The Recommended Daily Allowance (RDA) of magnesium for young adults is around 400 mg/day for men and 310 mg/day for women. For adults over 30, the RDA is 420 mg/day for men and 320 mg/day for women. However, despite how easy it is to find magnesium, studies have shown that between 68% and 75% of American adults are magnesium deficient.