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Nominal Size and Viewable Size
There are some things you need to take into account when assessing the size of a monitor. Sizes are normally stated as a single number, the width of the monitor in inches. The most popular sizes for monitors are 14", 15", 17", 20" and 21". This number represents the (alleged) diagonal width of the monitor, the distance from one corner of the screen to the opposite corner of the screen, and is the monitor's nominal size.
In reality, monitor screens are never the size that their manufacturers claim them to be. I do not know how this situation came about or why advertising authorities allow it to continue, but if you take a tape measure to your 17" monitor, you are likely to find that the screen itself is only, say, 15.8". And this doesn't take into account the fact that the screen image usually has black borders around its edge. The 15.8" number is the viewable size of the monitor. Many companies are getting better about advertising the viewable size along with the nominal size, but many still do not. Some unscrupulous ones exaggerate the stated viewable size of the monitor as well.
Some monitors have much more of a discrepancy between the nominal and viewable size than others do: most 15" monitors have a viewable size of 13.7" to 13.9" and most 14" monitors 13.1" to 13.3". I have, however, seen in advertisements, companies selling a 15" monitor with a viewable size of 13.7" and a 14" monitor with a viewable size of 13.5"! The monitors were the same make and model, had the same maximum resolution and dot pitch, and the 15" version was 35% more expensive. Which would you choose?
I personally find the whole matter of selecting monitors by their diagonal width to be confusing and rather silly. What really matters in the case of a monitor is the viewable surface area. If you have a 17" monitor that has a viewable image size of 15.8", and a 15" monitor with 13.9" viewable, is the 17" monitor 13.6% (15.8 divided by 13.9) larger? No, it is actually 29.4% larger, because the screen is larger in two dimensions, height and width. (This problem also occurs with pizza... A 15" pie is much more than 25% larger than a 12" one. :^) )
Most monitors use a 4:3 aspect ratio, which means the ratio of their width to their height is 4:3. This corresponds to the ratio used for most of the common video resolution modes such as 640x480, 800x600 and 1024x768 (but not all of them). Assuming a 4:3 aspect ratio, this table shows the typical viewable sizes for monitors of different nominal sizes, their viewable surface area, and just how much of an increase in real estate you get going from one size to the next:
This comparison table shows the percentage increase in surface area for each size compared to each other size. It should be read by looking at the row corresponding to the nominal size of the monitor under consideration. Then, the percentage value in each column is the percentage larger or smaller that size's surface area is compared to the other sizes. For example, a 20" monitor is 85.7% larger than a 15" monitor, but 9.8% smaller than a 21" monitor:
The actual image on the screen does not extend to the very edges of the viewable surface area, because there are black borders around the image. Therefore, your real image size will be smaller still than the viewable size of the monitor. The controls on the front of the monitor can be used to adjust the size of the actual image, which can help you make better use of the full screen. Beware however of some monitors that seriously distort the image when it approaches the edge of the screen, especially in the corners.