AQA 8525 · Section 3.3 · 3.3.6

Representing
Images

Pixels · Bit Depth · Resolution · Colour Depth · File Size

CSZoneAQA GCSE Computer Science 8525

Pixels and Bitmaps

How Images Are Stored

A bitmap image is made up of a grid of tiny coloured squares called pixels. Each pixel is stored as a binary number representing its colour. The more pixels, the higher the quality — but also the larger the file.

4×4 bitmap (16 pixels)

White = 1
Black = 0

In 1-bit colour:
1 pixel = 1 bit

Colour Depth & Resolution

Quality vs File Size

COLOUR DEPTH

Number of bits used per pixel. More bits = more possible colours.

1-bit → 2 colours (black/white)
8-bit → 256 colours
24-bit → 16.7 million colours

RESOLUTION

Number of pixels per unit area (e.g. pixels per inch — PPI). Higher resolution = sharper image, more pixels to store, larger file.

Calculating File Size

Image File Size Formula

File size (bits) = Width × Height × Colour depth

Example: 800 × 600 image, 24-bit colour depth

File size = 800 × 600 × 24
= 480,000 × 24
= 11,520,000 bits
= 11,520,000 ÷ 8 bytes
= 1,440,000 bytes
= 1,440,000 ÷ 1024 KB
≈ 1406 KB ≈ 1.37 MB

Exam Practice

Have a go at this question

AQA-style question

An image is 400 × 300 pixels and uses an 8-bit colour depth. Calculate the file size in kilobytes (KB). Show your working.

3 marks

Bits = 400 × 300 × 8 = 960,000 bits [1]
Bytes = 960,000 ÷ 8 = 120,000 bytes [1]
KB = 120,000 ÷ 1,024 ≈ 117.2 KB [1]

Key Takeaways

What to Remember

Images are grids of pixels; each pixel stored as a binary number

Colour depth: bits per pixel → 2^n possible colours

File size = Width × Height × Colour depth (in bits)

Higher resolution/colour depth = better quality but larger file size