Note: The SI (System International) Unit prefix "m" stands for "milli" and equal to 0.001
1 mg = 1 {milli}g = 1 {milli} x g = 0.001g
and opposite is true as well:
1 g = 1 {milli} x g / {milli} = 1mg / {milli} = 1 mg / 0.001 = 1000 mg
Find various SI prefixes at the following URL:
http://www.aqua-calc.com/page/si-units-prefixes