I figured I would do a blog explaining exactly what certain dimensions mean and what they represent mathematically.
It's all based on multiplication really. Let's look at some examples...
We've all seen the TV dimensions 480p, 720p, and 1080p. But, what do they mean? Well,...
480p is based on older televisions and older computer monitors which had a much lower resolution than the monitors we have today (1080p currently being the highest available resolution on a large commercial scale for TVs). YouTube stil uses 480p as the 2nd or 3rd quality available. They also have 240p, 360p, 720p, and 1080p.
480p is based on the 4x3 (aka 4:3) model. 4x3 serves as the base number for the "square" look on TVs and PCs.
4x3 can be multiplied as 4 x 160 = 640 and 3 x 160 = 480 (i.e. 480p)
Therefore, 480p on a "standard" "square" TV = 640x480 pixels
This same math can be applied to any number. The lowest dimensions being 4x3. So, what about a "square" video on a 720p or 1080p TV? It's simple...
4 x 240 = 960 and 3 x 240 = 720 (i.e. 720p)
Therefore 720p = 960x720
4 x 360 = 1440 and 3 x 360 = 1080 (i.e. 1080p)
Therefore 1080p = 1440x1080
Remember that 720p and 1080p refer to the maximum height of the image/screen in pixels. They do not mention the width because the width can be different on any set but the height usually has a common number of 720 or 1080 pixels high.
Now, let's look at a widescreen 16x9 (aka 16:9) screen. In this circumstance, 16x9 is the lowest dimension in the set of numbers. This is not to be confused with letterbox screens, which have the black bars on the top and bottom of the screen even though you're watching it on a widescreen 16x9 TV.
16 x 80 = 1280 and 9 x 80 = 720 (i.e. 720p)
Therefore, on a 16x9 widescreen TV, 720p = 1280x720
16 x 120 = 1920 and 9 x 120 = 1080 (i.e. 1080p)