Determine the size of a pixel by using a special calculator which can be found at the link http://monik.com.ua/size_pixel/. It also determines the number of dots per inch and aspect ratio for your screen resolution. Fill in the required fields and click "Calculate". Also there is attached a table with the most common modern versions of the diagonals and permits.
To measure the value of a pixel in a particular image, try it on the image on the screen with a ruler. Find the value of vertically and horizontally. Then divide the resulting number by the horizontal resolution and vertical pixelx, respectively. Thus, you can get the approximate size of a pixel in millimeters (or centimeters, depending on what you will measure).
Visit the website of the manufacturer of your monitor. There find the model you're interested. Then find passport data on it should be specified the size of the pixel characteristic of the monitor. Option is not easy and long, but works too.
Use a scale factor specified in the settings Windows. Based on this method, the change amount of the object from pixels to mm would look like this: width_mm:=width_pixels/PixelsPerInch*25.4 where width_pixels will be the length of the object at pixelx, and will PixelsPerInch property of the Screen object that specifies the scale factor of pixels per inch.
Use "Machomer", which you can download here http://blase.at.ua/load/9-1-0-3. With its help it is possible to measure the distance travelled by the mouse. The program can measure distance not only in the pixelx, but in any other units of measurement (centimeters, meters, kilometers, etc.).