Digital cameras and similar often quote viewfinder/screen resolutions in dots instead of actual pixel dimensions. This calculator is able to work out the approximate screen resolution from this dot figure.
If you are unsure of your screen's aspect ratio, try 3:2 for stills cameras, or 4:3 for video cameras (newer widescreen cameras may be 16:9). Square pixels are assumed for the calculation.
The round result option will round to the nearest standard horizontal or vertical resolution.
This calculator came about when I was looking into the resolution of the eye-viewfinder vs rear lcd on my mirrorless camera, trying to figure out if it would be possible to focus properly during 1080p video recording - how much of this resolution is actually shown in the viewfinder? I was intrigued to re-discover that the tiny screen in the viewfinder has higher resolution than the larger lcd. From the calculations, the lcd has 480 lines (NTSC SD resolution), while the eye-viewfinder works out exactly 1024 x 768 (a common computer resolution - probably used so some standard driver chip could be used)
You will find a good article covering some of this subject here.