The size of an image is expressed directly as width of image in pixels by…

The idea

An image's size is stated as its width in pixels followed by its height in pixels, written in the form width x height. So an image described as 800 x 600 is 800 pixels wide and 600 pixels high. The first number is always the width (horizontal count) and the second is the height (vertical count); the order matters, so 600 x 800 describes a different, taller-than-wide image. To find the total number of pixels, multiply width by height: 800 x 600 = 480 000 pixels. This pixel count, combined with colour depth, is what lets you work out file size. When a question gives dimensions, read which number is width and which is height before calculating, and keep the width-first order when writing your answer.

What earns the marks

• The first value in the notation is the width of the image in pixels.
• The second value in the notation is the height of the image in pixels.
• The notation is written as width x height, with width stated first.
• The total number of pixels is found by multiplying the width by the height.
• The order of the two values matters: swapping them describes a different image shape.

Common mistakes and how to put them right

• Writing height before width.

  Put it right: The notation is width x height, so width comes first.

• Adding width and height instead of multiplying.

  Put it right: Total pixels = width x height, a product not a sum.

• Assuming 600 x 800 and 800 x 600 are the same image.

  Put it right: They describe different shapes because the order of width and height differs.

• Confusing pixel dimensions with physical size in centimetres or inches.

  Put it right: The notation gives counts of pixels, not physical measurements.

Exam tips

• Underline which number is width and which is height before you calculate anything.
• Show the multiplication explicitly, for example 800 x 600 = 480 000, so your method is clear.
• Check your answer keeps width first when you restate the image size.