Monitors are unfortunately not all created equal. Some are physically larger than others, and some give a sharper and clearer image. Those that display a better-defined image probably do so because they have a smaller dot pitch or pixel resolution, which is the distance between the dots or pixels of the same colour that make up an image. This is known as build quality because these characteristics are determined by manufacture and cannot be changed. Dot pitch is a term traditionally applied to cathode ray tube (CRT) monitors, but is also sometimes applied to flat-panel monitors. The dot pitch for a flat panel monitor, more correctly known as the pixel resolution, is typically about 0.3mm, giving resolutions in the range 60 to 120 ppi. Larger and sharper monitors of course also tend to be more expensive. However, dot pitch and pixel resolution figures can be misleading. Some manufacturers quote horizontal dot pitch because it produces a lower figure than diagonal dot pitch (by a factor of 0.866). It is also important to realize that dot pitch is not the sole measure of image quality.

Monitors are measured diagonally, so a typical 17" flat panel will be about 13" x 10.5", and a typical 19" flat panel will measure about 15" x 12". According to these approximated dimensions for viewable areas, a 17" screen with a resolution of 100ppi might be expected to display about 1,300 x 1,050 dots, and a 19" screen with a similar resolution 1,500 x 1,200 dots. Figures for particular monitors vary with manufacturer, build quality and price. Widescreen TFT monitors may have quite different sizes such as 1440 x 990. However, in practice the situation is much more complicated because software enables the effective screen resolution to be changed by the user to one of several standard formats - typically , 800 x 600, 1024 x 768, 1152 x 864, 1280 x 768, 1280 x 800, or 1280 x 1024 pixels. Graphics cards may also limit the maximum achievable resolution.

As the screen resolution is increased, so the size of the elements displayed decreases. A GIF image such as the one on this page will appear smaller as the resolution increases. So on a 17" screen set to a resolution of 1024 x 768 pixels it will appear larger than when displayed on a 19" monitor set to 1280 x 1024 pixels. The physical size on a monitor of the white square on the right will be dependent upon a number of factors. It might seem reasonable to expect that its displayed dimensions would be determined by dividing the 216 pixel dimension of the original graphic by the monitor resolution (eg100 ppi) - hence 216 / 100ppi = 2.16". However, effective screen resolution is dependent upon monitor build quality, software and user control. Some browsers also use screen resolution compensation mechanisms, so complicated algorithms may be at work. There are also differences between Microsoft and Apple systems. Perceived quality remains more or less the same although there are differences in colour and gamma. The reality is that the displayed dimensions of the white square may be different on almost every computer system.

To determine the actual display resolution of the monitor on which this page is displayed, use a ruler or tape measure to determine the horizontal length of the green bar below. Then read the corresponding display resolution from the table on the right. In the case of a relatively modern monitor the measurement should be in the region of three or four inches, giving a display resolution in the range of approximately 70 - 100 ppi. It is important to ensure that any zoom setting available within the browser software is reset to the default or normal setting before the measurement is made.

In Windows XP resolution may be checked and changed from the control panel. Go to "appearance and themes", select "change screen resolution" and move the slider as far as possible to the right. Under the "settings" tab, select "advanced" and then "general" to see the dpi setting. The current screen resolution on which this page is being viewed is shown below.