Know that the size of a bitmap image is measured in pixels (width x height).

The idea

A bitmap is a grid of pixels, and its size is described by the number of pixels across (width) and the number down (height), written as width × height. An image of 800 × 600 has 800 pixels per row and 600 rows, so it contains 800 × 600 = 480 000 pixels. This pixel count is the image resolution and is separate from colour depth, which is the bits per pixel. Total storage in bits is pixels × colour depth; for 480 000 pixels at 8 bits each that is 3 840 000 bits, which is 480 000 bytes. To answer, read the two dimensions, multiply them for the pixel count, and keep width and height in the correct order.

What earns the marks

• Bitmap size is measured as width × height in pixels.
• Width is the number of pixels across; height is the number of pixels down.
• Total pixels = width × height (for example 800 × 600 = 480 000 pixels).
• Pixel dimensions describe resolution and are distinct from colour depth.
• Total storage in bits = total pixels × colour depth.

Common mistakes and how to put them right

• Adding width and height instead of multiplying.

  Put it right: Total pixels = width × height.

• Confusing pixel dimensions with colour depth.

  Put it right: Dimensions give the pixel count; colour depth gives bits per pixel.

• Reversing width and height.

  Put it right: Width is horizontal (across), height is vertical (down).

Exam tips

• Multiply width by height to find the total number of pixels.
• Keep the width × height order when writing an image size.
• If storage is asked for, multiply total pixels by colour depth to get bits.