Text, Sound and Images

The standard exam formula for the size of an uncompressed bitmap is:

file size (bits) = image width (pixels) × image height (pixels) × colour depth (bits per pixel)

To convert to bytes, divide by 8.

Example — A bitmap image has a resolution of 800 × 600 pixels and a colour depth of 24 bits per pixel. Calculate the file size in bytes.

• Step 1: Total pixels = 800 × 600 = 480 000 pixels.
• Step 2: Apply the formula in bits: 480 000 × 24 = 11 520 000 bits.
• Step 3: Divide by 8 to convert to bytes: 11 520 000 ÷ 8 = 1 440 000 bytes (about 1.4 MB).

Example — An image is captured at 500 × 500 pixels with 24-bit colour depth, then re-captured at 1000 × 1000 pixels with the same colour depth. What happens to the file size?

• Original size in pixels: 500 × 500 = 250 000.
• New size in pixels: 1000 × 1000 = 1 000 000.
• The new image has four times as many pixels, so the file is four times larger.

The lesson: doubling each side multiplies the file size by 4 (because both width and height contribute to the pixel count).

Common exam answer: "The image has a higher quality / sharper image, and the file size is larger." Two marks if both halves are clear.

Example — An image of 1024 × 768 is stored first with 8-bit colour and then with 24-bit colour. Compare the two file sizes.

• 8-bit version: 1024 × 768 × 8 = 6 291 456 bits = 786 432 bytes.
• 24-bit version: 1024 × 768 × 24 = 18 874 368 bits = 2 359 296 bytes.
• The 24-bit version is 3 times the size of the 8-bit version, because the colour depth has tripled and resolution is unchanged.