of pixels wide and high (resolution/dimensions) |
| 4 | Each colour has a unique binary value |
| 5 | Binary values stored in sequence |
| 6 | File contains metadata (colour depth, resolution, dimensions) |
File size calculation
Worked example
Second example (colour depth in bytes)
Interactive file size calculator
Enter width, height and colour depth to calculate the file size.
Effect of changes
↑ Resolution
More pixels → more binary values to store → larger file size. Also: more detail, higher quality.
↑ Colour depth
More bits per pixel → more possible colours → larger file size. Also: more accurate colours, higher quality.
Quick check
What is colour depth?
Activity 1 — File Size Challenge
Calculate the file size for the given image.
Press New Question.
Activity 2 — Spot the Mistake
Press New Question.
Activity 3 — Effect Predictor
What happens to the file size when this change is made?
Press New Question.
Adaptive practice
Press New Question.
Cambridge-style questions
Press New Question.
Review quiz
Press New.
Exam traps
Memory triggers
🧠 "W × H × D ÷ 8"
Width × Height × Depth (colour depth in bits) = bits. Divide by 8 = bytes. That's the whole formula.
🧠 "Mosaic tiles"
Each pixel is one mosaic tile of one colour. More tiles = more detail = bigger file.
🧠 "6 points + metadata"
Image storage: pixels, one colour each, dimensions, unique binary values, sequence, metadata. Hit any 3 for full marks.