Representing images: Size = (bytes) = (W x H x D)/8

The idea

File size in bytes is found by first calculating the total bits as W × H × D, then dividing by 8 because one byte contains 8 bits. W is width in pixels, H is height in pixels and D is colour depth in bits per pixel. Example: a 4 × 3 image at 2-bit depth has 4 × 3 × 2 = 24 bits, and 24 ÷ 8 = 3 bytes. The division by 8 is a unit conversion, not a change to the image. If the result is not a whole number, keep the decimal or round as the question directs. For larger images you may then convert bytes to kilobytes or megabytes, but only if asked. Always confirm whether the required answer is in bits or bytes before dividing.

What earns the marks

• Calculate total bits as W × H × D using width, height and colour depth.
• Divide the bit total by 8 to convert bits into bytes.
• Explain that 8 bits equal 1 byte, which justifies the division.
• Worked example: 4 × 3 × 2 = 24 bits, then 24 ÷ 8 = 3 bytes.
• Keep the answer in bytes unless a further conversion to kB or MB is requested.

Common mistakes and how to put them right

• Dividing by 8 before multiplying by D.

  Put it right: The depth factor must be included in the bit total first, then converted.

• Multiplying by 8 instead of dividing.

  Put it right: Converting bits to bytes requires division because a byte is larger than a bit.

• Reporting bits when bytes were requested.

  Put it right: The final unit must match the question, so complete the division by 8.

Exam tips

• Show the bit calculation and the division by 8 as separate steps to protect method marks.
• Label the final answer with its unit (bytes) to show the conversion was completed.
• Only convert to kB or MB if the question explicitly asks for those units.