The Supplement Facts panel on a dietary supplement label declares the amount of elemental magnesium in the product, not the weight of the entire magnesium - containing compound. (ods.od.nih.gov)
Contains as much as 20 % more elemental magnesium than other magnesium supplements. (magnesium.ca)
Up to 800 mg / day of elemental magnesium is probably safe; however, people with kidney disease or severe heart disease should not supplement with magnesium without their doctor's approval. (afibbers.org)