In the USA B12 is measured in picograms/mL (pg/mL) while in Australia and other countries that use the SI system the units are picomoles/L (pmol/L).

According to the American Family Physician paper by Oh and Brown (2003)
http://www.aafp.org/afp/2003/0301/p979.html
the conversion factor is approximately 4/3.

To convert from pmol/L to pg/mL multiply by 4/3 or 1.355

To convert from pg/mL to pmol/L multiply by 3/4 or 0.738

Martin