If you are asking what you say you are asking, i.e. how many DPI is your screen, nobody can say without a bit more info. But if your screen displays (say) 1200x1024 pixels and is (say) 12" wide and 9" high then it's 100dpi horizontally and 1024/9 dpi vertically. Simply really. But like that article that someone else pointed you to, dpi isn't a very useful notion when it comes to displays, which makes me wonder if you should be asking the question you asked...