GCSE Computer Science Tool

Image File Size CalculatorWork out the storage size of an image step by step.

Calculate bitmap image file sizes using resolution, colour depth, and compression. Perfect for GCSE Computer Science exam questions.

Image Size Calculator

Example Calculations

Small monochrome icon (100×100 pixels, 1-bit colour)
100×100 pixels, 1-bit colour
Image file size: 1250 bytes (1.22 KB, 0.0012 MB)
VGA monitor screenshot (640×480 pixels, 24-bit colour)
640×480 pixels, 24-bit colour
Image file size: 921600 bytes (900.00 KB, 0.8789 MB)
HD photo (1920×1080 pixels, 24-bit colour)
1920×1080 pixels, 24-bit colour
Image file size: 6220800 bytes (6075.00 KB, 5.9326 MB)
HD photo with 50% JPEG compression
1920×1080 pixels, 24-bit colour, 50% compression
Image file size: 3110400 bytes (3037.50 KB, 2.9663 MB) with 50% compression

How to Calculate Image File Size

File Size Formula
Width × Height × Colour Depth ÷ 8

Resolution

Width × Height gives total pixels. Higher resolution = more pixels = larger file.

Colour Depth

Bits per pixel. More bits = more colours = larger file. 24-bit = 16.7 million colours.

Convert to Bytes

Divide by 8 to convert from bits to bytes. Computers store data in bytes.

What Is Colour Depth?

1

1-bit (monochrome)

Black and white only

Example: Old fax machines, simple line drawings

Colours: 2

8

8-bit (256 colours)

256 different colours

Example: Early computer graphics, GIF images

Colours: 256

16

16-bit (high colour)

65,536 colours

Example: Older digital cameras, some mobile graphics

Colours: 65,536

24

24-bit (true colour)

16.7 million colours

Example: Modern digital photos, web images

Colours: 16,777,216

32

32-bit (true colour + alpha)

16.7 million colours + transparency

Example: Modern graphics with transparency

Colours: 4,294,967,296

Lossless vs Lossy Compression

Lossless Compression

  • • No quality loss
  • • File can be perfectly restored
  • • Smaller reduction (10-50%)
  • • Examples: PNG, GIF, BMP
  • • Good for graphics with solid colours

Lossy Compression

  • • Some quality loss
  • • File cannot be perfectly restored
  • • Much smaller files (50-95% reduction)
  • • Examples: JPEG, MP3, MP4
  • • Good for photos and complex images

Frequently Asked Questions

Multiply the width by height to get total pixels, then multiply by colour depth (bits per pixel), then divide by 8 to convert bits to bytes.

24-bit colour means each pixel uses 24 bits (3 bytes) to store colour information. This gives 2^24 = 16,777,216 possible colours, which is 'true colour'.

Compression removes redundant data. Lossless compression (like PNG) reorganises data efficiently. Lossy compression (like JPEG) removes some detail that humans can't easily see.

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