Converting Vitamin D doses from mcg to IU and vice versa

To convert Vitamin D:
From IU to mcg: IU/40 = mcg
For example: 400 IU/40 = 10 mcg
From mcg to IU: mcg * 40 =IU

i.e. 400 IU of Vitamin = 10 mcg of Vitamin D

Source: https://dietarysupplementdatabase.usda.nih.gov/ingredient_calculator/help.php