How much iron is missing?
Iron deficiency is the most common cause of anemia worldwide. When the body lacks iron, it first uses up its stores, held mainly in the liver as ferritin, and then cannot make enough hemoglobin, the protein that carries oxygen in red blood cells. Correcting the anemia means replacing both the iron missing from hemoglobin and the iron missing from the stores. In 1970, the Swiss physician A. M. Ganzoni proposed a formula to estimate this total:
Total iron deficit (mg) = weight (kg) × (target Hb − actual Hb) × 2.4 + iron stores (mg)
- Target Hb is usually 15 g/dL for people weighing 35 kg (77 lb) or more.
- Iron stores are usually 500 mg above 35 kg, and 15 mg per kg below.
- 2.4 combines the blood volume (about 7% of body weight) and the iron content of hemoglobin (0.34%).
Worked example
An adult weighing 154 lb (69.9 kg) has a hemoglobin of 9 g/dL:
- Iron for hemoglobin: 69.9 × (15 − 9) × 2.4 = 1,006 mg.
- Iron for stores: 500 mg.
- Total deficit: 1,506 mg.
- With a maximum of 1,000 mg per infusion, that means 2 infusions.
Why the formula matters
Oral iron tablets are the first treatment for most people, but only a small fraction of the iron in each tablet is absorbed, and side effects are common. Intravenous iron is used when tablets fail or are not tolerated, when iron is lost faster than it can be absorbed, or before surgery. Because each IV product has a maximum single dose, knowing the total deficit helps plan the number of infusions.
Practical points
- Several IV iron products now use simplified dosing tables based on weight and hemoglobin, which may give a different total. The product’s labeling always takes precedence.
- In people with obesity, doctors often use ideal or adjusted body weight, since blood volume does not rise in proportion to fat mass.
- A target of 15 g/dL is high for many women; some prescribers use 13 to 14 g/dL.
- Hemoglobin rises slowly, by about 1 to 2 g/dL in 2 to 4 weeks after treatment. Ferritin and transferrin saturation are checked again a few weeks later.
- Finding the cause of the iron loss, such as heavy periods or bleeding in the digestive tract, is as important as replacing the iron.
This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.