Skip to content
VirtusAcademy

Image Size Calculations

FoundationHigherAQA

Practise Image Size Calculations for GCSE Computer Science with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA GCSE Computer Science (8525). Image file size is found from the resolution multiplied by the colour depth in bits.

Free downloads

These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA specification. Every worksheet comes with a full mark scheme.

Foundation

WorksheetFoundation · PDFMark SchemeFoundation · PDF

Higher

WorksheetHigher · PDFMark SchemeHigher · PDF

Topic overview

Image file size is calculated as resolution multiplied by colour depth, giving the answer in bits.

The formula is file size = width × height × colour depth. The result is in bits, so dividing by 8 converts to bytes, and dividing by a further 1000 converts to kilobytes.

Forgetting the unit conversion is the most frequent error. A 200 by 100 image at 8 bits per pixel gives 200 × 100 × 8 = 160 000 bits, which is 20 000 bytes, which is 20 kilobytes. Metadata adds a small amount on top, which questions sometimes ask you to include — so read carefully whether the metadata size has been given.

Revision notes

The formula

File size = width × height × colour depth.

The result is in BITS. Divide by 8 for bytes, then by 1000 for kilobytes. State the unit at every stage so the conversion is visible.

Working through an example

A 200 by 100 image at 8 bits per pixel: 200 × 100 = 20 000 pixels.

20 000 × 8 = 160 000 bits. 160 000 ÷ 8 = 20 000 bytes. 20 000 ÷ 1000 = 20 kilobytes.

Including metadata

Metadata adds a small fixed amount to the file size.

If a question gives the metadata size, add it after converting the pixel data to the same unit. Adding bits to bytes without converting is a common slip.

Key points

  • File size = width × height × colour depth.
  • The result is in bits.
  • Divide by 8 for bytes.
  • Divide by 1000 for kilobytes.
  • State the unit at every stage.
  • Add metadata after converting to matching units.

Worked examples

Example 1

An image is 300 by 200 pixels with a colour depth of 8 bits. Calculate the file size in bits. [3 marks]

Working

300 × 200 = 60 000 pixelsfind the number of pixels
60 000 × 8multiply by the colour depth
= 480 000 bitswork out the file size

Example 2

Convert a file size of 160 000 bits into kilobytes. [3 marks]

Working

160 000 ÷ 8 = 20 000 bytesconvert bits to bytes
20 000 ÷ 1000convert bytes to kilobytes
= 20 kilobyteswork out the answer

Example 3

An image is 50 by 40 pixels at 4 bits per pixel. Calculate the file size in bytes. [3 marks]

Working

50 × 40 = 2000 pixelsfind the number of pixels
2000 × 4 = 8000 bitsmultiply by the colour depth
8000 ÷ 8 = 1000 bytesconvert bits to bytes

Common mistakes

  • Forgetting the answer is initially in bits.

    Divide by 8 to get bytes.

  • Dividing by 1000 before dividing by 8.

    Convert bits to bytes first, then bytes to kilobytes.

  • Adding metadata in the wrong unit.

    Convert both to the same unit before adding.

  • Multiplying width and height incorrectly.

    Work out the pixel count as a separate step.

Exam tips

  • Work out the pixel count as a separate step.
  • Remember the formula gives bits.
  • Convert bits to bytes to kilobytes in order.
  • Label the unit at every stage.

Key terms

File size
The storage an image requires.
Colour depth
Bits per pixel.
Resolution
The pixel dimensions of the image.
Conversion
Dividing by 8 for bytes, then 1000 for kilobytes.

Related topics

Written and reviewed against the current AQA specification. Spotted an error? Let us know.