屏幕PPI计算: (White^2+Height^2)^0.5/屏幕大小英寸数

毫米和像素换算:    mm=(px/dpi)*25.4        px=(mm*dpi)/25.4

　　　　　　　　　英寸=px/dpi

　　　　　　　　　1英寸=25.4毫米

dpi获取:

方法1.

using (Graphics graphics = Graphics.FromHwnd(IntPtr.Zero))
            {
                float dpiX = graphics.DpiX;
                float dpiY = graphics.DpiY;
            }

方法2.

using (ManagementClass mc = new ManagementClass("Win32_DesktopMonitor"))
            {
                using (ManagementObjectCollection moc = mc.GetInstances())
                {

                    int PixelsPerXLogicalInch = 0; // dpi for x
                    int PixelsPerYLogicalInch = 0; // dpi for y

                    foreach (ManagementObject each in moc)
                    {
                        PixelsPerXLogicalInch = int.Parse((each.Properties["PixelsPerXLogicalInch"].Value.ToString()));
                        PixelsPerYLogicalInch = int.Parse((each.Properties["PixelsPerYLogicalInch"].Value.ToString()));
                    }

                    Console.WriteLine("PixelsPerXLogicalInch:" + PixelsPerXLogicalInch.ToString());
                    Console.WriteLine("PixelsPerYLogicalInch:" + PixelsPerYLogicalInch.ToString());
                    Console.Read();
                }
            }

获取DPI方法参考:http://www.cnblogs.com/dingli/p/3187217.html

像素和毫米的换算