A common problem in astronomy is the conversion of a quantity measured in some units into a quantity based on other units.There are also several `systems of measurement' in use in physics and astronomy. The common systems are mks, for meter-kilogram-second, cgs, for centimeter-gram-second, and SI or the System Internationale (SI). SI is essentially an expanded mks system that includes a large variety of other standard units for quantities such as energy, force and magnetic flux.
1 ly = 9.4605x1015 m = 9.4605x1012 km (light year)
1 pc = 3.0857x1016 m = 3.0857x1013 km = 3.2616 ly = 206265 AU (parsec)
1 Mpc = 106 pc = 3.2616x106 ly (Megaparsec)