It is possible to convert metric units simply by moving the decimal to the left or right from the starting prefix postition to the ending prefix position.
Kilo Hecta Deka Meter/Liter/Gram Deci Centi Milli
To convert from milligrams to grams move the decimal:
left 1
left 2
left 3
left 4
left 5
left 6
right 1
right 2
right 3
right 4
right 5
right 6
To convert from hectameters to millimeters move the decimal: