I normally design (mostly theatre posters etc) for print so the size of artwork in cms or mm always. However, when folks ask me design graphics for web they stipulate the pixel size i.e 200 pixels high x 200 pixels high. I have NO IDEA what this measurement is so cannot even begin to set up a size for the artwork. Is there method of calculating? Thanks
The measurement is exactly that: 200px x 200px.
So for example, in PhotoShop, go to open a new image and when it gives you the dialog for the dimensions, type 200 into height and width and set it to px.
There's often a DPI setting as well, which will determine how big it will be when you print it from photoshop, but that's irrelevant here, as only the pixel dimensions matter. Out of habit/tradition, you can set it to 72dpi, however.
There is no 'real world' measurement of a pixel because a pixel is not a fixed unit of measurement in relation to any physical unit other than the screen you happen to be viewing it on. And screens come with all different sizes of pixels. On a Retina iPhone, 1px = 1/326 of an inch. That's pretty small. On a scoreboard at the stadium, 1px may = 1 inch. That's pretty big.
A bit of trivia: The '72dpi' tradition for graphics on a screen goes back to the original Macintosh. Which supposedly was due to Jobs fascination with calligraphy and typography at the time. Its screen was exactly 72 ppi (which correlated to the 'point' unit of measure in type which was also 72 to the inch). The ImageWriter, which was the first Apple printer, was exactly 144 dpi. So for a brief time in DTP history, there was an actual relation between the pixel on screen and the printed image and a real world unit of measurement (1 inch).
DPI refers to the density in which a printer can squirt dots of ink. PPI controls the intended density of the graphics pixels meaning "how many of your pixels do you want to squeeze into one real world inch when it is printed".
For screen, both PPI and DPI are irrelevant, as a pixel is a pixel and the density is controlled by the screen you are viewing it through (See DA01's 3rd paragraph about retina vs scoreboard). Most people set screen content to 72ppi, but that is just a trend and makes absolutely no difference to an image set to 10,000ppi, providing the actual pixel dimensions remain the same. This would add some confusion though if it was ever sent to a printer...
In print you are probably used to setting your image at 300ppi, meaning in 1in x 1in of real-world space, the image is simply 300px x 300px. You are setting it as 300 pixels to be spread across an inch, the higher the PPI number the more you squeeze into that inch and the denser that space gets. An 8in x 10in photo, if set to 300ppi, is simply a multiple of that: 2400px x 3000px.