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Technologies K–10 · Years 7–8

Pixels, colour depth and image file size

Digital Technologies context (NSW Technology Mandatory 7–8, 2017, outgoing; the Technology 7–8 Syllabus (2023) Digital and communication technologies focus area has no content point on data representation); Digital Technologies: Knowledge and understanding, Data representation (ACARA v9)

Practical, model not builtLow risk

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

An uncompressed image stores an integer for every pixel, so its size is width × height × bits per pixel, plus a small header.

What you need

  • Computer with an image editor that saves BMP files (for example Microsoft Paint)
  • A photograph to edit
  • File Explorer (or equivalent) showing file size in exact bytes

How to do it

  1. Resize the image to exactly 1920 × 1080 pixels.
  2. Predict the size of a 24-bit colour copy: width × height × 3 bytes.
  3. Save it as a 24-bit BMP and read the exact size in bytes from the file properties.
  4. Save it as a 256-colour (8-bit) BMP; predict and check again, allowing for the colour table.
  5. Resize to 1921 × 1080, save as 24-bit BMP and explain the extra bytes.
  6. Save the 1920 × 1080 image as PNG and JPEG and compare their sizes with the BMP.

What you should see

The pixel data is 1920 × 1080 × 24 = 49,766,400 bits = 6,220,800 bytes (5.93 MiB). With the common 54-byte header (14-byte file header and 40-byte information header) the 24-bit BMP is 6,220,854 bytes. The 8-bit BMP stores one byte per pixel plus a 256-entry colour table of 1024 bytes: 2,074,678 bytes. At 1921 pixels wide each row of 5763 bytes is padded to a multiple of 4 bytes, so the file is 6,225,174 bytes. PNG and JPEG are far smaller because they compress. A different header size shows the editor wrote a newer header form. The learner knows it worked when the predicted and reported byte counts match.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

  • File size depends on what the picture shows (for an uncompressed BMP it depends only on size and colour depth).
  • Doubling width and height doubles the file size (it quadruples it).
  • JPEG keeps every pixel exactly (it throws some detail away to save space).

Safety card

Low riskLearners carry it out

Hazards

  • Copyright in downloaded images

Controls

  • Use the learner's own photos or images with a free licence

Note

No chemicals or heat.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

  • Technology Mandatory 7–8 Syllabus (2017), NESA. Outgoing: replaced by the Technology 7–8 Syllabus (2023) from 2026 and not available after December 2027. Code read from the official syllabus document (DOCX) on 2026-09-22.TE4-7DI
  • Australian Curriculum v9AC9TDI8K03

Sources

The pages the author read to write this activity.

  1. curriculum.nsw.edu.au/learning-areas/tas/technology-7-8-2023/outcomes
  2. www.nsw.gov.au/education-and-training/nesa/curriculum/tas/technology-mandatory-7-8-2017
  3. learn.microsoft.com/en-us/windows/win32/gdi/bitmap-storage
  4. www.digitaltechnologieshub.edu.au/plan-and-prepare/scope-and-sequence-f-10/years-7-8
  5. curriculum.nsw.edu.au/learning-areas/tas/technology-7-8-2023/content/stage-4/fa8e5fb98c

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