It is unfortunately impossible to convert millimeters into pixels, because the distance difference in millimeters on a screen depends on screen size in pixels, monitor size viewing area in centimeters and their ratio (Also known as resolution)

For instance, a 1 cm line on your screen will not be covered by the same number of pixels if you work in a 640x480 screen size or in 1600x1200 both with a 17inches monitor. In the first case, you have 640 pixels on 340mm, that is 1 pixel = 0.53mm (25 DPI). In the second case, you have 1600pixels still on 340mm, that is 1 pixel = 0.21mm (120 DPI).

To make it short, it is not possible to convert such so easily milimeters into pixels.

If you use the excellent Firefox as navigator, we suggest you to use an extension that allows you to calculate the number of pixel between two points (see link below)

In the Windows environment, one of the functions of screen capture software Faststone Captures allows the calculation of sizes in pixel anywhere in your graphic environment.