S2 ICT | Image and Media File Size — Lesson-Prep Guide for Tutors | Pixels, Colour Depth, 1024 Units and Compression

2026-11-11

Tutor lesson-prep guides · Secondary ICT

Students say a picture is big “because it is high quality” but cannot work out that a 1024 by 768 image at 24-bit colour is 2.25 MB. One formula and one division turn a feeling into a checkable number.

Objectives — Students can: (1) compute the size of a 1024 by 768, 24-bit bitmap as 2,359,296 bytes, which is 2,304 KB or 2.25 MB; (2) explain that bitmaps pixelate when enlarged while vectors stay sharp, with one use for each; (3) work out that three minutes of 44,100 Hz, 16-bit stereo audio is about 31.75 MB and say why it must be compressed.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Multiplication and units	Roughly what is 1024 times 768?	786,432, about 786 thousand
Bits and bytes	How many bits are in a byte?	Eight, so divide bits by eight to get bytes
Percentages	Ten percent means what fraction of the original?	One tenth, so the file keeps ten percent

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-5 min	Two files	Write “same photo: PNG 3 MB, JPG 300 KB” and ask why the ten-fold gap.	Most say “different formats” without mentioning compression
5-14 min	Pixels and depth	Write 1024 by 768 and 24-bit colour; explain that each pixel takes three bytes.	Say that 24 divided by 8 is 3
14-25 min	Compute once	Multiply 786,432 by 3, divide by 1024 twice to reach 2.25 MB.	Reach 2,304 KB themselves
25-33 min	Bitmap and vector	Draw both enlarged ten times; show the blocks and the smooth edges.	Give one use for each
33-40 min	Audio and compression	Compute 44,100 times 16 times 2, then times 180 seconds; compare with JPEG ratios.	Reach 31.75 MB and a compressed size
40-45 min	Exit check	Collect three answers and mark them on the spot.	Hand in three answers

03 Teaching script (read it out)

Scene 1: Act 1: A picture is a pile of pixels

“1024 times 768 is 786,432 pixels. Each pixel stores colour in 24 bits: eight each for red, green and blue. Twenty-four bits divided by eight is three bytes, so the image is 786,432 times 3, which is 2,359,296 bytes. Divide by 1024 for kilobytes, 2,304, and by 1024 again for megabytes, 2.25. The formula has only three factors: width, height and colour depth. Drop the depth from 24 bits to 8 and the file becomes a third, because each pixel then takes one byte. Write all three intermediate numbers; an answer of 2.25 MB alone earns no method marks.”

Width times height times depth divided by eight, then divide by 1024 twice.

Scene 2: Act 2: 1024 is not 1000

“Computers work in binary, so a kilobyte is 1024 bytes and a megabyte is 1024 kilobytes. Divide 2,359,296 by 1000 twice and you get 2.36 MB instead of the correct 2.25 MB. On a small file the difference looks trivial; on a 4 GB video it is hundreds of megabytes. Storage and memory questions always use 1024. Drive makers label in 1000, which is why a 1 TB drive shows about 931 GB in the computer, and that mismatch is itself examinable. State which base you used and the marker cannot count it wrong.”

Capacity uses 1024; the 1000 printed on the drive box is a separate convention.

Scene 3: Act 3: Audio and the compression trade-off

“CD quality samples 44,100 times a second at 16 bits per sample across two channels, so each second is 44,100 times 16 times 2, which is 1,411,200 bits, or 1,411.2 kilobits per second. Three minutes is 180 seconds, giving 31,752,000 bytes, about 30.28 MiB or 31.75 MB. One song fills thirty-odd megabytes, so a CD holds only a dozen. Hence compression: MP3 discards frequencies the ear barely registers, JPEG discards high-frequency detail the eye barely sees. Compression is lossy; the harder you squeeze, the smaller the file and the more detail is gone. Write the size and the quality together to show the trade-off.”

1,411.2 kbps means about 31.75 MB for three minutes, traded against quality.

