How many milligrams of vitamin d do you need per day?

It varies. Vitamin d is usually described in iu (international units) rather than in mg. 600 iu, the suggested intake for adults in the us, is equivalent to 15 micrograms (mcg), or 0.015 milligrams (mg). This may be affected by sunlight exposure or GI conditions causing decrease absorption. However, once a deficit exists, it make take thousands of ius per day to correct the deficiency.