points = (pixels / 96) * 72 on a standard XP/Vista/7 machine (factory defaults)
points = (pixels / 72) * 72 on a standard Mac running OSX (Factory defaults)
Windows runs as default at 96dpi (display) Macs run as default at 72 dpi (display)
72 POSTSCRIPT Points = 1 inch 12 POSTSCRIPT Points = 1 POSTSCRIPT Pica 6 POSTSCRIPT Picas = 72 Points = 1 inch
1 point = 1/72 inches = 25.4/72 mm = 0.3527 mm
DPI = Dots Per Inch PPI = Pixels Per Inch LPI = Lines per inch
More info if using em as measuring
16px = 1em (default for normal text) 8em = 16px * 8 Pixels/16 = em