04 Question types and worked steps

Type	Example	Steps	Answer
File size calculation	A 1024 by 768 bitmap at 24-bit colour, uncompressed: how many MB?	(1) Multiply width by height (2) Multiply by depth and divide by 8 (3) Divide by 1024 twice	786,432 times 24 divided by 8 equals 2,359,296 bytes, which is 2,304 KB or 2.25 MB
Unit conversion	A 4 GB video in 1024-based units: how many MB?	(1) Confirm 1 GB is 1024 MB (2) Multiply (3) State the binary base	4 times 1024 equals 4,096 MB
Bitmap or vector	A company needs to print a huge banner. Which format, and why?	(1) Describe behaviour when enlarged (2) Describe file characteristics (3) Match to the use	Vector, because it stays sharp and the file is small; a bitmap would pixelate
Audio calculation	44,100 Hz, 16-bit, stereo, 30 seconds of uncompressed audio: how many MB?	(1) Compute bits per second (2) Multiply by seconds (3) Divide by 8 (4) Divide by 1024 twice	44,100 times 16 times 2 is 1,411,200 bits per second; times 30 divided by 8 is about 5.29 MB

Sanity check (teach this): Sanity check: 1024 times 768 is 786,432, not 78,643; times 3 is 2,359,296 bytes, not 235,929; dividing by 1024 twice gives 2.25 MB while dividing by 1000 gives 2.36 MB; three minutes of audio is 31.75 MB, while a normal song file is 3 to 10 MB and the difference is compression. An image answer in the hundreds of megabytes, or audio in kilobytes, means the divide-by-eight or the times-two channels step was dropped.

05 Independent practice (3 questions)

Question	Answer	What to watch
An 800 by 600 image at 8-bit colour: how many KB uncompressed?	480,000 bytes, about 468.75 KB	Remembered that 8 bits is one byte
Why do vector images not pixelate when enlarged?	They store mathematical descriptions of lines and fills, not per-pixel colours	Says “not pixels” rather than “better quality”
The same song as WAV and MP3: which is bigger and why?	WAV; MP3 uses lossy compression that discards some frequencies	Distinguishes lossy from lossless

06 Common mistakes and what to say

Mistake	Student writes	Say this
Skipping the divide by eight	“2,359,296 bits, so about 2.3 MB.”	“Bits must be divided by eight to become bytes. Skip that step and your file is eight times too big.”
Using 1000 for conversion	“2,359,296 divided by 1000 twice is 2.36 MB.”	“Computers use 1024. Two divisions by 1024 give 2.25 MB. This half-step loses the most marks in capacity questions.”
Forgetting the channels	“44,100 times 16 is the size per second.”	“Stereo has two channels, so multiply by two as well. What you computed is mono, and the question says stereo.”
Treating dpi as pixel dimensions	“300 dpi means 300 by 300 pixels.”	“dpi is print density, not a pixel count. Pixel dimensions come from width and height; dpi only converts to a printed size.”

07 Homework (paper, section and question numbers)

Recompute the 1024 by 768 24-bit image and write all three intermediate values — Image chapter, practice 1
Compare the same image saved at 8-bit and 24-bit colour — Image chapter, practice 2
Work out how a 4 GB video displays in 1024-based units — Image chapter, practice 3 data row

08 Exit check (last 5 minutes)

Question	Answer	Decision
How many MB is a 1024 by 768, 24-bit uncompressed image?	2.25 MB (2,359,296 bytes)	Wrong: reteach Act 1
How many bytes are in a kilobyte?	1024	Wrong: reteach Act 2
How much bigger is stereo than mono?	Twice, because there are two channels	Wrong: reteach Act 3

09 Tutor FAQ

Must students memorise 44,100?

Yes, together with 16 bits and two channels as “1,411.2 kbps per second”, which is faster than re-deriving it.

Why teach image and audio together?

They share one pattern, rate times depth times seconds or channels, so the transfer is visible.

Is compression always bad?

No. Web pages and streaming depend on it; the skill is naming the method, what is lost and how much space is gained.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.