This key facts covers Image File Size Calculation within Images & Sound for GCSE Computer Science. Revise Images & Sound in 3.3 Data Representation for GCSE Computer Science with 18 exam-style questions and 16 flashcards. This topic shows up very often in GCSE exams, so students should be able to explain it clearly, not just recognise the term. It is section 5 of 12 in this topic. Use this key facts to connect the idea to the wider topic before moving on to questions and flashcards.
Image File Size Calculation
Formula:
File Size (bits) = Width × Height × Color Depth File Size (bytes) = (Width × Height × Color Depth) ÷ 8
Example 1: Simple Calculation
Image: 800 × 600 pixels, 24-bit color depth Step 1: Calculate total pixels 800 × 600 = 480,000 pixels Step 2: Calculate bits 480,000 pixels × 24 bits = 11,520,000 bits Step 3: Convert to bytes 11,520,000 bits ÷ 8 = 1,440,000 bytes Step 4: Convert to MB (optional) 1,440,000 bytes ÷ 1,048,576 = 1.37 MB
Example 2: HD Photo
Image: 1920 × 1080 pixels, 24-bit color depth 1920 × 1080 = 2,073,600 pixels 2,073,600 × 24 = 49,766,400 bits 49,766,400 ÷ 8 = 6,220,800 bytes ≈ 6.2 MB
Example 3: Different Color Depths
Same image (640 × 480 pixels) at different color depths: 1-bit (B&W): 640 × 480 × 1 ÷ 8 = 38,400 bytes (37.5 KB) 8-bit (256 colors): 640 × 480 × 8 ÷ 8 = 307,200 bytes (300 KB) 24-bit (True Color): 640 × 480 × 24 ÷ 8 = 921,600 bytes (900 KB) 24-bit is 24× larger than 1-bit!
Worked method: converting units and reversing the formula
Convert bits to MB the exam way, then work backwards to a missing value.
- Convert bits to KB and MB the GCSE wayDivide bits by 8 to get bytes, then by 1000 for KB and by 1,000,000 for MB. AQA and OCR both use the decimal (SI) convention at GCSE — do NOT divide by 1024.
- Spot the 1024 trapDividing by 1024 instead of 1000 gives a smaller number that looks plausible but is marked wrong. If a question just says 'convert to MB', assume 1000 unless the paper states otherwise.
- Metadata is not in the formulaWidth × height × colour depth calculates only the raw pixel data. Saved files also store a header with metadata, so the real file on disk is slightly larger than your calculated answer. Mention this if asked to explain a size difference.
- Rearrange to work backwardsGiven a total size and asked for colour depth: colour depth = file size (bits) ÷ (width × height). Always convert the file size into bits first — this is where reverse-calculation marks are lost.
An uncompressed image is 2400 × 1800 pixels and the file is 2,160,000 bytes. (a) State the size in MB. (b) Calculate the colour depth.
(a) 2,160,000 ÷ 1,000,000 = 2.16 MB. (b) Convert to bits: 2,160,000 × 8 = 17,280,000 bits. colour depth = 17,280,000 ÷ (2400 × 1800) = 17,280,000 ÷ 4,320,000 = 4 bits per pixel.
Most marks are lost in (b) by forgetting to convert back into bits before dividing, or dividing by width only instead of width × height.
Practice questions for Images & Sound
What does colour depth refer to in a digital image?
Explain the effect of increasing colour depth on a digital image. Refer to both file size and image quality in your answer.