The average adult body contains only a few grams of iron, mostly packed into the blood.

An adult human body holds roughly 3 to 4 grams of iron in total, a little less than the weight of a small paperclip. About two-thirds of it is bound up in hemoglobin, the protein inside red blood cells that grabs oxygen in the lungs and carries it to tissues throughout the body.

The rest is stored in the liver, spleen, and bone marrow, or tucked into other proteins involved in cellular energy production. Because the body has no efficient way to excrete excess iron, it carefully controls how much gets absorbed from food in the intestines, and even small chronic imbalances — too little or too much — can lead to conditions like anemia or iron overload.