C#

PointF ConvertMillimtersToPixel(Graphics grfx, PointF pointF)
{
    pointF.X *= grfx.DpiX / 25.4f;
    pointF.Y *= grfx.DpiY /25.4f;

    return pointF;
}

public sealed class Display
{
  static Display()
  {
    SystemEvents.DisplaySettingsChanged += new EventHandler(Update);
  }
  private Display() {} // Prevent instantiation

  [DllImport("gdi32.dll")]
  private static extern IntPtr GetDeviceCaps(IntPtr hdc, IntPtr index);

  [DllImport("user32.dll")]
  private static extern IntPtr GetDC(IntPtr hwnd);

  [DllImport("user32.dll")]
  private static extern IntPtr ReleaseDC(IntPtr hwnd, IntPtr hdc);

  private static void Update(object sender, EventArgs e)
  {
    Setup(true);
  }
  private static void Setup(bool force)
  {
    if (initialized && !force) return;
    twipsPerPixelX  = twipsPerPixelY = 0; // Initial values
    IntPtr hdc = GetDC(IntPtr.Zero);
    try
    {
      if (!hdc.Equals(IntPtr.Zero))
      {
        IntPtr value;
        value = GetDeviceCaps(hdc, new IntPtr(88)); // LOGPIXELSX
        twipsPerPixelX = 1440 / value.ToInt32();
        value = GetDeviceCaps(hdc, new IntPtr(90)); // LOGPIXELSY
        twipsPerPixelY = 1440 / value.ToInt32();
        ReleaseDC(IntPtr.Zero, hdc);
      }
    }
    catch {}
    if (twipsPerPixelX == 0) twipsPerPixelX = 15; // Default for 96 DPI
    if (twipsPerPixelY == 0) twipsPerPixelY = 15; // Default for 96 DPI
    initialized = true;
  }
  private static bool initialized;
  private static int twipsPerPixelX;
  private static int twipsPerPixelY;
  public static int TwipsPerPixelX
  {
    get
    {
      Setup(false);
      return twipsPerPixelX;
    }
  }
  public static int TwipsPerPixelY
  {
    get
    {
      Setup(false);
      return twipsPerPixelY;
    }
  }
  public static int FromPixelsX(int x, ScaleMode mode)
  {
    switch (mode)
    {
      case ScaleMode.Points:
        return PixelsToTwipsX(x) / 20;
      case ScaleMode.Inches:
        return PixelsToTwipsX(x) / 1440;
      default:
        throw new ArgumentException("Invalid scale mode.", "mode");
    }
  }
  public static int FromPixelsY(int y, ScaleMode mode)
  {
    switch (mode)
    {
      case ScaleMode.Points:
        return PixelsToTwipsY(y) / 20;
      case ScaleMode.Inches:
        return PixelsToTwipsY(y) / 1440;
      default:
        throw new ArgumentException("Invalid scale mode.", "mode");
    }
  }
  public static int ToPixelsX(int x, ScaleMode mode)
  {
    switch (mode)
    {
      case ScaleMode.Points:
        return TwipsToPixelsX(x * 20);
      case ScaleMode.Inches:
        return TwipsToPixelsX(x * 1440);
      default:
        throw new ArgumentException("Invalid scale mode.", "mode");
    }
  }
  public static int ToPixelsY(int y, ScaleMode mode)
  {
    switch (mode)
    {
      case ScaleMode.Points:
        return TwipsToPixelsY(y * 20);
      case ScaleMode.Inches:
        return TwipsToPixelsY(y * 1440);
      default:
        throw new ArgumentException("Invalid scale mode.", "mode");
    }
  }
  public static int PixelsToTwipsX(int x)
  {
    Setup(false);
    return x * twipsPerPixelX;
  }
  public static int PixelsToTwipsY(int y)
  {
    Setup(false);
    return y * twipsPerPixelY;
  }
  public static int TwipsToPixelsX(int x)
  {
    Setup(false);
    return x / twipsPerPixelX;
  }
  public static int TwipsToPixelsY(int y)
  {
    Setup(false);
    return y / twipsPerPixelY;
  }
}

// A couple of examples
public enum ScaleMode
{
  Points,
  Inches
}